4.16: Extensions of the Laws of Inheritance
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- Ying Liu
- City College of San Francisco
Mendel studied traits with only one mode of inheritance in pea plants. The inheritance of the traits he studied all followed the relatively simple pattern of dominant and recessive alleles for a single characteristic. There are several important modes of inheritance, discovered after Mendel’s work, that do not follow the dominant and recessive, single-gene model.
Alternatives to Dominance and Recessiveness
Mendel’s experiments with pea plants suggested that: 1) two types of “units” or alleles exist for every gene; 2) alleles maintain their integrity in each generation (no blending); and 3) in the presence of the dominant allele, the recessive allele is hidden, with no contribution to the phenotype. Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.” Since then, genetic studies in other organisms have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelism.
Incomplete Dominance
Mendel’s results, demonstrating that traits are inherited as dominant and recessive pairs, contradicted the view at that time that offspring exhibited a blend of their parents’ traits. However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents. For example, in the snapdragon, Antirrhinum majus (Figure \(\PageIndex{1}\)), a cross between a homozygous parent with white flowers ( C W C W ) and a homozygous parent with red flowers ( C R C R ) will produce offspring with pink flowers ( C R C W ). (Note that different genotypic abbreviations are used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.) This pattern of inheritance is described as incomplete dominance, meaning that one of the alleles appears in the phenotype in the heterozygote, but not to the exclusion of the other, which can also be seen. The allele for red flowers is incompletely dominant over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses. In this case, the genotypic ratio would be 1 C R C R :2 C R C W :1 C W C W , and the phenotypic ratio would be 1:2:1 for red:pink:white. The basis for the intermediate color in the heterozygote is simply that the pigment produced by the red allele (anthocyanin) is diluted in the heterozygote and therefore appears pink because of the white background of the flower petals.
Codominance
A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance occurs in the ABO blood groups of humans. The A and B alleles are expressed in the form of A or B molecules present on the surface of red blood cells. Homozygotes ( I A I A and I B I B ) express either the A or the B phenotype, and heterozygotes ( I A I B ) express both phenotypes equally. The I A I B individual has blood type AB. In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct. However, the 1:2:1 genotypic ratio characteristic of a Mendelian monohybrid cross still applies (Figure \(\PageIndex{2}\)).
Multiple Alleles
Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level, such that many combinations of two alleles are observed. Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype in the natural population as the wild type (often abbreviated “+”). All other phenotypes or genotypes are considered variants (mutants) of this typical form, meaning they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.
An example of multiple alleles is the ABO blood-type system in humans. In this case, there are three alleles circulating in the population. The I A allele codes for A molecules on the red blood cells, the I B allele codes for B molecules on the surface of red blood cells, and the i allele codes for no molecules on the red blood cells. In this case, the I A and I B alleles are codominant with each other and are both dominant over the i allele. Although there are three alleles present in a population, each individual only gets two of the alleles from their parents. This produces the genotypes and phenotypes shown in Figure \(\PageIndex{3}\). Notice that instead of three genotypes, there are six different genotypes when there are three alleles. The number of possible phenotypes depends on the dominance relationships between the three alleles.
EVOLUTION IN ACTION: Multiple Alleles Confer Drug Resistance in the Malaria Parasite
Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae , and is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia. Plasmodium falciparum and P. vivax are the most common causative agents of malaria, and P. falciparum is the most deadly. When promptly and correctly treated, P. falciparum malaria has a mortality rate of 0.1 percent. However, in some parts of the world, the parasite has evolved resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by geographic region.
In Southeast Asia, Africa, and South America, P. falciparum has developed resistance to the anti-malarial drugs chloroquine, mefloquine, and sulfadoxine-pyrimethamine. P. falciparum , which is haploid during the life stage in which it is infective to humans, has evolved multiple drug-resistant mutant alleles of the dhps gene. Varying degrees of sulfadoxine resistance are associated with each of these alleles. Being haploid, P. falciparum needs only one drug-resistant allele to express this trait.
In Southeast Asia, different sulfadoxine-resistant alleles of the dhps gene are localized to different geographic regions. This is a common evolutionary phenomenon that comes about because drug-resistant mutants arise in a population and interbreed with other P. falciparum isolates in close proximity. Sulfadoxine-resistant parasites cause considerable human hardship in regions in which this drug is widely used as an over-the-counter malaria remedy. As is common with pathogens that multiply to large numbers within an infection cycle, P. falciparum evolves relatively rapidly (over a decade or so) in response to the selective pressure of commonly used anti-malarial drugs. For this reason, scientists must constantly work to develop new drugs or drug combinations to combat the worldwide malaria burden. 1
Sex-Linked Traits
In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes—one pair of non-homologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains fewer genes. When a gene being examined is present on the X, but not the Y, chromosome, it is X-linked.
Eye color in Drosophila , the common fruit fly, was the first X-linked trait to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in 1910. Like humans, Drosophila males have an XY chromosome pair, and females are XX. In flies the wild-type eye color is red (X W ) and is dominant to white eye color (X w ) (Figure \(\PageIndex{4}\)). Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous, in that they have only one allele for any X-linked characteristic. Hemizygosity makes descriptions of dominance and recessiveness irrelevant for XY males. Drosophila males lack the white gene on the Y chromosome; that is, their genotype can only be X W Y or X w Y. In contrast, females have two allele copies of this gene and can be X W X W , X W X w , or X w X w .
In an X-linked cross, the genotypes of F 1 and F 2 offspring depend on whether the recessive trait was expressed by the male or the female in the P generation. With respect to Drosophila eye color, when the P male expresses the white-eye phenotype and the female is homozygously red-eyed, all members of the F 1 generation exhibit red eyes (Figure \(\PageIndex{5}\)). The F 1 females are heterozygous (X W X w ), and the males are all X W Y, having received their X chromosome from the homozygous dominant P female and their Y chromosome from the P male. A subsequent cross between the X W X w female and the X W Y male would produce only red-eyed females (with X W X W or X W X w genotypes) and both red- and white-eyed males (with X W Y or X w Y genotypes).  The F 1 generation would exhibit only heterozygous red-eyed females (X W X w ) and only white-eyed males (X w Y). Half of the F 2 females would be red-eyed (X W X w ) and half would be white-eyed (X w X w ). Similarly, half of the F 2 males would be red-eyed (X W Y) and half would be white-eyed (X w Y).
ART CONNECTION
What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color?
Discoveries in fruit fly genetics can be applied to human genetics. When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to 100 percent of her male offspring, because the males will receive the Y chromosome from the male parent. In humans, the alleles for certain conditions (some color-blindness, hemophilia, and muscular dystrophy) are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, X-linked traits appear more frequently in males than females.
 This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in whom they are hemizygous.
Linked Genes Violate the Law of Independent Assortment
Although all of Mendel’s pea plant characteristics behaved according to the law of independent assortment, we now know that some allele combinations are not inherited independently of each other. Genes that are located on separate, non-homologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string. The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair. However, because of the process of recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. To understand this, let us consider the biological basis of gene linkage and recombination.
Homologous chromosomes possess the same genes in the same order, though the specific alleles of the gene can be different on each of the two chromosomes. Recall that during interphase and prophase I of meiosis, homologous chromosomes first replicate and then synapse, with like genes on the homologs aligning with each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material (Figure \(\PageIndex{6}\)). This process is called recombination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.
When two genes are located on the same chromosome, they are considered linked, and their alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will tend to go together into a gamete and the short and yellow alleles will go into other gametes. These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles. If you create a Punnett square with these gametes, you will see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply. As the distance between two genes increases, the probability of one or more crossovers between them increases and the genes behave more like they are on separate chromosomes. Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a measure of how far apart genes are on a chromosome. Using this information, they have constructed linkage maps of genes on chromosomes for well-studied organisms, including humans.
