8.2: Acids-Properties and Examples
Learning Objectives
- Examine properties of acids.
Many people enjoy drinking coffee. A cup first thing in the morning helps start the day. But keeping the coffee maker clean can be a problem. Lime deposits from hard water build up after a while and slow down the brewing process. The best cure for this is to put vinegar (dilute acetic acid) in the pot and run it through the brewing cycle. The vinegar dissolves the deposits of hard water and cleans the maker, which will speed up the brewing process back to its original rate. Just be sure to run water through the brewing process after the vinegar, or you will get some really horrible coffee.
Properties of Acids
Acids are very common in some of the foods that we eat. Citrus fruits such as oranges and lemons contain citric acid and ascorbic acid. Ascorbic acid is better known as vitamin C. Carbonated sodas contain carbonic acid. Vinegar contains acetic acid. Your own stomach utilizes hydrochloric acid to digest food. Acids are a distinct class of compounds because of the properties of their aqueous solutions as outlined below:
- Aqueous solutions of acids are electrolytes, meaning that they conduct electrical current. Some acids are strong electrolytes because they ionize completely in water, yielding a great many ions. Other acids are weak electrolytes that exist primarily in a non-ionized form when dissolved in water.
- Acids have a sour taste. Lemons, vinegar, and sour candies all contain acids.
- Acids change the color of certain acid-base indicators. Two common indicators in the laboratory are litmus and phenolphthalein. Blue litmus turns red in the presence of an acid, while phenolphthalein turns colorless. Dye extracted from purple cabbage is an excellent indicator to qualitatively determine the acid or base properties of household chemicals.
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Acids react with active metals to yield hydrogen gas. Metals will replace the hydrogen from an acid in a single-replacement reaction, as shown below:
\[\ce{Zn} \left( s \right) + \ce{H_2SO_4} \left( aq \right) \rightarrow \ce{ZnSO_4} \left( aq \right) + \ce{H_2} \left( g \right) \label{eq1}\] - Acids react with bases to produce a salt compound and water. When equal moles of an acid and a base are combined, the acid is neutralized by the base. The products of this reaction are an ionic compound, which is labeled as a salt, and water.
Common Acids
Common binary acids are formed from monoatomic anions of the halogen family (group 7A). The anions fluoride (F - ), chloride (Cl - ), bromide (Br - ) and iodide (I - ) combine with one hydrogen ion, (H + ) to form hydrofluoric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid. The acids are called binary because they are made of two elements.
hydrofluoric acid HF (aq)
hydrochloric acid HCl (aq)
hydrobromic acid HBr(aq)
hydroiodic acid HI (aq)
Common ternary acids are formed from polyatomic anions such as sulfate (SO 4 2 - ), nitrate (NO 3 - ), carbonate (CO 3 2 - ) and phosphate (PO 4 3 - ). The anions sulfate and carbonate combine with two hydrogen ions and form sulfuric acid and carbonic acid. The nitrate ion combine with one hydrogen ion and form nitric acid, and the phosphate anion combine with three hydrogen ions to form phosphoric acid. The acids are called ternary because they are made of three elements.
Nitric acid HNO 3 (aq)
Sulfuric acid H 2 SO 4 (aq)
Carbonic acid H 2 CO 3 (aq)
Phosphoric acid H 3 PO 4 (aq)
There are other common acids based on the carboxylic acid functional group. Examples are acetic acid in vinegar, citric acid in citrus fruits, and ascorbic acid in vitamin C. Here's another partial list of some common acids, along with some chemical formulas and uses.
|
Chemist Name
|
Common Name | Uses |
|---|---|---|
|
hydrochloric acid, HCl |
muriatic acid (used in pools) and stomach acid is HCl | Used in cleaning (refining) metals, in maintenance of swimming pools, and for household cleaning. |
|
sulfuric acid, H 2 SO 4 |
Used in car batteries, and in the manufacture of fertilizers. | |
|
nitric acid, HNO 3 |
Used in the manufacture of fertilizers, explosives and in extraction of gold. | |
|
acetic acid, HC 2 H 3 O 2 |
vinegar | Main ingredient in vinegar. |
|
carbonic acid, H 2 CO 3 |
responsible for the "fizz" in carbonated drinks | As an ingredient in carbonated drinks. |
|
citric acid, C 6 H 8 O 7 |
Used in food and dietary supplements. Also added as an acidulant in creams, gels, liquids, and lotions. | |
|
acetylsalicylic acid, C 6 H 4 (OCOCH 3 )CO 2 H |
aspirin | The active ingredient in aspirin. |
Hydrochloric Acid
Hydrochloric acid is a corrosive, strong mineral acid with many industrial uses. A colorless, highly pungent solution of hydrogen chloride (HCl) in water. Hydrochloric acid is usually prepared by treating \(\ce{HCl}\) with water.
\[ \ce{\displaystyle HCl (g) + H2O (l) \longrightarrow H_3O^{+}(aq) + Cl^{-} (aq) }\]
Hydrochloric acid can therefore be used to prepare chloride salts. Hydrochloric acid is a strong acid, since it is completely dissociated in water. Hydrochloric acid is the preferred acid in titration for determining the amount of bases.
Sulfuric Acid
Sulfuric acid is a highly corrosive strong mineral acid with the molecular formula \(\ce{H2SO4}\). Sulfuric acid is a diprotic acid and has a wide range of applications including use in domestic acidic drain cleaners, [ as an electrolyte in lead-acid batteries, and in various cleaning agents. It is also a central substance in the chemical industry.
The hydration of sulfuric acid is highly exothermic and the affinity of it for water is sufficiently strong. So sulfuric acid is an excellent dehydrating agent. Concentrated sulfuric acid has a very powerful dehydrating property, removing water (\(\ce{H2O}\)) from other compounds including sugar and other carbohydrates and producing carbon, heat, steam. Sulfuric acid behaves as a typical acid in its reaction with most metals by generating hydrogen gas (Equation \ref{Eq1}).
\[\ce{Zn + H2SO4 → ZnSO4 + H2 } \label{Eq1}\]
Nitric Acid
Nitric acid (\(\ce{HNO3}\)) is a highly corrosive mineral acid and is also commonly used as a strong oxidizing agent. Nitric acid is normally considered to be a strong acid at ambient temperatures. Nitric acid can be made by reacting nitrogen dioxide (\(\ce{NO_2(g)}\)) with water.
\[\ce{3 NO2(g) + H2O (l)→ 2 HNO3 (ag) + NO(g)}\]
Nitric acid reacts with most metals, but the details depend on the concentration of the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in its reaction with most metals (e.g., nitric acid with magnesium, manganese or zinc will liberate \(\ce{H2}\) gas):
\[\ce{Mg + 2 HNO3 → Mg(NO3)2 + H2 }\]
\[\ce{Mn + 2 HNO3 → Mn(NO3)2 + H2 }\]
\[\ce{Zn + 2 HNO3 → Zn(NO3)2 + H2 }\]
Nitric acid is a corrosive acid and a powerful oxidizing agent. The major hazard posed by it is chemical burns as it carries out acid hydrolysis with proteins (amide) and fats (ester) which consequently decomposes living tissue (Figure \(\PageIndex{2}\)). Concentrated nitric acid stains human skin yellow due to its reaction with the keratin
Carbonic Acid
Carbonic acid is a chemical compound with the chemical formula \(\ce{H2CO3}\) and is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of \(\ce{H2CO3(aq)}\). Carbonic acid, which is a weak acid, forms two kinds of salts: the carbonates and the bicarbonates. In geology, carbonic acid causes limestone to dissolve, producing calcium bicarbonate—which leads to many limestone features such as stalactites and stalagmites. Carbonic acid is a polyprotic acid, specifically it is diprotic, meaning it has two protons which may dissociate from the parent molecule.
When carbon dioxide dissolves in water it produces carbonic acid:
\[\ce{CO2 + H2O <=> H2CO3} \]
The reaction can be pushed to favor the reactants to generate \(\ce{CO2(g)}\) from solution, which is key to the bubbles observed in carbonated beverages (Figure \(\PageIndex{3}\)).
Formic Acid
Formic acid (\(\ce{HCO2H}\)) is the simplest carboxylic acid and is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. The word "formic" comes from the Latin word for ant, formica, referring to its early isolation by the distillation of ant bodies. Formic acid occurs widely in nature as its conjugate base formate.
Citric Acid
Citric acid (\(\ce{C6H8O7}\)) is a weak organic tricarboxylic acid that occurs naturally in citrus fruits. The citrate ion is an intermediate in the citric acid cycle ( Krebs cycle ), a central metabolic pathway for animals, plants and bacteria. Because it is one of the stronger edible acids, the dominant use of citric acid is used as a flavoring and preservative in food and beverages, especially soft drinks.
Acetylsalicylic Acid
Acetylsalicylic acid (also known as aspirin) is a medication used to treat pain, fever, and inflammation. Aspirin, in the form of leaves from the willow tree, has been used for its health effects for at least 2,400 years.
Aspirin is a white, crystalline, weakly acidic substance.
Summary
A brief summary of key aspects of several acids commonly encountered by students was given. Acids are a distinct class of compounds because of the properties of their aqueous solutions.
Contributions & Attributions
- Deboleena Roy, Ph.D. Dept. of Science and Engineering @ American River College.
-
Peggy Lawson (Oxbow Prairie Heights School). Funded by Saskatchewan Educational Technology Consortium.
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Henry Agnew (UC Davis)


3.2: Ionic Bonding
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Learning Objectives
- To describe the characteristics of ionic bonding.