Mendel’s seminal publication makes no mention of linkage, and many researchers have questioned whether he encountered linkage but chose not to publish those crosses out of concern that they would invalidate his independent assortment postulate. The garden pea has seven chromosomes, and some have suggested that his choice of seven characteristics was not a coincidence. However, even if the genes he examined were not located on separate chromosomes, it is possible that he simply did not observe linkage because of the extensive shuffling effects of recombination.
Epistasis
Mendel’s studies in pea plants implied that the sum of an individual’s phenotype was controlled by genes (or as he called them, unit factors), such that every characteristic was distinctly and completely controlled by a single gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.
CONCEPT IN ACTION
Eye color in humans is determined by multiple alleles. Use the Eye Color Calculator to predict the eye color of children from parental eye color.
In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes expressed simultaneously affect a phenotype. An apparent example of this occurs with human skin color, which appears to involve the action of at least three (and probably more) genes. Cases in which inheritance for a characteristic like skin color or human height depend on the combined effects of numerous genes are called polygenic inheritance.
Genes may also oppose each other, with one gene suppressing the expression of another. In epistasis, the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. “Epistasis” is a word composed of Greek roots meaning “standing upon.” The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking. Often the biochemical basis of epistasis is a gene pathway in which expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.
An example of epistasis is pigmentation in mice. The wild-type coat color, agouti ( AA ) is dominant to solid-colored fur ( aa ). However, a separate gene C , when present as the recessive homozygote ( cc ), negates any expression of pigment from the A gene and results in an albino mouse (Figure \(\PageIndex{7}\)). Therefore, the genotypes AAcc , Aacc , and aacc all produce the same albino phenotype. A cross between heterozygotes for both genes ( AaCc x AaCc ) would generate offspring with a phenotypic ratio of 9 agouti:3 black:4 albino (Figure \(\PageIndex{7}\)). In this case, the C gene is epistatic to the A gene.
Section Summary
Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles for a gene to exist in a population. In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two.
According to Mendel’s law of independent assortment, genes sort independently of each other into gametes during meiosis. This occurs because chromosomes, on which the genes reside, assort independently during meiosis and crossovers cause most genes on the same chromosomes to also behave independently. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel's law of independent assortment. However, recombination serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on a given chromosome are not always inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chromosomal space.
Whether or not they are sorting independently, genes may interact at the level of gene products, such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis.
Art Connections
Figure \(\PageIndex{5}\): What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color?
- Answer
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Half of the female offspring would be heterozygous (X W X w ) with red eyes, and half would be homozygous recessive (X w X w ) with white eyes. Half of the male offspring would be hemizygous dominant (X W Y) with red eyes, and half would be hemizygous recessive (X w Y) with white eyes.
Footnotes
- 1 Sumiti Vinayak et al., “Origin and Evolution of Sulfadoxine Resistant Plasmodium falciparum,” PLoS Pathogens 6 (2010): e1000830.
Glossary
- codominance
- in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic
- epistasis
- an interaction between genes such that one gene masks or interferes with the expression of another
- hemizygous
- the presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant
- incomplete dominance
- in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype
- linkage
- a phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together
- recombination
- the process during meiosis in which homologous chromosomes exchange linear segments of genetic material, thereby dramatically increasing genetic variation in the offspring and separating linked genes
- wild type
- the most commonly occurring genotype or phenotype for a given characteristic found in a population
- X-linked
- a gene present on the X chromosome, but not the Y chromosome
Contributors and Attributions
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Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4-e119a8aafbdd ).


18.3: Nursing Care During the Second Stage of Labor
By the end of this section, you will be able to:
- Obtain assessment data on the birthing person and fetus during the second stage of labor
- Discuss nursing interventions during the second stage of labor
- Explain the nursing actions when monitoring the fetus during the second stage of labor
- Explain the nursing actions when monitoring the birthing person for complications during the second stage of labor
- Explain the nursing actions when preparing for a vaginal delivery
Nursing care during the second stage of labor involves assisting the birthing person with pain management, positioning, and efforts to bear down during the contractions and birth. The laboring person is now completely dilated and feeling the urge to push. Successful coordination of the person’s bearing-down efforts and contraction pattern is an essential part of the nurse’s role during the second stage of labor. At the same time, the nurse also continues to monitor the birthing person’s progress, emotional and physical response, and fetal adaptation while preparing for the birth of the newborn.
Assessment of the Laboring Person and Fetus during the Second Stage of Labor
During the second stage of labor, nursing care is focused on providing essential support for both the laboring person and the fetus. The nurse will closely monitor the progression of contractions, the descent of the fetus through the birth canal, and the fetal heart rate. The nurse is a source of encouragement and guidance through the second stage and supports the birthing person with pain management. Ensuring the birthing person’s and support person’s comfort, safety, and informed decision making remains a priority for the nurse, as the health-care team readies for the safe delivery of the baby. Continuous assessment, communication, and collaboration are key components during the second stage of labor.
Monitoring the Physical Response of the Person during the Second Stage of Labor
Maternal vital signs are routinely assessed at least every 30 minutes during the second stage of labor unless otherwise determined by order or protocol (Table 18.6). The nurse should assist the patient into a position of the birthing person’s choosing and provide support and encouragement, or guidance with pushing. If the birthing person desires, the nurse may provide the birthing person with sips of water or ice chips (Milton, 2024).
| Assessment Data | Second Stage of Labor |
|---|---|
| Vital signs |
Every 30 minutes
Temperature
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| Contraction pattern | Intermittently or continuously every 15 minutes, documented at the same interval as FHR |
| Progress in descent | As indicated based on pressure, perineal bulging or crowning, abnormalities in the FHR |
| Pain | Hourly, or more frequently as needed |
| Emotional response | Continuous |
| Intake and output | Continuous |
The nurse continues to monitor contraction strength and frequency during the second stage of labor. The contraction pattern is typically assessed every 15 minutes. During the strongest part of the contractions, the nurse coaches the birthing person through the bearing down effort s and monitors for fatigue. If necessary, additional instructions for bearing down are provided between contractions when indicated.
The nurse also monitors the descent of the fetus through the pelvis by assessing the fetal station and presence of crowning. As the fetus descends, the nurse provides updates to the birthing person and their support persons. These updates aid in keeping the birthing person motivated to keep the bearing-down efforts most effective (Milton, 2024).
Monitoring the Fetal Response to the Second Stage of Labor
The fetal heart rate is monitored during the second stage of labor, assessing for patterns that can indicate fetal compromise (Table 18.7). Abnormal patterns are associated with hypoxemia. which may lead to hypoxia, resulting in metabolic acidosis. The type and frequency of fetal monitoring are determined by provider order, protocol, and clinical situation (AWHONN, 2018). Fetal monitoring may be done intermittently, continuously with an external fetal monitor, or internally with a fetal scalp electrode . The nurse should be in regular contact with the provider, relaying critical information about the fetal monitoring findings and implementing interventions when compromise is identified.
| Fetal Assessment Data | Second Stage of Labor | |
|---|---|---|
| Low risk without oxytocin | With risk factors or oxytocin | |
|
FHR baseline
FHR variability Periodic changes |
Every 5–15 minutes | Every 5–15 minutes |
Nursing Interventions during the Second Stage of Labor
In many cases, the nurse is at the bedside frequently during the second stage of labor and exclusively during pushing. The role of the nurse during the second stage of labor is to assess the labor progress, monitor the fetal heart rate and contraction patterns, guide and support the birthing person, and aid the provider.
Encouraging Bearing-Down Efforts during the Second Stage of Labor
The direct contact of the fetus on the pelvic floor usually initiates the maternal urge to push. This sensation may be dulled when the birthing person has an epidural in place. The role of the nurse during this stage of labor is to provide motivation and encouragement for the birthing person. The strategies implemented may include emphasizing to the birthing person the benefits of relaxation of the perineum or coordinating bearing-down efforts with the contraction pattern via the tocodynamometer in a birthing person with an epidural. The nurse will coach the birthing person to maximize pushing efforts and coordinate pushing and breathing efforts. The nurse will also provide feedback to the birthing person about their pushing efforts and maintain continuous communication with the provider about the pushing progress.
In open glottis pushing , the laboring person follows their body’s spontaneous, natural urges to push without specific instructions or holding their breath or can be directed to bear down while exhaling. Directed open glottis pushing contrasts with the traditional closed glottis or Valsalva pushing technique, in which the laboring person is instructed to hold their breath and push forcefully for a specified duration during each contraction (Barasinski et al., 2020). During open glottis pushing, the laboring person continues to breathe normally. They take deep breaths in and exhale naturally as they feel the urge to bear down and push. There is no forced breath-holding. Open glottis pushing allows for more natural pushing efforts. Studies suggest that there is no difference in the incidence of perineal lacerations between open and closed glottis pushing (Barasinski et al., 2020).
Changing positions during labor can be highly beneficial in aiding the descent of the fetus through the birth canal (Huang et al., 2019). Different positions can help optimize the alignment of the fetal head with the pelvis, facilitate uterine contraction s, and reduce pressure on specific areas, potentially making the labor process more efficient and comfortable. It’s important to note that the effectiveness of positions may vary for each laboring person, and the choice of position should be based on comfort and what feels most effective during labor. Continuous communication with the health-care team and a supportive birthing environment are essential to ensure safety and to optimize the labor process.
Promoting Relaxation and Rest during the Second Stage of Labor
The role of the nurse in promoting relaxation and rest during the second stage of labor may vary based on the pain control decisions of the birthing person. For a birthing person who has an epidural, the nurse can assist the person with positioning and creating a calm, quiet environment. For a birthing person who has chosen analgesia or nonpharmacologic methods, the nurse can support the person with positioning, bearing down, and breathing techniques (Heim & Makuch, 2023).
Providing Comfort Measures during the Second Stage of Labor
The second stage of labor can bring discomfort that the nurse can assist in alleviating. Warm compresses may be applied to the perineum or lower back, and the nurse or support person may apply pressure to or massage the lower back. Cool cloths can also be applied to the birthing person’s forehead or neck. To encourage family-centered care, the nurse can teach the family or friends of the birthing person to apply these techniques.
Monitoring for Complications during the Second Stage of Labor
The second stage of labor brings about the potential for complications that may affect the laboring person or the fetus. The nurse continues to monitor maternal vital signs, discomfort, and pain while supporting pushing. The nurse should continually assess for complications of the laboring person, including bladder distention , perineal tears, excessive bleeding, and slow or lack of progress in descent (Milton, 2024). These complications are outlined in (Table 18.8). (Please see further discussion of shoulder dystocia in Chapter 19 Complications of Labor and Birth.)
| Complication | Nursing Assessment |
|---|---|
| Prolonged second stage | Assess cervical dilation and descent of the fetus. |
| Monitor uterine contractions for frequency and strength. | |
| Evaluate the laboring person’s pushing efforts. | |
| Observe the fetal heart rate for signs of distress. | |
| Assess for signs of exhaustion or distress in the laboring person. | |
| Assess for distended bladder. | |
| Fetal distress | Continuously monitor the fetal heart rate. |
| Document heart rate patterns (e.g., decelerations). | |
| Evaluate maternal vital signs for signs of infection or fever. | |
| Assess amniotic fluid color, odor, and meconium presence. | |
| Notify the health-care provider of the presence and associated symptoms. | |
| Shoulder dystocia | Assist with maneuvers such as McRoberts and suprapubic pressure, as directed by the health-care provider. |
| Document the time of occurrence and actions taken. | |
| Assist with the evaluation for maternal and fetal injuries. | |
| Rapid delivery | Notify the health-care provider. |
| Support the baby’s head and guide the shoulders if the health-care provider is not present. | |
| Ensure a safe and controlled birth process. | |
| Document the time of birth and any complications. |
The second stage of labor is a time of increased stress on the fetus. The nurse must monitor the fetal heart rate to ensure the safe delivery of the newborn. The nurse should monitor for an abnormal fetal heart rate with pushing and notify the health provider when indicated. The nurse must also monitor the birthing person’s progress for any risk of shoulder dystocia , a situation in which the head of the newborn is delivered but a shoulder is trapped behind the birthing person’s pubic bone during the delivery. If a shoulder dystocia does occur, the nurse must be prepared to react and assist the provider in repositioning maneuvers, or cesarean birth, to deliver the newborn (Milton, 2024). (Please see further discussion of shoulder dystocia in Chapter 19 Complications of Labor and Birth.)
Communication with the Health-Care Provider
Communication with a health-care provider who is not at the bedside should continue regularly and consistently throughout the second stage of labor and during pushing. The nurse is responsible for the continued assessment of the birthing person and the fetal heart rate pattern and descent of the fetus through the pelvis. The provider should be aware of any potential maternal complications, abnormal fetal heart rate patterns, progress of the second stage, and pushing efforts. Any changes in maternal or fetal status should be reported immediately to the health-care provider.
Perineal Hygiene
Perineal hygiene is recommended during the second stage. Hygiene includes keeping the perineum clean using warm water poured over the perineum or using wet washcloths (Figure 18.2). The standardized method of perineal hygiene requires the nurse to cleanse the perineum from the pubic hair line to the anus (front to back). The nurse should be aware of any hospital policies for perineal cleansing and will assist the provider when appropriate and required.
Perineal Cleansing
Perineal cleansing during delivery is an important aspect of hygiene and infection prevention. Here are the steps for perineal cleansing:
- Gather supplies: Ensure that you have all the necessary supplies ready, including clean gloves, warm water, mild soap or antiseptic solution (per facility policy), clean washcloths or sterile sponges, and a clean towel.
- Verify if the patient has any allergies.
- Hand hygiene: Perform proper hand hygiene by washing your hands thoroughly with soap and water and drying them with a clean towel or disposable paper towels. Don sterile gloves to maintain aseptic technique.
- Position the patient: Assist the laboring person into a comfortable position for perineal cleansing, such as a lithotomy position (lying on the back with knees bent and feet in stirrups) or a position of their choice that allows easy access to the perineal area.
- Privacy and consent: Ensure the patient’s privacy and obtain their consent for the procedure.
- Warm water: Using warm water, wet a clean washcloth or sterile sponge. Squeeze out excess water to avoid excessive dripping.
- Cleansing motion: Gently cleanse the perineal area from front to back, starting with the pubic hair line and moving toward the rectal area. Use a separate area of the washcloth or sponge for each stroke, maintaining a one-way cleaning motion to prevent contamination from the rectum to the vaginal area.
- Pay attention to folds: Pay special attention to the folds and creases in the perineal area, ensuring thorough cleansing. Avoid excessive scrubbing or irritation, as the perineal area may be sensitive during labor.
- Rinse: After cleansing, use a clean, damp washcloth or sponge to ensure the area is clean and any soap or antiseptic solution is removed from the perineum. Again, use a front-to-back motion.
- Pat dry: Gently pat the perineal area dry with a clean towel or sterile gauze. Avoid rubbing, as this can irritate the skin.
- Dispose of supplies: Properly dispose of any used supplies, such as gloves and disposable washcloths, in accordance with health-care facility protocols.
- Reassure and comfort: Throughout the procedure, provide emotional support, reassurance, and explanations to the patient, maintaining open communication and addressing any concerns.
Perineal cleansing is essential for maintaining hygiene and preventing infection during labor and delivery. It is also an opportunity to provide comfort and support to the laboring person. Nurses and health-care providers should follow facility policies and guidelines while performing perineal cleansing to ensure safe and effective care.