- To quantitatively describe the energetic factors involved in the formation of an ionic bond.
Ions are atoms or molecules which are electrically charged. Cations are positively charged and anions carry a negative charge. Ions form when atoms gain or lose electrons. Since electrons are negatively charged, an atom that loses one or more electrons will become positively charged; an atom that gains one or more electrons becomes negatively charged. Ionic bonding is the attraction between positively- and negatively-charged ions. These oppositely charged ions attract each other to form ionic networks (or lattices ). Electrostatics explains why this happens: opposite charges attract and like charges repel. When many ions attract each other, they form large, ordered, crystal lattices in which each ion is surrounded by ions of the opposite charge. Generally, when metals react with non-metals, electrons are transferred from the metals to the non-metals. The metals form positively-charged ions and the non-metals form negatively-charged ions.
Generating Ionic Bonds
Ionic bonds form when metals and non-metals chemically react. By definition, a metal is relatively stable if it loses electrons to form a complete valence shell and becomes positively charged. Likewise, a non-metal becomes stable by gaining electrons to complete its valence shell and become negatively charged. When metals and non-metals react, the metals lose electrons by transferring them to the non-metals, which gain them. Consequently, ions are formed, which instantly attract each other—ionic bonding.
In the overall ionic compound, positive and negative charges must be balanced, because electrons cannot be created or destroyed, only transferred. Thus, the total number of electrons lost by the cationic species must equal the total number of electrons gained by the anionic species.
Example \(\PageIndex{1}\): Sodium Chloride
For example, in the reaction of Na (sodium) and Cl (chlorine), each Cl atom takes one electron from a Na atom. Therefore each Na becomes a Na + cation and each Cl atom becomes a Cl - anion. Due to their opposite charges, they attract each other to form an ionic lattice. The formula (ratio of positive to negative ions) in the lattice is \(\ce{NaCl}\).
\[\ce{2Na (s) + Cl 2(g) \rightarrow 2NaCl (s)} \nonumber \]
These ions are arranged in solid NaCl in a regular three-dimensional arrangement (or lattice):
NaCl lattice. (left) 3-D structure and (right) simple 2D slice through lattes. Images used with permission from Wikipedia and Mike Blaber.
The chlorine has a high affinity for electrons, and the sodium has a low ionization energy. Thus the chlorine gains an electron from the sodium atom. This can be represented using ewis dot symbols (here we will consider one chlorine atom, rather than Cl 2 ):
The arrow indicates the transfer of the electron from sodium to chlorine to form the Na + metal ion and the Cl - chloride ion. Each ion now has an octet of electrons in its valence shell:
- Na +: 2s 2 2p 6
- Cl - : 3s 2 3p 6
Energetics of Ionic Bond Formation
Ionic bonds are formed when positively and negatively charged ions are held together by electrostatic forces. Consider a single pair of ions, one cation and one anion. How strong will the force of their attraction be? According to Coulomb's Law , the energy of the electrostatic attraction (\(E\)) between two charged particles is proportional to the magnitude of the charges and inversely proportional to the internuclear distance between the particles (\(r\)):
\[E \propto \dfrac{Q_{1}Q_{2}}{r} \label{Eq1a} \]
\[ E = k\dfrac{Q_{1}Q_{2}}{r} \label{Eq1b} \]
where each ion’s charge is represented by the symbol Q . The proportionality constant k is equal to 2.31 × 10 −28 J·m. This value of k includes the charge of a single electron (1.6022 × 10 −19 C) for each ion. The equation can also be written using the charge of each ion, expressed in coulombs (C), incorporated in the constant. In this case, the proportionality constant, k , equals 8.999 × 109 J·m/C 2 . In the example given, Q 1 = +1(1.6022 × 10 −19 C) and Q 2 = −1(1.6022 × 10 −19 C). If Q 1 and Q 2 have opposite signs (as in NaCl, for example, where Q 1 is +1 for Na + and Q 2 is −1 for Cl − ), then E is negative, which means that energy is released when oppositely charged ions are brought together from an infinite distance to form an isolated ion pair.
Energy is always released when a bond is formed and correspondingly, it always requires energy to break a bond.
As shown by the green curve in the lower half of Figure \(\PageIndex{1}\), the maximum energy would be released when the ions are infinitely close to each other, at r = 0. Because ions occupy space and have a structure with the positive nucleus being surrounded by electrons, however, they cannot be infinitely close together. At very short distances, repulsive electron–electron interactions between electrons on adjacent ions become stronger than the attractive interactions between ions with opposite charges, as shown by the red curve in the upper half of Figure \(\PageIndex{1}\). The total energy of the system is a balance between the attractive and repulsive interactions. The purple curve in Figure \(\PageIndex{1}\) shows that the total energy of the system reaches a minimum at r 0 , the point where the electrostatic repulsions and attractions are exactly balanced. This distance is the same as the experimentally measured bond distance .
Consider the energy released when a gaseous \(Na^+\) ion and a gaseous \(Cl^-\) ion are brought together from r = ∞ to r = r 0 . Given that the observed gas-phase internuclear distance is 236 pm, the energy change associated with the formation of an ion pair from an \(Na^+_{(g)}\) ion and a \(Cl^-_{(g)}\) ion is as follows:
\[ \begin{align*} E &= k\dfrac{Q_{1}Q_{2}}{r_{0}} \\[4pt] &= (2.31 \times {10^{ - 28}}\rm{J}\cdot \cancel{m} ) \left( \dfrac{( + 1)( - 1)}{236\; \cancel{pm} \times 10^{ - 12} \cancel{m/pm}} \right) \\[4pt] &= - 9.79 \times 10^{ - 19}\; J/ion\; pair \label{Eq2} \end{align*} \]
The negative value indicates that energy is released. Our convention is that if a chemical process provides energy to the outside world, the energy change is negative. If it requires energy, the energy change is positive. To calculate the energy change in the formation of a mole of NaCl pairs, we need to multiply the energy per ion pair by Avogadro’s number:
\[ E=\left ( -9.79 \times 10^{ - 19}\; J/ \cancel{ion pair} \right )\left ( 6.022 \times 10^{ 23}\; \cancel{ion\; pair}/mol\right )=-589\; kJ/mol \label{Eq3} \]
This is the energy released when 1 mol of gaseous ion pairs is formed, not when 1 mol of positive and negative ions condenses to form a crystalline lattice. Because of long-range interactions in the lattice structure, this energy does not correspond directly to the lattice energy of the crystalline solid. However, the large negative value indicates that bringing positive and negative ions together is energetically very favorable, whether an ion pair or a crystalline lattice is formed.
We summarize the important points about ionic bonding:
- At r 0 , the ions are more stable (have a lower potential energy) than they are at an infinite internuclear distance. When oppositely charged ions are brought together from r = ∞ to r = r 0 , the energy of the system is lowered (energy is released).
- Because of the low potential energy at r 0 , energy must be added to the system to separate the ions. The amount of energy needed is the bond energy.
- The energy of the system reaches a minimum at a particular internuclear distance (the bond distance).
Example \(\PageIndex{2}\): LiF
Calculate the amount of energy released when 1 mol of gaseous Li + F − ion pairs is formed from the separated ions. The observed internuclear distance in the gas phase is 156 pm.
Given: cation and anion, amount, and internuclear distance
Asked for: energy released from formation of gaseous ion pairs
Strategy:
Substitute the appropriate values into Equation \(\ref{Eq1b}\) to obtain the energy released in the formation of a single ion pair and then multiply this value by Avogadro’s number to obtain the energy released per mole.
Solution:
Inserting the values for Li + F − into Equation \(\ref{Eq1b}\) (where Q 1 = +1, Q 2 = −1, and r = 156 pm), we find that the energy associated with the formation of a single pair of Li + F − ions is
\[ \begin{align*} E &=k \dfrac{Q_1Q_2}{r_0} \\[4pt] &=\left(2.31 \times 10^{−28} J⋅\cancel{m} \right) \left(\dfrac{\text{(+1)(−1)}}{156\; pm \times 10^{−12} \cancel{m/pm}} \right)\\[4pt] &=−1.48 \times 10^{−18} \end{align*} \nonumber \]
Then the energy released per mole of Li + F − ion pairs is
\[ \begin{align*} E&= \left(−1.48 \times 10^{−18} J/ \cancel{\text{ion pair}}\right) \left(6.022 \times 10^{23} \cancel{\text{ion pair}}/mol\right)\\[4pt] &−891 \;kJ/mol \end{align*} \nonumber \]
Because Li + and F − are smaller than Na + and Cl − (see Section 7.3 ), the internuclear distance in LiF is shorter than in NaCl. Consequently, in accordance with Equation \(\ref{Eq1b}\), much more energy is released when 1 mol of gaseous Li + F − ion pairs is formed (−891 kJ/mol) than when 1 mol of gaseous Na + Cl − ion pairs is formed (−589 kJ/mol).
Exercise \(\PageIndex{2}\): Magnesium oxide
Calculate the amount of energy released when 1 mol of gaseous \(\ce{MgO}\) ion pairs is formed from the separated ions. The internuclear distance in the gas phase is 175 pm.