Nursing Actions to Prepare for the Birth and the Time of Birth
During the second stage of labor, the nurse will begin to prepare for the birth of the newborn. A delivery table is usually set up in the labor and birth suite before the onset of the second stage of labor (Milton, 2024). The delivery table will contain sterile drapes, delivery equipment used by the provider, and instruments to complete repairs to the perineum if necessary (Figure 18.3). The contents of the delivery table will vary based on hospital and provider preferences.
The infant warmer should be available during delivery, allowing the rapid initiation of resuscitation of the neonate if necessary. The warmer should be stocked before the birthing person is placed in the room, and its contents should be verified by the nurse before delivery (Ahmed et al., 2018). (See Table 18.9 for a list of equipment and supplies to have ready for delivery in the warmer.) The warmer should be turned on by the nursing staff before delivery, allowing enough time for the warmer to preheat to the desired temperature (Figure 18.4). The labor nurse should also make sure oxygen and suction are set up and working. The nurse will communicate with other nursing staff, the nursery nurse, and the neonatal intensive care unit (NICU) to coordinate and prepare for delivery/resuscitation as needed (Ahmed et al., 2018).
| Type | Equipment |
|---|---|
| Resuscitation equipment | Neonatal resuscitation bag and mask |
| Suction bulb or catheter | |
| Endotracheal tube s (various sizes) | |
| Laryngoscope with blades (sizes 0 and 1) | |
| Stylet for endotracheal tube | |
| Meconium aspirator (if indicated) | |
| Oxygen equipment | Oxygen source (wall outlet or portable) |
| Oxygen mask and tubing (neonatal sizes) | |
| Thermoregulation | Radiant warmer or heat lamp |
| Prewarmed blankets or towels | |
| Hat and warm clothing for the newborn | |
| Monitoring and assessment | Neonatal stethoscope |
| Blood pressure cuff (neonatal size) | |
| Pulse oximeter with neonatal probe | |
| Thermometer (preferably electronic) | |
| Apgar scoring materials (Apgar score sheet) | |
| Clock or timer for timing interventions | |
| Umbilical cord care | Sterile umbilical cord clamp s or ties |
| Sterile scissors | |
| Antiseptic solution (e.g., chlorhexidine) for cord care | |
| Suction and oral care | Bulb syringe or mechanical suction device |
| Sterile saline solution for suctioning | |
| Oral suction catheters (if indicated) | |
| Emergency equipment | Neonatal bag-valve-mask (BVM) resuscitator |
| Cardiopulmonary resuscitation (CPR) board or firm surface | |
| Emergency medications (e.g., epinephrine, naloxone) and syringes | |
| Miscellaneous | Newborn diapers |
| Identification bands for parents and baby | |
| Baby blanket or swaddle |
The delivery of the newborn results in the collaboration of multiple health-care providers, which may be overwhelming to the birthing person and their family (Weiseth, 2022). The nurse should discuss the need for each member of the team and their role with the birthing person. In addition to the primary nurse assigned to the laboring person and the delivery provider, an additional nurse should attend the delivery solely to support the newborn, complete a newborn assessment, and initiate resuscitation with the neonatal resuscitation team when warranted. It would not be uncommon to find at least two nurses, the primary provider, a respiratory therapist (per facility policy), and support staff at the bedside during the delivery, all available for the safe delivery and support of the birthing person and newborn during the initial moments following birth (Weiseth, 2022).


<IP_ADDRESS>: Mutability
Mutability
Mutability License: Public DomainWe are as clouds that veil the midnight moon;
How restlessly they speed and gleam and quiver,
Streaking the darkness radiantly! yet soon
Night closes round, and they are lost for ever:—
Or like forgotten lyres whose dissonant strings
Give various response to each varying blast,
To whose frail frame no second motion brings
One mood or modulation like the last.
We rest—a dream has power to poison sleep;
We rise—one wandering thought pollutes the day;
We feel, conceive or reason, laugh or weep,
Embrace fond woe, or cast our cares away:—
It is the same!—For, be it joy or sorrow,
The path of its departure still is free;
Man's yesterday may ne'er be like his morrow;
Nought may endure but Mutability.


24.5B: Ureters
The ureters are two tubes that drain urine from each of the kidneys into the bladder.
- Describe the role of ureters in the urinary system and their structure
Key Points
- The ureter contains transitional epithelium and an additional smooth muscle layer in the more distal one-third to assist with peristalsis.
- The ureters are usually 25–30 cm (10–12 in) long and 3–4 mm in diameter.
- Muscles in the walls of the ureters send the urine in small spurts into the bladder.
- Kidney stones are a common disease of the ureter, in which a stone made of minerals obstructs the ureter.
- The ureter has a few points of constriction where kidney stones are more commonly found.
Key Terms
- transitional epithelial tissues : Epithelial tissues that can change between columnar and squamous forms that are found in the ureters and the cervix.
- peristalsis : The rhythmic, wave-like contraction of smooth muscle tissue that can propel substances through the many muscular tubes of the body.
The ureters are tubes made of smooth muscle fibers that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually 25–30 cm (10–12 in) long and 3–4 mm in diameter. The ureter is one of the essential organs of urinary tract that controls urine transport.
Ureter Structure and Function
The ureters are two tubes that are made out of smooth muscle and transitional epithelial tissues, which are a type of epithelial tissue that may either be columnar or squamous. Each kidney has its own ureter through which urine drains into.
Human urinary system : 1) Human urinary system. 2) Kidney. 3) Renal pelvis. 4) Ureter. 5) Urinary bladder. 6) Urethra (left side with frontal section). 7) Adrenal gland vessels. 8) Renal artery and vein. 9) Inferior vena cava. 10) Abdominal aorta. 11) Common iliac artery and vein with transparency. 12) Liver. 13) Large intestine. 14) Pelvis.
The ureters are long tubes that have a few points of constriction, where obstructions are more common. The ureters receive a blood supply from a few different major arteries including the renal and illiac artery derivatives, and have a relatively dense nerve supply as well.
Muscles in the walls of the ureters send the urine in small spurts into the bladder, in a process called peristalsis. After the urine enters the bladder from the ureters, small folds in the bladder mucosa act like valves to prevent the backward flow of the urine; these are called the ureteral valves. The ureteral valves function similarly to the semilunar valves in the veins of the body, but are structurally different, consisting of transverse mucosal epithelial folds.
Ureter Pathology
Kidney stones and cancer are common diseases of the ureter. A kidney stone can move from the kidney and become lodged inside the ureter, which can block the flow of urine, as well as cause a sharp cramp in the back, side, or lower abdomen. The affected kidney could then develop hydronephrosis, should a part of the kidney become swollen due to blocked flow of urine.
Kidney stones are very common and are usually clumps of aggregated minerals that are most often found at the constriction points in the ureter. Ureter cancer is often due to a malignant transformation of of the transitional epithelial tissue, which is more vulnerable to developing cancer cells compared to other tissues.


11.6: Moment of Inertia
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In order to model how an object rotates about an axis, we use Newton’s Second Law for rotational dynamics:
\[\begin{aligned} \vec\tau^{ext} = I \vec \alpha\end{aligned}\]
where \(\vec\tau^{ext}\) is the net external torque exerted on the object about the axis of rotation, \(\vec \alpha\) is the angular acceleration of the object, and \(I\) is the moment of inertia of the object (about the axis). If we consider the object as being made of many particles of mass \(m_i\) each located at a position \(\vec r_i\) relative to the axis of rotation, the moment of inertia is defined as:
\[\begin{aligned} I = \sum_i m_i r_i^2\end{aligned}\]
Consider, for example, the moment of inertia of a uniform rod of mass \(M\) and length \(L\) that is rotated about an axis perpendicular to the rod that pass through one of the ends of the rod, as depicted in Figure \(\PageIndex{1}\).