- Answer
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−3180 kJ/mol = −3.18 × 10 3 kJ/mol
Electron Configuration of Ions
How does the energy released in lattice formation compare to the energy required to strip away a second electron from the Na + ion? Since the Na + ion has a noble gas electron configuration, stripping away the next electron from this stable arrangement would require more energy than what is released during lattice formation (Sodium I 2 = 4,560 kJ/mol). Thus, sodium is present in ionic compounds as Na + and not Na 2 + . Likewise, adding an electron to fill a valence shell (and achieve noble gas electron configuration) is exothermic or only slightly endothermic. To add an additional electron into a new subshell requires tremendous energy - more than the lattice energy. Thus, we find Cl - in ionic compounds, but not Cl 2 - .
| Compound | Lattice Energy (kJ/mol) |
|---|---|
| LiF | 1024 |
| LiI | 744 |
| NaF | 911 |
| NaCl | 788 |
| NaI | 693 |
| KF | 815 |
| KBr | 682 |
| KI | 641 |
| MgF 2 | 2910 |
| SrCl 2 | 2130 |
| MgO | 3938 |
This amount of energy can compensate for values as large as I 3 for valence electrons (i.e. can strip away up to 3 valence electrons). Because most transition metals would require the removal of more than 3 electrons to attain a noble gas core, they are not found in ionic compounds with a noble gas core. A transition metal always loses electrons first from the higher 's' subshell, before losing from the underlying 'd' subshell. (The remaining electrons in the unfilled d subshell are the reason for the bright colors observed in many transition metal compounds!) For example, iron ions will not form a noble gas core:
- Fe: [Ar]4s 2 3d 6
- Fe 2 + : [Ar] 3d 6
- Fe 3 + : [Ar] 3d 5
Some metal ions can form a pseudo noble gas core (and be colorless), for example:
- Ag: [Kr]5s 1 4d 10 Ag + [Kr]4d 10 Compound: AgCl
- Cd: [Kr]5s 2 4d 10 Cd 2 + [Kr]4d 10 Compound: CdS
The valence electrons do not adhere to the "octet rule" in this case (a limitation of the usefulness of this rule). Note: The silver and cadmium atoms lost the 5s electrons in achieving the ionic state.
When a positive ion is formed from an atom, electrons are always lost first from the subshell with the largest principle quantum number
Polyatomic Ions
Not all ionic compounds are formed from only two elements. Many polyatomic ions exist, in which two or more atoms are bound together by covalent bonds. They form a stable grouping which carries a charge (positive or negative). The group of atoms as a whole acts as a charged species in forming an ionic compound with an oppositely charged ion. Polyatomic ions may be either positive or negative, for example:
- NH 4 + (ammonium) = cation
- SO 4 2 - (sulfate) = anion
The principles of ionic bonding with polyatomic ions are the same as those with monatomic ions. Oppositely charged ions come together to form a crystalline lattice, releasing a lattice energy. Based on the shapes and charges of the polyatomic ions, these compounds may form crystalline lattices with interesting and complex structures.
Summary
The amount of energy needed to separate a gaseous ion pair is its bond energy. The formation of ionic compounds are usually extremely exothermic . The strength of the electrostatic attraction between ions with opposite charges is directly proportional to the magnitude of the charges on the ions and inversely proportional to the internuclear distance. The total energy of the system is a balance between the repulsive interactions between electrons on adjacent ions and the attractive interactions between ions with opposite charges.
Contributors and Attributions
- Wikibooks [en.wikibooks.org]
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Modified by Joshua Halpern ( Howard University )


4: The Integumentary System
By the end of this section, you will be able to:
- Identify the components of the integumentary system
- Describe the layers of the skin and the functions of each layer
- Identify and describe the hypodermis and deep fascia
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- 4.1: Layers of the Skin
- This page explains the skin as a vital organ in the integumentary system, composed of three main layers: the avascular epidermis, vascular dermis, and hypodermis. The epidermis has multiple cell layers, while the dermis includes blood vessels and nerves, along with hair follicles and sweat glands. Additionally, it details hair and nails, which are keratin-based accessory structures that protect the body, with nails originating from specialized epidermal cells at the tips of fingers and toes.
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- 4.2: Exercises
- This page details lab exercises centered on the anatomy and physiology of skin and related structures. It includes tasks for labeling components of the integumentary system like hair follicles, sebaceous glands, epidermal layers, and nails. The exercises focus on skin accessories and histological features, enhancing understanding of skin structure and function. Each task is designed to progressively build knowledge by identifying specific parts and their roles.


 The following equation is shown at the bottom of the sheet:
\(=\frac{v_{2}^{2}-v_{1}^{2}}{2a}\)
along with the following description:
- \(v_{1} =\) initial measured velocity of the ball m/s - various measurements.
- \(v_{2} =\) final measured velocity of the ball m/s - seems to be zero every time.
- \(a =\) acceleration of the ball \((−9.8\text{m/s}^{2})\).
Unfortunately, the students spilled ketchup on the left hand side of their equation, making it illegible. Luckily, you are proficient in dimensional analysis. What were the students trying to calculate, based on this model?
- Answer
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We can use their equation to determine the dimension of the quantity on the left hand side:
\([?]=\frac{[v_{2}^{2}]-[v_{1}^{2}]}{[a]}=\frac{\frac{L}{T}^{2}-\frac{L}{T}^{2}}{\frac{L}{T^{2}}}=L\)
Thus, the dimension of the unknown quantity is length. Given the context, they were likely attempting to model the height at which a vertically thrown ball would travel before stopping.
Exercise \(\PageIndex{2}\)
Chelsea is preparing meticulously for her upcoming trip to Europe. Being a self-proclaimed “shop-a-holic” and physics lover, she wants to figure out how many pairs of shoes she can buy on vacation that will physically fit in her closet. Her closet is a walk-in closet with two entrance doors. Estimate the number of pairs of shoes that can fit in Chelsea’s closet.
- Answer
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We start by estimating the volume of Chelsea’s closet as well as that of a pair of shoes. Chelsea’s closet is a “walk-in closet’ with two double doors. If we know the dimensions of the door, we can estimate the width and height of the closet. Estimating the average size of a large door to be \(1\)m\(×2\)m, one face of the close will have an area of \(4\text{m}^{2}\). If we estimate the depth of Chelsea’s closet to be about \(3\text{m}\), the volume of her closet is \(12\text{m}^{3}\)
Next, we can estimate the size of an average pair of shoes, by modeling a shoe as a rectangular box. A single shoe has a height and width of about \(5\) cm and a length of about \(25\) cm. A pair of shoes will thus be equivalent to box with dimensions \(5\)cm\(×10\)cm\(×25\)cm\(=1250\)cm\(^{3}\). This is equivalent to \(0.00125\text{m}^{3}\). We can now determine how many pairs of shoes, \(N\), would fit in the closet:
\(N=\frac{(12\text{m}^{3})}{(0.00125\text{m}^{3})}=9600\approx 10,000\)
We find that Chelsea can buy about \(10,000\) new pairs of shoes on her trip, and still fit them all into her closet. Time to get shopping, Chelsea!


2.2: Trailblazer
By the end of this section, you will be able to:
- Explain how authors weave identity into their compositions.
- Articulate how genre conventions are shaped by purpose, culture, and expectation.
Identity Trailblazer: Cathy Park Hong
"Sometimes you need to explain your experiences
in order to understand them yourself."
Figure \(2.3\) Cathy Park Hong ( https://openstax.org/r/ cathyparkhong) writes about racism from the perspective of Asians and Asian Americans. (credit: “Racism is not an option” by GoToVan/flickr, CC BY 2.0)
Fear: The Enemy Within
Cathy Park Hong (b. 1976) is an Asian American poet, writer, and educator committed to exploring living art.
Born to Korean parents in Los Angeles, California, Hong studied at Oberlin College and at the Writers’ Workshop at the University of Iowa, where she earned a master of fine arts degree. She has received numerous fellowships, including a Fulbright scholarship, a Guggenheim Fellowship, a National Endowment for the Arts Fellowship, and a New York Foundation for the Arts Fellowship. Hong has taught at Sarah Lawrence College and Rutgers University and served as poetry editor for the New Republic .
In her work, Hong explores her search for identity as a first-generation Asian American, specifically her struggles with feeling alienated from Anglo-American culture. Hong’s first publication, the book of poems Translating Mo’um (2002), examines the often tenuous challenges experienced by first-generation Americans, specifically regarding language. Her second book of poems, Dance Dance Revolution: Poems (2007), won the Barnard Women Poets Prize. The work incorporates the idea of “code switching,” a technique in which people switch between languages or language forms, such as formal and colloquial.
Hong most recently published a collection of essays titled Minor Feelings: An Asian American Reckoning (2020), a memoir that takes on the lens of cultural criticism. Writing from her personal experiences as an Asian American woman growing up and living in America, Hong delves into deeply painful and often unseen racial trauma experienced by Asians and Asian Americans. Published as anti-Asian hate crimes gained increasing national attention, Hong’s essays help readers understand the curious place that Asian Americans inhabit in American race relations, where they are viewed as caricatures of the “model minority” and face invisible racism.
The title of Minor Feelings comes from the word han , which Koreans use to describe emotions that include anger, melancholy, envy, and shame. Hong believes these same emotions are shared by minorities in America today. She expresses difficulty in using the pronoun we in her writing because of the diversity of Asian Americans. Yet she notes that what this diverse population has in common is that even as second- and third-generation Americans, Asian Americans find they still don’t enjoy first-class status in American life in the same way that White Americans do. She proposes thinking of the label Asian Americans as “less of an identity, and more as a coalition” (Hong, “Why”) in order to seek common ground with others who share similar experiences. Hong believes that one key is cross-cultural community building among Asian communities and between Asian, Black, Latina/Latino, and Indigenous communities.
Hong’s poetry and essays also explore the racism she has experienced in the literary world, from graduate school to literary circles. Perhaps the most maddening part of those experiences, she notes, is that her perception of racism is constantly “questioned or dismissed” (Hong, Minor 55). Although writing about racial experiences was discouraged as “anti-academic” when she was a student, Hong has made a career of sharing these experiences through poetry and prose. She hopes that her work not only helps Asian Americans feel recognized and acknowledged but also encourages readers of all ethnicities to practice self-interrogation.
Discussion Questions
- Cathy Park Hong recalls not having an outlet to express the racism she experienced growing up. How has writing provided that outlet for her as an adult?
- Race informs Hong’s writing, though the academic circles she was a part of discouraged this. How can art and language be influenced by identity?
- How has Hong’s work helped her explore her own culture and provided a window for others to understand it?
- The myth of the model minority isolates Asian Americans from other people of color. How does Hong’s writing work to overcome this isolation?


1.10: Writing in College
- Describe effective methods for approaching different kinds of college writing assignments
The Importance of Writing in the Workplace
Consider this: a recent survey of employers conducted by the Association of American Colleges and Universities found that 89 percent of employers say that colleges and universities should place more emphasis on “the ability to effectively communicate orally and in writing.” [1] It was the single-most favored skill in this survey.
In addition, several of the other valued skills are grounded in written communication:
- “Critical thinking and analytical reasoning skills” (81 percent)
- “The ability to analyze and solve complex problems” (75 percent)
- “The ability to locate, organize, and evaluate information from multiple sources” (68 percent).
This emphasis on communication probably reflects the changing reality of work. Employers also reported that employees will have to “take on more responsibilities,” “use a broader set of skills,” “work harder to coordinate with other departments,” face “more complex” challenges, and mobilize “higher levels of learning and knowledge.” [2]
The pay-off from improving your writing comes much sooner than graduation. Suppose you complete about 40 classes for a 120-credit bachelors’ degree, and—averaging across writing-intensive and non-writing-intensive courses—you produce about 2,500 words of formal writing per class. Even with that low estimate, you’ll write 100,000 words during your college career. That’s roughly equivalent to a 330-page book.
Spending a few hours sharpening your writing skills will make those 100,000 words much easier and more rewarding to write. Even your non-English professors care about and appreciate good writing.
Understanding the Assignment
There are four kinds of analysis you need to do in order to fully understand an assignment: determining its purpose, understanding how to answer an assignment’s questions, recognizing implied questions in the assignment, and recognizing the expectations of the assignment, which vary depending on the discipline and subject matter. Always make sure you fully understand an assignment before you start writing!
Determine the Purpose
The wording of an assignment should suggest its purpose. Any of the following might be expected of you in a college writing assignment:
- Summarizing information
- Analyzing ideas and concepts
- Taking a position and defending it
- Combining ideas from several sources and creating your own original argument.
Understand How to Respond
College writing assignments will ask you to answer a how or why question – questions that can’t be answered with just facts. For example, the question “ What are the names of the presidents of the US in the last twenty years?” needs only a list of facts to be answered. The question “ Who was the best president of the last twenty years and why?” requires you to take a position and support that position with evidence.
Sometimes, a list of prompts may appear with an assignment. Usually, your instructor will not expect you to answer all of the questions listed. They are simply offering you some ideas so that you can think of your own questions to ask.
Recognize Implied Questions
A prompt may not include a clear ‘how’ or ‘why’ question, though one is always implied by the language of the prompt. For example:
“Discuss the effects of the No Child Left Behind Act on special education programs” is asking you to write how the act has affected special education programs.
“Consider the recent rise of autism diagnoses” is asking you to write why the diagnoses of autism are on the rise.
Before beginning the writing process, always establish the following:
- Is there an assigned topic or are you free to choose your own?
- What about your subject interests you?
- Why is your subject worth reading about?
- Double-check that your subject is not too broad – narrow it down if necessary.
- Determine the purpose of the work.
- Determine the readers of the work and their level of knowledge about the topic.
- Determine where your evidence will come from.
- Decide what kind of evidence would best serve your argument.
- Identify the required style (MLA, APA, etc.) of the paper.
- Be aware of length specifications.
- Consider if visuals might be helpful in your paper.
- Will someone be reviewing drafts of your paper? Who?
- Note your deadline and how much time you have for each stage of the writing process.
This Assignment Calculator can help you plan ahead for your writing assignment. Just plug in the date you plan to get started and the date it is due, and it will help break it down into manageable chunks.
Recognize Expectations
Depending on the discipline in which you are writing, different features and formats of your writing may be expected. Always look closely at key terms and vocabulary in the writing assignment, and be sure to note what type of evidence and citations style your instructor expects.
All college classes will expect you to do your own work. Using another person’s words, images, or other original creations without giving proper credit is called plagiarism .
Oftentimes, as we prepare to address an assignment, we look at other material to help us with our thinking. This is research, and it’s a great thing! Professors always do research! When we do research to help us clarify our thinking, however, we need to be sure to acknowledge the sources that we have consulted and the ways in which our ideas have been influenced by others.
We use different citation guides (like APA and MLA) to format citations and lists of references. Sometimes students aren’t sure how to do these references correctly, and so they leave out the citations altogether. That’s never a good idea. Even if you aren’t sure how to create a perfect citation, always include references to all the material you’ve consulted. Otherwise, you could be committing plagiarism: taking someone else’s work (words and/or ideas) and presenting it as your own—the equivalent of cheating on a test. In order to be sure you don’t accidentally leave out a source, remember to keep track of what you consult as you begin research for a project or assignment.
The Writing Process
Have you ever received a writing assignment, thought “this won’t take long” and then stayed up all night writing the night before your assignment was due because it ended up taking a lot longer than you thought it would? If you have, you’re not alone. Many beginning writers struggle to plan well when it comes to a writing assignment, and this results in writing that is just not as good as it could be. When you wait until the last minute and fail to engage in every step of the writing process, you’re not doing your best work.
Writing itself is a process through which you ask questions; create, develop, hone, and organize ideas; argue a point; search for evidence to support your ideas…and so on. The point here is that writing really involves creative and critical thinking processes. Like any creative process, it often starts in a jumble as you develop, sort, and sift through ideas. But it doesn’t need to stay in disarray. Your writing will gain direction as you start examining those ideas. It just doesn’t happen all at once. Writing is a process that happens over time. And like any process, there are certain steps or stages.
These are some of the major stages in a strong writing process:
- Thinking about your assignment
- Prewriting
- Gathering information and evidence
- Organizing and drafting
- Revising and editing
The writing process is not linear, but recursive, meaning you will need to move forward through some steps and then circle back to redo previous steps. In other words, while we still think of writing as a process taking place in a series of steps, we now understand that good writers tend to switch frequently among the different steps as they work. An insight gained while editing one chapter might convince the writer that an additional chapter is needed; as a result, she might start another drafting phase—or even decide to divide one chapter into two or three, and begin reorganizing and developing new drafts.
In short, while it is very useful to think of writing as a process, the process is not a clear, always-the-same series of steps. Instead, it is a sometimes messy, forward-and-backward process in which you strive for simplicity but try to appeal to your audience, create but also organize, enjoy yourself, if possible, but also follow some rules, and eventually create a finished product that works.
practice: purpose and process
Writing Through Fear
Writing is an activity that can cause occasional anxiety for anyone, even professional writers. Start early and use strategies, like those mentioned below, to help you work through the anxiety.
The following essay about writing anxiety, by Hillary Wentworth, from the Walden Writing Center, offers insight about how to handle issues surrounding writer’s block:
I suppose fall is the perfect time to discuss fear. The leaves are falling, the nights are getting longer, and the kids are preparing ghoulish costumes and tricks for Halloween.
So here’s my scary story: A few weeks ago, I sat down at my computer to revise an essay draft for an upcoming deadline. This is old hat for me; it’s what I do in my personal life as a creative writer, and it’s what I do in my professional life as a Walden Writing Center instructor. As I was skimming through it, though, a feeling of dread settled in my stomach, I began to sweat, and my pulse raced. I was having full-on panic. About my writing.
This had never happened to me before. Sure, I have been disappointed in my writing, frustrated that I couldn’t get an idea perfectly on paper, but not completely fear-stricken. I Xed out of the Word document and watched Orange Is the New Black on Netflix because I couldn’t look at the essay anymore. My mind was too clouded for anything productive to happen. The experience got me thinking about the role that fear plays in the writing process. Sometimes fear can be a great motivator. It might make us read many more articles than are truly necessary, just so we feel prepared enough to articulate a concept. It might make us stay up into the wee hours to proofread an assignment. But sometimes fear can lead to paralysis. Perhaps your anxiety doesn’t manifest itself as panic at the computer; it could be that you worry about the assignment many days—or even weeks—before it is due.
Here are some tips to help:
- Interrogate your fear . Ask yourself why you are afraid. Is it because you fear failure, success, or judgment? Has it been a while since you’ve written academically, and so this new style of writing is mysterious to you?
- Write through it . We all know the best way to work through a problem is to confront it. So sit at your desk, look at the screen, and write. You might not even write your assignment at first. Type anything—a reflection on your day, why writing gives you anxiety, your favorite foods. Sitting there and typing will help you become more comfortable with the prospect of more.
- Give it a rest . This was my approach. After realizing that I was having an adverse reaction, I called it quits for the day, which ultimately helped reset my brain.
- Find comfort in ritual and reward . Getting comfortable with writing might involve establishing a ritual (a time of day, a place, a song, a warm-up activity, or even food or drink) to get yourself into the writing zone. If you accomplish a goal or write for a set amount of time, reward yourself.