We introduce the linear mass density of the rod, \(\lambda\) , as the mass per unit length:
\[\begin{aligned} \lambda = \frac{M}{L}\end{aligned}\]
We model the rod as being made of many small mass elements of mass \(\Delta m\) , of length \(\Delta r\) , at a location \(r_i\) , as illustrated in Figure \(\PageIndex{1}\). Using the linear mass density, the mass element, \(\Delta m\) , has a mass of:
\[\begin{aligned} \Delta m = \lambda \Delta r\end{aligned}\]
The rod is made of many such mass elements, and the moment of inertia of the rod is thus given by:
\[\begin{aligned} I &= \sum_i \Delta m r_i^2 =\sum_i \lambda \Delta r r_i^2\end{aligned}\]
If we take the limit in which the length of the mass element is infinitesimally small ( \(\Delta r \to dr\) ) the sum can be written as an integral over the dimension of the rod:
\[\begin{aligned} I &= \int_0^L\lambda r_i^2dr = \frac{1}{3}\lambda L^3 = \frac{1}{3}\left( \frac{M}{L} \right)L^3 \\[4pt] &=\frac{1}{3} ML^2\end{aligned}\]
where we re-expressed the linear mass density in terms of the mass and length of the rod. In general, we can write the moment of inertia of a continuous object as:
\[\begin{aligned} I = \int r^2 dm \end{aligned}\]
where \(dm\) is a small mass element that makes up the object, \(r\) is the distance from that mass element to the axis of rotation, and the integral is over the dimension of the object. As we did above, we would usually set up this integral so that \(dm\) is expressed in terms of \(r\) so that we can take an integral over \(r\) .
Example \(\PageIndex{1}\)
Calculate the moment of inertia of a uniform thin ring of mass \(M\) and radius \(R\) , rotated about an axis that goes through its center and is perpendicular to the disk.
Solution
We take a small mass element \(dm\) of the ring, as shown in Figure \(\PageIndex{2}\).
The moment of inertia is given by:
\[\begin{aligned} I = \int dm r^2\end{aligned}\]
In this case, each mass element around the ring will be the same distance away from the axis of rotation. The value \(r^2\) in the integral is a constant over the whole ring, and so can be taken out of the integral:
\[\begin{aligned} I = \int dm r^2 = R^2\int dm\end{aligned}\]
where we used the fact that the ring has a radius \(R\) , so the distance \(r\) of each mass element to the axis of rotation is \(R\) . The integral:
\[\begin{aligned} \int dm\end{aligned}\]
just means “sum all of the mass elements, \(dm\) ”, and is thus equal to \(M\) , the total mass of the ring. The moment of inertia of the ring is thus:
\[\begin{aligned} I = R^2\int dm = MR^2\end{aligned}\]
The parallel axis theorem
The moment of inertia of a solid object can be difficult to calculate, especially if the object is not symmetric. The parallel axis theorem allows us to determine the moment of inertia of an object about an axis, if we already know the moment of inertia of the object about an axis that is parallel and goes through the center of mass of the object.
Consider an object for which we know the moment of inertia, \(I_{CM}\) , about an axis that goes through the object’s center of mass. We define a coordinate system such that the origin is located at the center of mass, and the \(z\) axis is parallel to the axis about which we know the moment of inertia, as illustrated in Figure \(\PageIndex{3}\).
We wish to determine the moment of inertia for the object for an axis that is parallel to the \(z\) axis, but goes through a point with coordinates \((x_0,y_0)\) located a distance \(h\) away from the center of mass. The moment of inertia about an axis parallel to the \(z\) axis and that goes through that point, \(I_h\) is given by:
\[\begin{aligned} I_h = \sum_i m_i r_i^2\end{aligned}\]
where \(m_i\) is a mass element of the object located at a distance \(r_i\) from the axis of rotation. If the mass element is located at a position \((x_i,y_i)\) relative to the center of mass, we can write the distance \(r_i\) in terms of the position of the mass element, and of the position of the axis of rotation:
\[\begin{aligned} r_i^2 = (x_i-x_0)^2+(y_i-y_0)^2 = x_i^2-2x_ix_0+x_0^2+y_i^2-2y_iy_0+y_0^2\end{aligned}\]
Note that:
\[\begin{aligned} x_0^2 + y_0^2 = h^2\end{aligned}\]
The moment of inertia, \(I_h\) , can thus be written as:
\[\begin{aligned} I_h &= \sum_i m_i r_i^2 =\sum_i (m_i(x_i^2+ y_i^2)-2x_0m_ix_i-2y_0m_iy_i+m_ih^2)\\[4pt] &=\sum_i m_i(x_i^2+ y_i^2) + h^2\sum_i m_i - 2x_0 \sum_im_ix_i- 2y_0 \sum_im_iy_i\end{aligned}\]
where we broke the sum up into several sums, and factored constant terms ( \(h\) , \(x_0\) , \(y_0\) ) out of the sums, since these constants do not depend on which mass element we are considering. The first term is the moment of inertia about the center of mass, since \(x_i^2+y_i^2\) is the distance to the center of mass. The second term is \(h^2\) times the total mass of the object, since the sum of all the \(m_i\) is just the mass, \(M\) , of the object. Now consider the term:
\[\begin{aligned} -2x_0 \sum_im_ix_i\end{aligned}\]
The sum, \(\sum m_i x_i\) is the numerator in the definition of the \(x\) coordinate of the center of mass! The sum is thus zero, because we choose the origin to be located at the center of mass. The last two terms in the sum are thus identically zero, because they correspond to the \(x\) and \(y\) coordinates of the center of mass!
We can thus write the parallel axis theorem:
\[I_{h}=I_{CM}+Mh^{2}\]
where \(I_{CM}\) is the moment of inertia of an object of mass \(M\) about an axis that goes through the center of mass and, \(I_h\) , is the moment of inertia about a second axis that is parallel to the first and a distance \(h\) away.
Example \(\PageIndex{2}\)
In the previous section, we calculated the moment of inertia of a rod of length \(L\) and mass \(M\) through an axis that is perpendicular to the rod and through one of its ends, and found that it was given by:
\[\begin{aligned} I=\frac{1}{3}ML^2\end{aligned}\]
What is the moment of inertia of the rod about an axis that is perpendicular to the rod and goes through its center of mass?
Solution
In this case, we know the moment of inertia through an axis that does not go through the center of mass. The center of mass is located a distance \(h=L/2\) away from the point about which we know the moment of inertia, \(I_h\) .
Using the parallel axis theorem, we can find the moment of inertia through the center of mass:
\[\begin{aligned} I_{CM} &= I_h - Mh^2\\[4pt] &=\frac{1}{3}ML^2 - M \left( \frac{L}{2}\right)^2 = \frac{1}{12}ML^2\end{aligned}\]
Discussion
We find that the moment of inertia about the center of mass is smaller than the moment of inertia about the end of the rod. This makes sense because when rotating the rod about its end, more of its mass is further away from the axis of rotation, which results in a larger moment of inertia.


3.2: Valence Bond Theory- Lewis Dot Structures, the Octet Rule, Formal Charge, Resonance, and the Isoelectronic Principle
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Atoms and Molecules
Atomism, because it was dismissed by Aristotle, enjoyed a long sleep in scientific discourse until it was reconsidered by Galileo, Decartes, and Gassendi in the 1600s. Dalton postulated the modern atomic theory in 1808 based on his observation that elements such as hydrogen and oxygen combined in specific ratios (the Law of Definite Proportions), but the atomic theory remained contentious throughout most of the 19th century. Thompson, Rutherford, Bohr, and others around the turn of the 20th century established that matter was indeed composed of atoms that contained heavy nuclei and light electrons, and that atoms could exist in excited states that could be interpreted as excitations of their electrons to different energy levels. However the atomic theory did not provide a ready explanation for the bonded states of atoms in molecules.