- Remember that knowledge is power. Sometimes the only way to assuage our fear is to know more. Perhaps you want to learn about the writing process to make it less intimidating. Check out the Writing Center’s website for tips and tutorials that will increase your confidence. You can also always ask your instructor questions about the assignment.
- Break it down. If you feel overwhelmed about the amount of pages or the vastness of the assignment, break it up into small chunks. For example, write one little section of the paper at a time.
- Buddy up. Maybe you just need someone with whom to share your fears—and your writing. Ask a classmate to be a study buddy or join an eCampus group.
The writing centers at the University of North Carolina at Chapel Hill and the University of Richmond , as well as the news site Inside Higher Ed , also have helpful articles on writing anxiety.
Contributors and Attributions
- The Writing Process Image. Provided by : Lumen Learning. License : CC BY: Attribution
- Modification, adaptation, and original content. Provided by : Lumen Learning. License : CC BY: Attribution
- Understanding the Assignment and Assessing the Writing Situation . Authored by : Robin Jeffrey. Located at : https://openoregon.pressbooks.pub/aboutwriting/chapter/understanding-the-assignment/ . Project : About Writing: A Guide. License : CC BY: Attribution
- Paragraph on recursive writing. Authored by : Marianne Botos, Lynn McClelland, Stephanie Polliard, Pamela Osback . Located at : https://pvccenglish.files.wordpress.com/2010/09/eng-101-inside-pages-proof2-no-pro.pdf . Project : Horse of a Different Color: English Composition and Rhetoric . License : CC BY: Attribution
- The Writing Process. Authored by : Excelsior OWL. Located at : https://owl.excelsior.edu/writing-process/ . License : CC BY: Attribution
- Really? Writing? Again?. Authored by : Amy Guptill. Provided by : The College at Brockport, SUNY Open Textbooks. License : CC BY-NC-SA: Attribution-NonCommercial-ShareAlike
- Writing Through Fear. Authored by : Hillary Wentworth. Provided by : Walden Writing Center. Located at : http://waldenwritingcenter.blogspot.com/2013/10/writing-through-fear.html . License : CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
- Image of tree. Authored by : Broo_am (Andy B). Located at : https://flic.kr/p/dHcmy5 . License : CC BY-NC-ND: Attribution-NonCommercial-NoDerivatives
- Avoiding Plagiarism. Authored by : Cengage Learning. Located at : https://florida.theorangegrove.org/og/file/86100f79-955b-41f8-ac7f-7f3906ea0128/1/CengageOpenNow_CollegeSuccessNarrative.pdf . Project : Open Now College Success. License : CC BY: Attribution


6.1: Introduction
Epidemiology is the study of diseases, health-states, and health outcomes. Epidemiologists are especially interested in how diseases and conditions are distributed throughout populations, and finding those factors that increase or decrease the risk of a particular disease - or in fact actually cause the disease or condition. The word “epidemiology” comes from the Greek word “epidemia”, which means the “prevalence of a disease”. So naturally those studying diseases have a lot of questions, such as:
- Does a disease affect one population more than another? Do the number of people affected increase or decrease over time?
- What are the associations between biological factors, exposures to pathogens or toxins, the environment, and behaviors that could influence disease states and health outcomes?
- Are there ways to increase the average lifespan and make those years healthy and enjoyable?
- Can we still identify and stop a disease or condition from hurting more people even if we don’t know the exact cause?
All of these questions - and others - spark the curiosity of someone interested in epidemiology. Historically, epidemiology was first used to help identify causes and risk factors for infectious diseases. This is still an essential role for epidemiologists around the world, especially as we experience new and re-emerging contagious diseases. However, chronic diseases, behavioral health problems, injuries, and violence are now also studied using epidemiology techniques. In this chapter, we’ll go over some of the basic tools that epidemiologists use to understand health problems, and potentially how to solve them.


22.6: The Hall Effect
Learning Objectives
By the end of this section, you will be able to:
- Describe the Hall effect.
- Calculate the Hall emf across a current-carrying conductor.
We have seen effects of a magnetic field on free-moving charges. The magnetic field also affects charges moving in a conductor. One result is the Hall effect, which has important implications and applications.
Figure \(\PageIndex{1}\) shows what happens to charges moving through a conductor in a magnetic field. The field is perpendicular to the electron drift velocity and to the width of the conductor. Note that conventional current is to the right in both parts of the figure. In part (a), electrons carry the current and move to the left. In part (b), positive charges carry the current and move to the right. Moving electrons feel a magnetic force toward one side of the conductor, leaving a net positive charge on the other side. This separation of charge creates a voltage \(\varepsilon\), known as the Hall emf , across the conductor. the creation of a voltage across a current-carrying conductor by a magnetic field is known as the Hall effect , after Edwin Hall, the American physicist who discovered it in 1879.
One very important use of the Hall effect is to determine whether positive or negative charges carries the current. Note that in Figure 1b, where positive charges carry the current, the Hall emf has the sign opposite to when negative charges carry the current. Historically, the Hall effect was used to show that electrons carry current in metals and it also shows that positive charges carry current in some semiconductors. The Hall effect is used today as a research tool to probe the movement of charges, their drift velocities and densities, and so on, in materials. In 1980, it was discovered that the Hall effect is quantized, an example of quantum behavior in a macroscopic object.
The Hall effect has other uses that range from the determination of blood flow rate to precision measurement of magnetic field strength. To examine these quantitatively, we need an expression for the Hall emf, \(\varepsilon\), across a conductor. Consider the balance of forces on a moving charge in a situation where \(B\), \(v\), and \(l\) are mutually perpendicular, such as shown in Figure \(\PageIndex{2}\). Although the magnetic force moves negative charges to one side, they cannot build up without limit. The electric field caused by their separation opposes the magnetic force, \(F = qvB\), and the electric force, \(F_{e} = qE\), eventually grows to equal it. That is,
\[qE = qvB \nonumber\]
or
\[E = vB. \nonumber\]
Note that the electric field \(E\) is uniform across the conductor because the magnetic field \(B\) is uniform, as is the conductor. For a uniform electric field, the relationship between electric field and voltage is \(E = \varepsilon / l \), where \(l\) is the width of the conductor and \(\varepsilon\) is the Hall emf. Entering this into the last expression gives
\[\frac{\varepsilon}{l} = vB.\nonumber\]
Solving this for the Hall emf yields
\[\varepsilon = Blv \label{22.7.4}\]
if \(B\), \(v\), and \(l\) are mutually perpendicular.
where \(\varepsilon\) is the Hall effect voltage across a conductor of width \(l\) through which charges move at a speed \(v\).
One of the most common uses of the Hall effect is in the measurement of magnetic field strength \(B\). Such devices, called Hall probes , can be made very small, allowing fine position mapping. Hall probes can also be made very accurate, usually accomplished by careful calibration. Another application of the Hall effect is to measure fluid flow in any fluid that has free charges (most do) (Figure \(\PageIndex{3}\)). A magnetic field applied perpendicular to the flow direction produces a Hall emf \(\varepsilon\) as shown. Note that the sign of \(\varepsilon\) depends not on the sign of the charges, but only on the directions of \(B\) and \(v\). The magnitude of the Hall emf is \(\varepsilon = Blv\), where \(l\) is the pipe diameter, so that the average velocity \(v\) can be determined from \(\varepsilon\) providing the other factors are known.
Example \(\PageIndex{1}\): Calculating the Hall emf - Hall Effect for Blood Flow
A Hall effect flow probe is placed on an artery, applying a 0.100-T magnetic field across it, in a setup similar to that in Figure \(\PageIndex{3}\). What is the Hall emf, given the vessel’s inside diameter is 4.00 mm and the average blood velocity is 20.0 cm/s?
Strategy:
Because \(B\), \(v\), and \(l\) are mutually perpendicular, the Equation \ref{22.7.4} can be used to find \(\varepsilon\).
Solution:
Entering the given values for \(B\), \(v\), and \(l\) gives
\[\begin{align*} \varepsilon &= Blv \\[4pt] &= \left(0.100 T\right) \left(4.00 \times 10^{-3} m\right) \left(0.200 m/s\right) \\[5pt] &= 80.0 \mu V \end{align*}\]
Discussion:
This is the average voltage output. Instantaneous voltage varies with pulsating blood flow. The voltage is small in this type of measurement. \(\varepsilon\) is particularly difficult to measure, because there are voltages associated with heart action (ECG voltages) that are on the order of millivolts. In practice, this difficulty is overcome by applying an AC magnetic field, so that the Hall emf is AC with the same frequency. An amplifier can be very selective in picking out only the appropriate frequency, eliminating signals and noise at other frequencies.
Summary
- The Hall effect is the creation of voltage \(\varepsilon\), known as the Hall emf, across a current-carrying conductor by a magnetic field.
- The Hall emf is given by \[\varepsilon = Blv\nonumber\] with \(B\), \(v\), and \(l\) all mutually perpendicular for a conductor of width \(l\) through which charges move at a speed \(v\).
Glossary
- Hall effect
- the creation of voltage across a current-carrying conductor by a magnetic field
- Hall emf
- the electromotive force created by a current-carrying conductor by a magnetic field, \(ε=Blv\)


5.2: The Center of the Universe
The Center of the Universe
Simply stated, the Universe is huge. It is hard for most of us to comprehend the scale and size of our own Earth-Moon system, the distance to the Sun, our Solar System, or the Milky Way Galaxy. It is understandable that ancient peoples had challenges comprehending the arrangement and make-up of even the Solar System, nonetheless the Milky Way Galaxy and the Universe.