In 1916, still more than a decade before modern quantum theory would adequately describe the shapes of atomic orbitals, Lewis proposed the octet theory based on the empirically observed rules of valence, i.e., the combining ratios of atoms in molecules. [1] This theory, in hindsight, can be rationalized for s- and p-block elements by observing that main-group atoms can use their four valence orbitals (s, p x , p y , and p z ) to accommodate up to eight electrons, some or all of which may be shared with other atoms. In Lewis' model, the valence electrons of an atom were situated at the corners of a cube, and the cubes could share edges or faces to complete their octets. Lewis developed a shorthand notation for these structures based on dots that represented the valence electrons, as illustrated in Fig. \(\PageIndex{1}\). A pair of electrons shared between atoms constitutes a chemical bond, and can also be represented as a line joining the atoms. Four electrons shared between atoms, represented by two lines, is a double bond, and so forth. Any pairs of electrons not involved in bonding form "lone pairs" that belong to one atom only and are thus not involved in bonding.
The Lewis picture is powerful in its simplicity. It can be readily used to rationalize or predict the combining ratios of atoms, to sort plausible and unlikely structures for molecules (including large ones), and to rationalize the acid-base properties of many molecules. It is important to remember that the model is built on a shaky, pre-quantum mechanical description of atoms and so with it, we will make mistakes. Nevertheless it is useful to see how far we can go with the Lewis model.
Constructing the octet valence bond picture for a molecule
We can construct an octet picture for any molecule using the N-V method:
- Number of electrons needed to make separate octets around all atoms = N
- (N = 8 for non-H atoms, N = 2 for H)
- Number of valence electrons = V (add up using group numbers; don't forget to add or subtract for charges on ions)
- Number of shared electrons = S = N-V; and therefore the number of bonds = S/2
- Fill in lone pairs everywhere else to complete octets
Example: nitrate anion, NO 3 -
- N = 4 atoms x 8 electrons = 32
- V = 5 + 6 + 6 + 6 + 1 = 24 (N + O + O + O + charge)
- S = 32-24 = 8 shared electrons = 4 bonds
- The number of lone pairs is obtained by difference: (V-S)/2 = (24-8)/2 = 8
- Now draw the molecule, starting with the four bonds and adding eight lone pairs to complete the octets:
- Count the shared and unshared electrons around each atom in the nitrate ion and you will see that all atoms are octet and that the total number of valence electrons (V) is 24. If you do the N-V calculation according to the rules above, you should always get an octet structure.
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The charges on atoms in a molecule (or an extended solid such as SiO 2 ) can be estimated from X-ray photoelectron spectra (XPS). Energetic X-rays ionize atoms by kicking out electrons from their core orbitals. By conservation of energy, the kinetic energy (KE) of an emitted electron is related to its binding energy (BE) and the energy of the incoming X-ray photon (hν) by BE = hν - KE. Electrons are harder to remove from positively charged atoms (higher BE), and easier to remove from negatively charged atoms (lower BE), relative to the neutral element. |
The formal charge distribution is assigned by dividing the shared (bonding) electrons equally between atoms. Thus, the singly bonded O atoms each possess 7 electrons, and because O is in group 6, their formal charge is -1. The doubly bonded O has a formal charge of zero. The N atom has a formal charge of +1 because it "owns" 4 valence electrons and is in group 5.
Note that the formal charge is not the same thing as the oxidation number (or oxidation state ). In the nitrate ion, the oxidation state of nitrogen is +5 and the oxidation state of oxygen is -2. The formal charge is typically closer to the "real" charge on the atom (as measured, e.g., by X-ray photoelectron spectroscopy). Oxidation states are a useful bookkeeping device for keeping track of oxidation-reduction reactions, as we will discuss in Chapter 4. Like oxidation states, the formal charges on the atoms in a molecule or ion must add up to its overall charge .
We can similarly draw the Lewis structure for ammonia as shown below:
When we divide the shared electrons equally between the atoms, we see that the N atom has five electrons and each H atom has one. These are the same as their group numbers, and thus, all the formal charges in the ammonia molecule are zero.
Octet structures of the Lewis acid-base adduct NH 3 BF 3 , the hydronium ion H 3 O + , and the sulfate anion SO 4 2 - are shown below. In this case (and in many Lewis structures we will draw), we leave off the implied lone pairs around the peripheral atoms. Try calculating the number of bonds in each of these molecules using the N-V method, and fill in the lone pairs that are not explicitly drawn in.
In cases where more than one valence bond structure is possible, we can use formal charges to decide which structures should be more or less stable. The rules are:
- The formal charges on atoms are minimized in stable structures, zero being the best case.
- Negative formal charges should be placed on the most electronegative atom(s).
- Positive formal charges should be placed on the least electronegative atom(s).
- It is unfavorable to place like charges (++ or --) on neighboring atoms in a molecule.
Examples of these rules are shown below for alternative structures of BF 3 and ONF.
In the BF 3 case, the structure on the left is non-octet, because there are only six electrons (three bonds) in the valence shell of B. Such structures are said to be electron deficient. An octet structure (right) can be drawn, but it places a positive formal charge on F, the most electronegative atom in the molecule. Thus, neither structure is completely "happy," but the formal charge rules tell us that the electron-deficient structure on the left is more stable. The electron deficiency of BF 3 makes it a powerful Lewis acid.
In the case of ONF, the structure on the left is unfavorable for two reasons. First, it places a positive formal charge on F, the most electronegative atom. Second, there is another possible structure (right) that has zero formal charge, which is preferable to one with non-zero charge. This means that the structure on the right, with a double bond between N and O, is more reasonable.
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Life outside of the ocean owes its existence to the presence of ozone (O 3 ) in the stratosphere. Ozone absorbs the ultraviolet light in the solar spectrum, which otherwise would cause catastrophic damage to DNA and other biological molecules. Because ozone is produced photochemically from O 2 , which is itself generated by photosynthesis, the spectroscopic signature of ozone in the atmosphere of distant planets is one possible way to look for extraterrestrial life. |
Resonance structures
The ozone (O 3 ) molecule has two equivalent octet structures, shown below:
In both cases, the Lewis dot diagram suggests that there are three kinds of oxygen atoms in the molecule, with +1, 0, and -1 formal charges. These structures also suggest that ozone should have one single and one double bond. Experimentally (by electron diffraction), however, we find that the molecule is symmetric, with both O-O bond lengths the same. The real (instantaneous) structure is the average of the two forms, as shown below. In the classical Lewis picture of the molecule, we can rationalize resonance by observing that electrons, being thousands of times lighter than the nuclei of atoms, move very fast on the timescale of molecular vibrations. Thus, in the time it takes for the oxygen atoms to adjust their positions, the electrons can move back and forth many times. In the quantum mechanical MO picture (Chapter 2), we will see that resonance involves electrons that are fully delocalized over the atoms in the molecule. We represent resonance structures with a double-headed arrow to signify that the only difference between the Lewis structures is the distribution of electrons.
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Susan Solomon discovered the heterogeneous catalytic mechanism whereby ozone is decomposed by chlorofluorocarbons (freons), creating the Antarctic ozone hole. Her work formed the basis of the U.N. Montreal Protocol, an international agreement to protect the ozone layer by regulating damaging chemicals. |
In the instantaneous structure of ozone, the formal (-) charge is shared between the two terminal oxygen atoms and therefore each one has a formal charge of -1/2. The O-O bonds are the average of a single and a double bond, i.e., each O-O bond order is 1.5.
Similarly, the nitrate anion has three resonance structures, and experimentally (by X-ray crystallography of nitrate salts) we find that all the N-O bonds in the ion are the same. In this case, in the averaged structure, each O atom has a formal charge of -2/3 and the N-O bond order is 4/3 (=1.33...).
Inequivalent resonance structures
The rules of resonance also apply to inequivalent structures, which in general will have different energies from each other. In this case the structure of the molecule represents a weighted average of the low energy structures. A good example is the cyanate ion, OCN - . We can write three inequivalent octet structures for the molecule:
The first two are reasonable resonance structures, although we suspect the one on the left is the best because it puts the negative formal charge on the most electronegative atom. The last one is clearly a bad resonance structure, because the formal charges are high and there is a positive charge on the oxygen atom. The real structure is thus a weighted average of the first two.
No-bond resonance
An interesting and useful kind of inequivalent resonance structure is one in which there is a bond order of zero between two of the atoms. This concept of no-bond resonance is important in understanding the bonding in many halogen- and hydrogen-containing compounds. The idea is illustrated below for the generic molecule X-Y-Z, where Z might be an electronegative halogen atom such as F. By moving the bonding electrons from the Y-Z bond onto the Z atom, and moving a lone pair from X into the X-Y bond, we generate the resonance structure shown on the right in which all atoms remain octet, but there is no bond between Y and Z.
It is important to recognize in this example that the no-bond form is only one resonance structure, and therefore the Z atom is still bound to Y. If the two resonance structures in this example have the same energy, we would expect the X-Y bond order to be 1.5 and the Y-Z bond order to be 0.5. Therefore the Y-Z bond should be longer than it is in a compound where the bond order is one, and it should be relatively easy to break the Y-Z bond.
Some molecular examples of no-bond resonance are shown below. In the molecule ONF 3 , the N-F bond is unusually long relative to the N-F bond in NF 3 , in which the bond order is 1. This can be explained by the no-bond resonance forms shown on the right. Similarly, in the Lewis acid-base adduct formed by combining BH 3 with CO, we can explain the long B-H bonds using no-bond resonance forms that place a partial positive charge on the H atoms.
No-bond resonance is often used to provide an octet bonding picture for so-called hypervalent compounds , which are compounds that appear to have more than 8 valence electrons in the bonding shell of the central atom. For example, we can consider two different valence bond structures for the triiodide ion, I 3 - , which is formed by reaction of I - with I 2 in water:
In this structure, the central iodine atom has 10 electrons in its valence shell, in violation of the octet rule. Raman spectra of the triiodide ion show that the I-I bond is weaker than the I-I single bond in I 2 , suggesting that this picture is not an accurate description of the bonding. A better representation of I 3 - can be obtained with no-bond resonance structures, as shown below. We will see that this picture is consistent with the MO description of I 3 - in Chapter 2:
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The deep blue color of starch-iodine solutions results from the complexation of linear polyiodide ions (I n - ) by the starch (amylose) left-handed helix. The interior of the helix is hydrophobic. In electron donor solvents such as ethanol and water, I 2 and salts of I 3 - have a brown color, the result of a charge transfer interaction between the solvent and solute. [2] [3]
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We can draw a similar picture for the XeF 2 molecule, which has the same number of valence electrons as I 3 - . This picture is consistent with XPS data, which show a partial negative charge on the F atoms, as well as vibrational spectra, which show that the Xe-F bond is weaker in XeF 2 than it is in the singly-bonded cation Xe-F + .
Other well known examples of hypervalent compounds are PF 5 , P(CH 3 ) 5 , and SF 6 , as well as oxyacids such as H 2 SO 4 and HClO 4 . The hypervalent structure is often drawn for these molecules, with the explanation that d-orbitals on the central atom contribute to the bonding in dsp 3 and d 2 sp 3 hybrids for 5- and 6-coordinate molecules, respectively. However, realistic molecular orbital calculations show that the phosphorus and sulfur 3d orbitals are too high in energy to contribute significantly to bonding in PF 5 and SF 6 . For these molecules, we can use no-bond resonance to make reasonable octet structures that predict polar bonds between the central atom and F.
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Space-filling model of sulfur hexafluoride. SF 6 is surprisingly unreactive with water, relative to other compounds that contain S-F bonds. The tight packing of F atoms around S prevents nucleophilic attack by water. |
In other cases, such as P(CH 3 ) 5 , the octet structure is unreasonable because it suggests a polar bond between P and C, with a partial negative charge on C. Further, in the case of oxyacids such as H 2 SO 4 and HClO 4 , X-ray crystallographic data establish that the S-O and Cl-O bonds are shorter for the oxygen atoms not bonded to hydrogen, which is more consistent with the hypervalent picture:
The question of whether hypervalency and the octet rule are really useful descriptions of the bonding in these compounds has been considered in a number of computational studies, which have used increasingly accurate quantum mechanical calculations to determine the number of electrons associated with the central atom. In a 2002 study, Gillespie and Silvi [4] found that the population of the valence shell is greater than eight for compounds with electropositive ligands, such as P(CH 3 ) 5 , and less than eight for compounds such as PF 5 . They concluded that these valence electron shell populations depend primarily on the coordination numbers and electronegativities of the central atoms and their ligands, and that there is no fundamental difference between the bonding in hypervalent and non-hypervalent (Lewis octet) molecules. This reminds us that the octet rule is not a law of nature, but rather an empirical rule that is useful within certain limits.
The isoelectronic principle
In calculating the octet structures of molecules using the N-V method, we needed to know only the number of atoms and the number of electrons, not the identities of the atoms themselves. This means that we will get the same answer (and the same set of octet and resonance structures) for any molecule or ion that contains (a) the same number of non-hydrogen atoms, and (b) the same total number of valence electrons. Such molecules are said to be isoelectronic. This is a powerful conclusion because, once we have determined the electronic structure of one molecule, we can write down the same solution for all other molecules that are isoelectronic. For example, we noted above that I 3 - and XeF 2 , which both have 22 valence electrons, have the same valence bond structure. We can further expect that isoelectronic molecules will have the same shapes and, very often, similar physical properties.
The four molecules and ions below all contain three non-hydrogen atoms and 16 valence electrons. All of them are linear molecules with two double bonds. The four molecules and ions shown below them all contain four non-hydrogen atoms and 24 valence electrons. These four have a trigonal planar shape. While BF 3 is a member of this isoelectronic series, we do not write it in the resonance form that contains a B=F double bond, because that would put a positive formal charge on F.
Similarly we can show that CH 4 , NH 3 , NH 4 + , H 2 O, H 3 O + , HF, F - , and OH - are all isoelectronic with one non-H atom and eight valence electrons. In the next section we will see that this gives rise to nearly identical electronic shapes for these molecules.
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Nitrous oxide (N 2 O), or laughing gas, is isoelectronic with CO 2 . The two molecules have the same shape (linear) and similar physical properties. CO 2 sublimes at -78 °C, whereas N 2 O melts at -91 °C and boils at -88 °C. |
Isoelectronic solids
The isoelectronic principle works not just for molecules but for extended solids as well. One technologically important set of isoelectronic solids are the p-block semiconductors. The group 14 element Si is the most widely used semiconductor for electronics, but, as we will discuss later, it is not a good light emitter. Light-emitting diodes (LEDs), which are used in lasers, high efficiency lighting, and display technologies, are made from compounds that are isoelectronic with Si and Ge, especially GaAs, GaP, AlAs, and GaN (all contain four valence electrons per atom). CdTe and CuIn 1-x Ga x Se 2 (CIGS) are promising solar cell materials that also have the same number of valence electrons per atom. Like Si and Ge, these compounds have tetrahedrally bonded structures in the solid state and absorb light across most of the solar spectrum, as we will discuss in more detail in Chapters 8 and 10.
The isoelectronic principle is also a powerful tool in materials research, because it provides guidance about where to look for new materials with similar and perhaps improved properties. For example, the discovery that 8.5% efficient thin layer solar cells could be made with the compound CsSnI 3 [5] stimulated the exploration of many isoelectronic ABX 3 compounds with the same perovskite crystal structure. Very recently thin film solar cells based on light absorbers in this structural family (MA)PbI 3-x Cl x and FA x MA 1−x PbBr y I 3−y (MA + = methylammonium, CH 3 NH 3 + ; FA + = formamidinium, HC(NH 2 ) 2 + ) have been reported with efficiencies as high as 22%. [6] [7] [8]


8: Sources of Magnetic Fields
In this chapter, we examine how magnetic fields are created by arbitrary distributions of electric current, using the Biot-Savart law. Then we look at how current-carrying wires create magnetic fields and deduce the forces that arise between two current-carrying wires due to these magnetic fields. We also study the torques produced by the magnetic fields of current loops. We then generalize these results to an important law of electromagnetism, called Ampère’s law.