Most early Solar System models placed Earth as the center of the Solar System — and most-often the Universe. The reasoning made sense: everything appeared to go around Earth. Aristotle in the 4 th Century BC was one of the earliest proponents of an Earth-centered Solar System — a geocentric model. Yet, one of the earliest Solar System models, that of Aristarchus in the 3 rd Century BC, did correctly place the Sun in the center of the Solar System. Most accepted the Earth-centered Solar System which was refined by the Greek philosopher Claudius Ptolemy in the 2 nd Century AD. This geocentric Solar System model was accepted by both scientific and religious circles until the work of Nicolas Copernicus (1543) and observations with the telescope of Galileo Galilei (1609) showed the Sun was the center of the Solar System, a heliocentric model. (1)
- Authored by : Florida State College at Jacksonville. License : CC BY: Attribution


4.2: Student Sample- Narrative Essay
Instructions: Read the student essay, “Melon Harvest,” by J. Workman. First read the NON-ANNOTATED student essay, and then compare it to the ANNOTATED version of the same essay. The purpose of annotation is to help you think deeply about a text as you read it. Notice how the annotated comments analyze and respond to the essay.
This is a model for how to engage with and annotate the writing you will encounter in this course.
Melon Harvest (Non-Annotated Version)
By J. Workman
At two in the afternoon I drove the five miles from our apartment complex into downtown Texarkana to drop off voter registration forms and mail paperwork about my recent change in residency to my employer. My husband’s job had brought me to this small city straddling the border of Texas and Arkansas, where everything still felt quaint and unfamiliar.
Driving home down Jefferson Avenue on the Arkansas side of town I passed a grocery store parking lot where a farmer had parked his beat-up truck and shoddy trailer under a faded awning. The trailer was loaded up with the last of his watermelon harvest—late season, probably sweet as they come. The truck bed held early season sweet potatoes. End of the summer fare and beginning of the winter stores, all in one load. The farmer’s back was facing me as he helped a woman buying watermelons. His arms were huge and his shoulders wider than a yardstick. He was tall, big, sunburned on his neck and upper arms.
 Although this farmer was a stranger to me, I recognized his profile. I knew his stories, including the toil, drudgery and poverty that too often mark a hard-working farmer’s life.  Great Grandpa George was a farmer and cowhand and blacksmith and sheep shearer and whatever else he could do to put food on the table.
Although rural communities surrounded Texarkana, somehow I was surprised to see this hardscrabble farmer here, now today. He was the real thing. This man wasn’t like the hippie organic growers at the Santa Monica farmer’s market I visited every Saturday before I moved from L.A. He wasn’t a part-timer coming to town only seasonally with a load of grapes or melons. This man was no immigrant laborer from Mexico or El Salvador or Guatemala who came in the 1990s to work the farms on behalf of the white owners. He was not the descendent of Latin American or Asian laborers who came three generations ago and now own farms themselves.
This was a homegrown, Caucasian, American farmer, but the not-quite-Norman Rockwell kind. He was a “dirt farmer,” as my new neighbor Billy described the type, himself the son of a sharecropper. He grew whatever he could coax out of the land, working those acres generation after generation to hold onto the family homestead. This farmer was young, early thirties perhaps, close to my own age (although you can’t really tell after awhile because farmers take on the age of the earth). Seeing him hit close to home for me, too much a reminder of the hardship and poverty of my own family’s roots, uncomfortably manifest today in another family’s ongoing hardship.
 I thought about it, but I didn’t really want to have to eat a whole watermelon on my own. More than that, I didn’t know how I would explain to him the tears I knew would stream from my eyes as I hefted the melon and counted out the bills.
Turning the corner into our apartment complex I passed the Greyhound station just in time to be a voyeur viewing a private moment between a mother and son.  He was a big, overweight, sloppy but good-natured looking boy. For the second time tears sprang to my eyes, and I couldn’t say quite say why. After all, my parents put me on a Greyhound bus to go back to college after the holidays one winter. My father said it was a mandatory life experience, taking the bus to school. When I complained, he told me it was good motivation to finish college and get a good job. I can’t say he was wrong about that.
But here in Texarkana my tears flowed again for that mother and son and the harshness of the big city world out beyond this place. I cried for the harshness of my citified eyes judging these homespun people in their quaint small place with its oddities and familiarities, its first names and slow-moving afternoons and churches bigger than stadiums.
I cried for how God must love the humility of a place like this, a town that unobtrusively preserves a Bible in the courthouse and trusts this nation could never do wrong. Perhaps in some ways this is a better place—better than me and the world I’m from. But in some ways it’s also a dying place, dying like a field of vines after the melons are harvested, wilting in the sunlight of America’s finest hour.
Annotated Version
Click on the link to view the annotated version of the essay: “Melon Harvest,” by J. Workman


12.2: Gastrointestinal Basics
It is important for the nurse to be aware of the underlying structures of the abdomen when completing a gastrointestinal or genitourinary assessment. See Figure \(\PageIndex{1}\) [1] for an illustration of the gastrointestinal system and the bladder. See Figure \(\PageIndex{2}\) [2] for an illustration of the male urinary system. Know the position of the organs within the abdominal cavity as you learn to auscultate, palpate, and percuss the abdomen.
For a detailed overview of the gastrointestinal system, common GI disorders, and related GI medications, visit the “ Gastrointestinal ” chapter of the Open RN Nursing Pharmacology textbook. Specific sections of the chapter include the following:
- GI System Review for additional review of the anatomy of the GI system
- Antidiarrheals and Laxatives for more information about treating diarrhea and constipation
- Antiemetics for more information about treating nausea and vomiting
- Hyperacidity and Acid Controlling Medication for more information about treating acid reflux and gastric ulcers
- “abdomen-intestine-large-small-1698565” by bodymybody is licensed under CC0 ↵
- “ Urinary_System_(Male).png ” by BruceBlaus is licenced under CC BY-SA 4.0 ↵


33.1.2: Superphylum Lophotrochozoa
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Learning Objectives
- Describe the phylogenetic position and basic features of lophotrochozoa
Lophotrochozoans
Animals belonging to superphylum Lophotrochozoa are protostomes: the blastopore (or the point of involution of the ectoderm or outer germ layer) becomes the mouth opening to the alimentary canal. This is called protostomy or “first mouth.” In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band which then splits internally to form the coelom; this protostomic coelom is termed schizocoelom.
As lophotrochozoans, the organisms in this superphylum possess either lophophore or trochophore larvae. The exact relationships between the different phyla are not entirely certain. The lophophores include groups that are united by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth. Lophophorata include the flatworms and several other phyla, including the Bryozoa, Entoprocta, Phoronida, and Brachiopoda. These clades are upheld when RNA sequences are compared. Trochophore larvae are characterized by two bands of cilia around the body. Previously, these were treated together as the Trochozoa, together with the arthropods, which do not produce trochophore larvae, but were considered close relatives of the annelids because they are both segmented. However, they show a number of important differences. Arthropods are now placed separately among the Ecdysozoa. The Trochozoa include the Nemertea, Mollusca, Sipuncula, and Annelida.
The lophotrochozoans are triploblastic, possessing an embryonic mesoderm sandwiched between the ectoderm and endoderm found in the diploblastic cnidarians. These phyla are also bilaterally symmetrical: a longitudinal section will divide them into right and left sides that are symmetrical. They also show the beginning of cephalization: the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.
Key Points
- Lophotrochozoa have a blastopore, which is an involution of the ectoderm that forms a rudimentary mouth opening to the alimentary canal, a condition called protostomy or “first mouth”.
- The Lophotrochozoa are comprised of the trochozoans and the lophophorata, although the exact relationships between the different phyla are not clearly determined.
- Lophophores are characterized by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth; they include the flatworms and several other phyla whose relationships are upheld by genetic evidence.
- Trochophore larvae are distinguished from the lophophores by two bands of cilia around the body; they include the Nemertea, Mollusca, Sipuncula, and Annelida.
- The lophotrochozoans have a mesoderm layer positioned between the ectoderm and endoderm and are bilaterally symmetrical, which signals the beginning of cephalization, the concentration of nervous tissues and sensory organs in the head of the organism.
Key Terms
- blastopore : the opening into the archenteron
- lophophore : a feeding organ of brachiopods, bryozoans, and phoronids
- cephalization : an evolutionary trend in which the neural and sense organs become centralized at one end (the head) of an animal


3.1: Heat, Work, and the First Law
The box below gives two forms of the first law of thermodynamics .
The equation \(\dif U=\dq+\dw\) is the differential form of the first law, and \(\Del U=q+w\) is the integrated form.
The heat and work appearing in the first law are two different modes of energy transfer. They can be defined in a general way as follows.
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An infinitesimal quantity of energy transferred as heat at a surface element of the boundary is written \(\dq\), and a finite quantity is written \(q\) (Sec. 2.5). To obtain the total finite heat for a process from \(q=\int\!\dq\) (Eq. 2.5.3), we must integrate over the total boundary surface and the entire path of the process.
An infinitesimal quantity of work is \(\dw\), and a finite quantity is \(w=\int\!\dw\). To obtain \(w\) for a process, we integrate all kinds of work over the entire path of the process.
The first-law equation \(\Del U=q+w\) sets up a balance sheet for the energy of the system, measured in the local frame, by equating its change during a process to the net quantity of energy transferred by means of heat and work. Note that the equation applies only to a closed system. If the system is open, energy can also be brought across the boundary by the transport of matter.
An important part of the first law is the idea that heat and work are quantitative energy transfers. That is, when a certain quantity of energy enters the system in the form of heat, the same quantity leaves the surroundings. When the surroundings perform work on the system, the increase in the energy of the system is equal in magnitude to the decrease in the energy of the surroundings. The principle of conservation of energy is obeyed: the total energy (the sum of the energies of the system and surroundings) remains constant over time.
Strictly speaking, it is the sum of the energies of the system, the surroundings, and any potential energy shared by both that is constant. The shared potential energy is usually negligible or essentially constant (Sec. G.5).
Heat transfer may occur by conduction, convection, or radiation. (Some thermodynamicists treat radiation as a separate contribution to \(\Del U\), in addition to \(q\) and \(w\).) We can reduce conduction with good thermal insulation at the boundary, we can eliminate conduction and convection with a vacuum gap, and we can minimize radiation with highly reflective surfaces at both sides of the vacuum gap. The only way to completely prevent heat during a process is to arrange conditions in the surroundings so there is no temperature gradient at any part of the boundary. Under these conditions the process is adiabatic, and any energy transfer in a closed system is then solely by means of work.
3.1.1 The concept of thermodynamic work
Appendix G gives a detailed analysis of energy and work based on the behavior of a collection of interacting particles moving according to the principles of classical mechanics. The analysis shows how we should evaluate mechanical thermodynamic work. Suppose the displacement responsible for the work comes from linear motion of a portion of the boundary in the \(+x\) or \(-x\) direction of the local frame. The differential and integrated forms of the work are then given by \begin{equation} \dw=F\sur_x\dx \qquad w=\int_{x_1}^{x_2}\!\!F\sur_x\dx \tag{3.1.1} \end{equation} (These equations are Eq. G.6.11 with a change of notation.) Here \(F\sur_x\) is the component in the \(+x\) direction of the force exerted by the surroundings on the system at the moving portion of the boundary, and \(\dx\) is the infinitesimal displacement of the boundary in the local frame. If the displacement is in the same direction as the force, \(\dw\) is positive; if the displacement is in the opposite direction, \(\dw\) is negative.
The kind of force represented by \(F\sur_x\) is a short-range contact force. Appendix G shows that the force exerted by a conservative time-independent external field, such as a gravitational force, should not be included as part of \(F\sur_x\). This is because the work done by this kind of force causes changes of potential and kinetic energies that are equal and opposite in sign, with no net effect on the internal energy (see Sec. 3.6).
Newton’s third law of action and reaction says that a force exerted by the surroundings on the system is opposed by a force of equal magnitude exerted in the opposite direction by the system on the surroundings. Thus the expressions in Eq. 3.1.1 can be replaced by \begin{equation} \dw=-F\sups{sys}_x\dx \qquad w=-\int_{x_1}^{x_2}\!\!F\sups{sys}_x\dx \tag{3.1.2} \end{equation} where \(F\sups{sys}_x\) is the component in the \(+x\) direction of the contact force exerted by the system on the surroundings at the moving portion of the boundary.
An alternative to using the expressions in Eqs. 3.1.1 or 3.1.2 for evaluating \(w\) is to imagine that the only effect of the work on the system’s surroundings is a change in the elevation of a weight in the surroundings. The weight must be one that is linked mechanically to the source of the force \(F\sur_x\). Then, provided the local frame is a stationary lab frame, the work is equal in magnitude and opposite in sign to the change in the weight’s potential energy: \(w = -mg\Del h\) where \(m\) is the weight’s mass, \(g\) is the acceleration of free fall, and \(h\) is the weight’s elevation in the lab frame. This interpretation of work can be helpful for seeing whether work occurs and for deciding on its sign, but of course cannot be used to determine its value if the actual surroundings include no such weight.
The procedure of evaluating \(w\) from the change of an external weight’s potential energy requires that this change be the only mechanical effect of the process on the surroundings, a condition that in practice is met only approximately. For example, Joule’s paddle-wheel experiment using two weights linked to the system by strings and pulleys, described latter in Sec. 3.7.2, required corrections for (1) the kinetic energy gained by the weights as they sank, (2) friction in the pulley bearings, and (3) elasticity of the strings (see Prob. 3.10).
In the first-law relation \(\Del U=q+w\), the quantities \(\Del U\), \(q\), and \(w\) are all measured in an arbitrary local frame. We can write an analogous relation for measurements in a stationary lab frame: \begin{equation} \Del E\sys=q\lab+w\lab \tag{3.1.3} \end{equation} Suppose the chosen local frame is not a lab frame, and we find it more convenient to measure the heat \(q\lab\) and the work \(w\lab\) in a lab frame than to measure \(q\) and \(w\) in the local frame. What corrections are needed to find \(q\) and \(w\) in this case?
If the Cartesian axes of the local frame do not rotate relative to the lab frame, then the heat is the same in both frames: \(q=q\lab\) (Sec. G.7).
The expressions for \(\dw\lab\) and \(w\lab\) are the same as those for \(\dw\) and \(w\) in Eqs. 3.1.1 and 3.1.2, with \(\dx\) interpreted as the displacement in the lab frame. There is an especially simple relation between \(w\) and \(w\lab\) when the local frame is a center-of-mass frame—one whose origin moves with the system’s center of mass and whose axes do not rotate relative to the lab frame (Eq. G.8.12): \begin{equation} w = w\lab - \onehalf m \Del\!\left(v^2\cm\right) - mg\Del z\cm \tag{3.1.4} \end{equation} In this equation \(m\) is the mass of the system, \(v\cm\) is the velocity of its center of mass in the lab frame, \(g\) is the acceleration of free fall, and \(z\cm\) is the height of the center of mass above an arbitrary zero of elevation in the lab frame. In typical thermodynamic processes the quantities \(v\cm\) and \(z\cm\) change to only a negligible extent, if at all, so that usually to a good approximation \(w\) is equal to \(w\lab\).
When the local frame is a center-of-mass frame, we can combine the relations \(\Del U=q+w\) and \(q=q\lab\) with Eqs. 3.1.3 and 3.1.4 to obtain \begin{equation} \Del E\sys = \Del E\subs{k} + \Del E\subs{p} + \Del U \tag{3.1.5} \end{equation} where \(E\subs{k} = \onehalf mv^2\cm\) and \(E\subs{p} = mgz\cm\) are the kinetic and potential energies of the system as a whole in the lab frame.
A more general relation for \(w\) can be written for any local frame that has no rotational motion and whose origin has negligible acceleration in the lab frame (Eq. G.7.3): \begin{equation} w = w\lab - mg \Del z\subs{loc} \tag{3.1.6} \end{equation} Here \(z\subs{loc}\) is the elevation in the lab frame of the origin of the local frame. \(\Del z\subs{loc}\) is usually small or zero, so again \(w\) is approximately equal to \(w\lab\). The only kinds of processes for which we may need to use Eq. 3.1.4 or 3.1.6 to calculate a non-negligible difference between \(w\) and \(w\lab\) are those in which massive parts of the system undergo substantial changes in elevation in the lab frame.
Simple relations such as these between \(q\) and \(q\lab\), and between \(w\) and \(w\lab\), do not exist if the local frame has rotational motion relative to a lab frame.
Hereafter in this e-book, thermodynamic work \(w\) will be called simply work . For all practical purposes you can assume the local frames for most of the processes to be described are stationary lab frames. The discussion above shows that the values of heat and work measured in these frames are usually the same, or practically the same, as if they were measured in a local frame moving with the system’s center of mass. A notable exception is the local frame needed to treat the thermodynamic properties of a liquid solution in a centrifuge cell. In this case the local frame is fixed in the spinning rotor of the centrifuge and has rotational motion. This special case will be discussed in Sec. 9.8.2.
3.1.2 Work coefficients and work coordinates
If a process has only one kind of work, it can be expressed in the form \begin{equation} \dw = Y\dif X \qquad \tx{or} \qquad w = \int_{X_1}^{X_2} \! Y\dif X \tag{3.1.7} \end{equation} where \(Y\) is a generalized force called a work coefficient and \(X\) is a generalized displacement called a work coordinate . The work coefficient and work coordinate are conjugate variables. They are not necessarily actual forces and displacements. For example, we shall see in Sec. 3.4.2 that reversible expansion work is given by \(\dw=-p\dif V\); in this case, the work coefficient is \(-p\) and the work coordinate is \(V\).
A process may have more than one kind of work, each with its own work coefficient and conjugate work coordinate. In this case the work can be expressed as a sum over the different kinds labeled by the index \(i\): \begin{equation} \dw = \sum_i Y_i \dif X_i \qquad \tx{or} \qquad w = \sum_i \int_{X_{i,1}}^{X_{i,2}} \! Y_i \dif X_i \tag{3.1.8} \end{equation}
3.1.3 Heat and work as path functions
Consider the apparatus shown in Fig. 3.1. The system consists of the water together with the immersed parts: stirring paddles attached to a shaft (a paddle wheel) and an electrical resistor attached to wires. In equilibrium states of this system, the temperature and pressure are uniform and the paddle wheel is stationary. The system is open to the atmosphere, so the pressure is constrained to be constant. We may describe the equilibrium states of this system by a single independent variable, the temperature \(T\). (The angular position of the shaft is irrelevant to the state and is not a state function for equilibrium states of this system.)
Here are three experiments with different processes. Each process has the same initial state defined by \(T_1 = 300.0\K\), and each has the same final state.
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Although the paths in the three experiments are entirely different, the overall change of state is the same. In fact, a person who observes only the initial and final states and has no knowledge of the intermediate states or the changes in the surroundings will be ignorant of the path. Did the paddle wheel turn? Did an electric current pass through the resistor? How much energy was transferred by work and how much by heat? The observer cannot tell from the change of state, because heat and work are not state functions. The change of state depends on the
sum
of heat and work. This sum is the change in the state function \(U\!\), as expressed by the integrated form of the first law, \(\Del U = q + w\).
It follows from this discussion that neither heat nor work are quantities possessed by the system. A system at a given instant does not have or contain a particular quantity of heat or a particular quantity of work. Instead, heat and work depend on the path of a process occurring over a period of time. They are path functions.
3.1.4 Heat and heating
In everyday speech the noun heat is often used somewhat differently. Here are three statements with similar meanings that could be misleading:
“Heat is transferred from a laboratory hot plate to a beaker of water.”
“Heat flows from a warmer body to a cooler body.”
“To remove heat from a hot body, place it in cold water.”
Statements such as these may give the false impression that heat is like a substance that retains its identity as it moves from one body to another. Actually heat, like work, does not exist as an entity once a process is completed. Nevertheless, the wording of statements such as these is embedded in our everyday language, and no harm is done if we interpret them correctly. This e-book, for conciseness, often refers to “heat transfer” and “heat flow,” instead of using the technically more correct phrase “energy transfer by means of heat.”
Another common problem is failure to distinguish between thermodynamic “heat” and the process of “heating.” To heat a system is to cause its temperature to increase. A heated system is one that has become warmer. This process of heating does not necessarily involve thermodynamic heat; it can also be carried out with work as illustrated by experiments 1 and 2 of the preceding section.
The notion of heat as an indestructible substance was the essence of the caloric theory. This theory was finally disproved by the cannon-boring experiments of Benjamin Thompson (Count Rumford) in the late eighteenth century, and in a more quantitative way by the measurement of the mechanical equivalent of heat by James Joule in the 1840s (see Sec. 3.7.2).
3.1.5 Heat capacity
The heat capacity of a closed system is defined as the ratio of an infinitesimal quantity of heat transferred across the boundary under specified conditions and the resulting infinitesimal temperature change: \begin{gather} \s{ \tx{heat capacity} \defn \frac{\dq}{\dif T} } \tag{3.1.9} \cond{(closed system)} \end{gather} Since \(q\) is a path function, the value of the heat capacity depends on the specified conditions, usually either constant volume or constant pressure. \(C_V\) is the heat capacity at constant volume and \(C_p\) is the heat capacity at constant pressure . These are extensive state functions that will be discussed more fully in Sec. 5.6.
3.1.6 Thermal energy
It is sometimes useful to use the concept of thermal energy . It can be defined as the kinetic energy of random translational motions of atoms and molecules relative to the local frame, plus the vibrational and rotational energies of molecules. The thermal energy of a body or phase depends on its temperature, and increases when the temperature increases. The thermal energy of a system is a contribution to the internal energy.
It is important to understand that a change of the system’s thermal energy during a process is not necessarily the same as energy transferred across the system boundary as heat. The two quantities are equal only if the system is closed and there is no work, volume change, phase change, or chemical reaction. This is illustrated by the three experiments described in Sec. 3.1.3: the thermal energy change is the same in each experiment, but only in experiment 3 is the work negligible and the thermal energy change equal to the heat.
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Although the paths in the three experiments are entirely different, the overall change of state is the same. In fact, a person who observes only the initial and final states and has no knowledge of the intermediate states or the changes in the surroundings will be ignorant of the path. Did the paddle wheel turn? Did an electric current pass through the resistor? How much energy was transferred by work and how much by heat? The observer cannot tell from the change of state, because heat and work are not state functions. The change of state depends on the
sum
of heat and work. This sum is the change in the state function \(U\!\), as expressed by the integrated form of the first law, \(\Del U = q + w\).


9.7: Review Questions
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- Last updated
- Save as PDF
1.
Based on guidelines from the Centers for Disease Control and Prevention, how many hours of sleep a night do adolescents need for optimal functioning?
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five to six hours
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six to seven hours
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eight to ten hours
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eleven to thirteen hours
2 .
The cycle that governs the sleep wake cycle on a rotating, twenty-four-hour pattern is called the ________ rhythm.
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circadian
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hypothalamic
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diurnal-nocturnal
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anterograde
3.
What dimensional/proportional changes are typically seen in a female as she goes through adolescence?
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Her waist increases relative to her shoulders and bust.
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Her hips increase relative to her shoulders and waist.
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Her shoulders increase relative to all lower regions of her body.
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Her hips and shoulders will increase in a one-to-one ratio, both more than her waist.
4 .
By the end of adolescence, what is true of the human brain?
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It will be at its adult weight and size.
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It will not have grown much since the midteens.
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It will be at 60 to 70 percent of its final dimensions.
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It will be thinner and lighter, but more lateralized, than it was prior to puberty.
5.
Which option is an example of a primary sex characteristic?
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facial hair
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enlargement of the uterus
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development of breast buds
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growth of pubic hair
6 .
Which of the following statements is true regarding U.S. adolescents and sexual intercourse?
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More than half of all high school students report having had sexual intercourse.
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Most teens—over 80 percent—remain virgins until college.
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Fifteen percent of adolescent females have had sexual intercourse.
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Just under one-third of teens report having had sexual intercourse.
7.
What group has reported the highest rate of experiencing sexual violence?
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LGBQ+ teens
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Hispanic teens
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Native American teens
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White teens
8 .
The adolescent birth rate has __________ over the past few decades.
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increased sharply
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increased steadily
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stayed the same
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declined steadily
9.
In regard to the timing of the onset of puberty, who is more at risk for a depressed mood, negative body image, eating disorders, substance use, promiscuity, and school problems?
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a male who experiences late-onset puberty
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a female who experiences late-onset puberty
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a boy who experiences early-onset puberty
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a girl who experiences early-onset puberty
10 .
What is a property of formal operational thought?
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believing that inanimate objects are alive and can have humanlike thoughts and feelings
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the ability to use abstract, hypothetical, and deductive reasoning skills
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the recognition that only physical appearance represents reality
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the ability to consider multiple perspectives at the same time
11.
Jamarcus realized that the method he used to study for his history test last time didn’t work. He decides to try a new approach. Which cognitive skill is Jamarcus demonstrating?
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assimilation
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perceptual speed
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metacognition
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propositional logic
12 .
Which of these concepts can be described as “interactionist”?
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the study of animal relationships in different species
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a mathematical equation that can only be solved one way
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the environment and the individual reinforcing each other
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the psychological study of the family environment
13.
Elise has a sense that her problems are unique, that nobody has ever experienced them, and that her pain is a special kind of suffering that nobody has
ever
felt. This is an example of the concept of __________.
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the imaginary audience
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adolescent egocentrism
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metacognition
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the personal fable
14 .
Which substance that two-thirds of teens report having tried before twelfth grade is the most commonly used in this age group?
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alcohol
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cannabis
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heroin
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tobacco
15.
Which hormone increases during puberty, which in turn can contribute to the formation of attachment and social relationships?
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glutamate
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oxytocin
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gamma-aminobutyric acid (GABA)
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norepinephrine


40.11: Mammalian Heart and Blood Vessels - The Cardiac Cycle
- Summarize the caridac cycle and explain the role of the SA node and the AV node in regulating the its rhythm
The Cardiac Cycle
The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the cardiac cycle. The cardiac cycle is the coordination of the filling and emptying of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day. In each cardiac cycle, the heart contracts (systole), pushing out the blood and pumping it through the body. This is followed by a relaxation phase (diastole), where the heart fills with blood. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of the atrioventricular valves produces a monosyllabic “lup” sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the pulmonary artery (which transports blood to the lungs). Closing of the semilunar valves produces a monosyllabic “dup” sound.
The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that comprise the heart muscle. Cardiomyocytes are distinctive muscle cells that are striated like skeletal muscle, but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks exclusive to cardiac muscle. Cardiomyocytes are self-stimulated for a period of time; isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.
The autonomous beating of cardiac muscle cells is regulated by the heart’s internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions are intimately intertwined. The internal pacemaker starts at the sinoatrial (SA) node, which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node, causing the two atria to contract in unison. The pulse reaches a second node, the atrioventricular (AV) node, between the right atrium and right ventricle, where it pauses for approximately 0.1 seconds before spreading to the walls of the ventricles. This pause allows the blood in the atria to empty completely into the ventricles before the ventricles pump out the blood. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, causing the ventricles to contract. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an electrocardiogram (ECG): a recording of the electrical impulses of the cardiac muscle.
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Key Points
- During the cardiac cycle, the heart contracts via systole, pushing blood out of the heart, and relaxes via diastole, filling the heart with blood.
- Cardiomyocytes, or cardiac cells, are striated and are responsible for the pumping of the heart; they are the only muscle cells with intercalated disks.
- The heart’s internal pacemaker regulates and times the beating of the heart via electrical signals.
- Electrical signals start at the SA node, causing atria contraction, and then move on to AV node, delaying electrical impulses to allow blood from the atrium to fill the ventricles.
- Signals move from the AV node to the bundle of His and then to the Prukinje fibers, which then allows the ventricles to contract.
Key Terms
- bundle of His : specialized heart muscle cells that transmit electrical impulses from the AV node in the heart to the muscle cells of the heart wall, which contract in response producing the heart beat
- sinoatrial (SA) node : impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm
- diastole : relaxation and dilation of the heart chambers, between contractions, during which they fill with blood
- systole : rhythmic contraction of the heart, by which blood is driven through the arteries
- atrioventricular (AV) node : part of the electrical control system of the heart that coordinates the top of the heart; electrically connects atrial and ventricular chambers