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- 8.1: Prelude to Sources of Magnetic Fields
- In the preceding chapter, we saw that a moving charged particle produces a magnetic field. This connection between electricity and magnetism is exploited in electromagnetic devices, such as a computer hard drive. In fact, it is the underlying principle behind most of the technology in modern society, including telephones, television, computers, and the internet.
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- 8.2: The Biot-Savart Law
- We have seen that mass produces a gravitational field and also interacts with that field. Charge produces an electric field and also interacts with that field. Since moving charge (that is, current) interacts with a magnetic field, we might expect that it also creates that field—and it does.
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- 8.3: Magnetic Field due to a Thin Straight Wire
- How does the shape of wires carrying current affect the shape of the magnetic field created? We know that a current loop created a magnetic field similar to that of a bar magnet, but what about a straight wire? We can use the Biot-Savart law to answer all of these questions, including determining the magnetic field of a long straight wire.
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- 8.4: Magnetic Force between Two Parallel Currents
- You might expect that two current-carrying wires generate significant forces between them, since ordinary currents produce magnetic fields and these fields exert significant forces on ordinary currents. But you might not expect that the force between wires is used to define the ampere. It might also surprise you to learn that this force has something to do with why large circuit breakers burn up when they attempt to interrupt large currents.
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- 8.5: Magnetic Field of a Current Loop
- We can use the Biot-Savart law to find the magnetic field due to a current. We first consider arbitrary segments on opposite sides of the loop to qualitatively show by the vector results that the net magnetic field direction is along the central axis from the loop. From there, we can use the Biot-Savart law to derive the expression for magnetic field.
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- 8.6: Ampère’s Law
- A fundamental property of a static magnetic field is that, unlike an electrostatic field, it is not conservative. A conservative field is one that does the same amount of work on a particle moving between two different points regardless of the path chosen. Magnetic fields do not have such a property. Instead, there is a relationship between the magnetic field and its source, electric current. It is expressed in terms of the line integral B and is known as Ampère’s law.
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- 8.8: Magnetism in Matter
- Why are certain materials magnetic and others not? And why do certain substances become magnetized by a field, whereas others are unaffected? To answer such questions, we need an understanding of magnetism on a microscopic level. Within an atom, every electron travels in an orbit and spins on an internal axis. Both types of motion produce current loops and therefore magnetic dipoles. For a particular atom, the net magnetic dipole moment is the vector sum of the magnetic dipole moments.


2.7: Field Technique Tips for Measuring Tree Height
Estimating total tree height is very easy to do correctly, but does require that we think about how we are taking our measurements. Here are some tips.
1. Always walk to where one can clearly see the tree top. This can be tough to do, especially in dense stands, on extremely foggy days, or in stands with heavy brush. Dragging a tape around in brush trying to get to that special spot where the tree tip comes into view can be frustrating, but it needs to be done. It is actually pretty rare to have a tree whose crown is impossible to see somewhere within 150 feet. This is why it is important to remain flexible in selecting horizontal distances. Commonly, errors in measuring tree heights come from measuring a branch instead of the tip, measuring the wrong tree top, or guessing where the tree top is in a dense canopy.
2. Do not always rely on that magical distance of 100 feet (Figure 2.15). Once you get comfortable with the instruments in the field, find a method that allows you to be flexible in choosing your horizontal distance from the tree. Discover a fast way to adjust your heights with varying horizontal distances. Some people have to follow a formula. I don’t want to mess with the formula, so I just think of 100 (for %slope) and 66 (for topographic slope) as “calibration” figures. For example:
- At 33 feet, I am 1/2 of one chain (66 feet) from the tree, so tree height is half of my topog slope readings.
- Likewise, at 50 feet, I am 50% of the way to 100; so the tree height is 50% of my slope readings. At 90 feet, the tree height is 90% of my slope readings, etc.
- Let’s say I get +74% to the top, and –22% to the stump. 74+22 = 98. If I am 60 feet from the tree (60% of 100), then tree height is 60% of 90 = 54 feet. If I am 80 feet from the tree (80% of 100), then tree height is 80% of 90 = 72 feet. If I am 120 feet from the tree (120% of 100), then tree height is 120% of 90 = 108 feet.


1.14: Crafting the Plot
Every story you tell will be creating a world where you present a problem and over the course of the story you solve it. In the process of solving the problem the hero is transformed and through the character’s journey the audience learns an important essential truth. However, in order to tell this story you need to first chart the course of events that will take place so that you can maximize the impact of your narrative.
First begin by establishing your Inciting Incident. Ask yourself “what happens that sets the play in motion?” Then ask yourself “What should be the climax or final event?” Once you have these two items figured out, you will know where your story truly begins and what you are building to. The story of your play is the journey from Inciting Incident to Climax.
Create an outline where you have your Inciting Incident on one end and the Climax on the other. If you are creating a full length play then you will need roughly 7-10 scenes. Scene 1 is where you introduce the world of your play, the main characters, establish a normal day for the characters, and at the end of the scene introduce the conflict with the Inciting Incident. Your final scene is the final confrontation of Hero and Villain and the Denouement or falling action where things return to as normal as possible after the events of the play. All of the scenes between are the events that build in tension and lead to the inevitable climax.
Crafting your plot allows you to sidestep writers block, troubleshoot problems in advance, and build tension throughout your story. When you know what needs to be accomplished in the scene you are able to write to that objective and since you have already established a strong intention for each character, once you set them in the scene they will move toward that objective and the scene will write itself. If you experience problems you can take a break from writing the scene since you already know where the scene ends and you can begin writing a later scene and stay productive.
The Following are 2 possible guides helping you craft your plot.
Example 1
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Act 1/Scene 1: Exposition, Foreshadowing, Inciting Incident.
- Establish characters, basic relationships, dreams/goals
- World of the play: Rules, Givens, Setting
- Conflict is introduced
- Inciting incident happens
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Scene 2: Rising action step 1
- Relationships are further defined
- New relationships begin to form based out of conflict
- Conflict is furthered and additional obstacles are introduced
- Introduction of additional characters
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Scene 3: Rising action step 2
- Conflict is furthered
- Relationships continue to develop
- Protagonist is able to form an idea of how to overcome the obstacle
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Scene 4: Rising Action step 3
- Stronger relationships begin to get stronger and the weak ones get weaker.
- At least one obstacle is overcome
- Conflict is furthered as dark twist is revealed
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Act 2/ Scene 5: Rising Action step 4
- Relationships continue to develop stronger relationships are formed and weaker ones continue to deteriorate.
- Protagonist is able to overcome multiple obstacles
- Conflict is furthered as the Antagonist becomes aware of the threat.
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Scene 6: Rising Action step 5
- Conflict is furthered as Antagonist introduces additional obstacles
- Protagonist feels defeated
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Scene 7: Rising Action step 6
- Relationships strengthen through character revelations
- Turning point as protagonist decides to continue fighting
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Scene 8: Climax and Denouement
- Conflict rises and reaches a climax as Protagonist and Antagonist collide.
- Conflict is resolved
Example 2
Act 1
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Scene 1: Setup/Catalyst
- Opening Image
- Set up / Prologue / Exposition
- Catalyst or Inciting Incident
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Scene 2: Rising Action
- Debate
- Decision
- Fun and Games or New World is introduced
-
Scene 3: Rising Action/Fun and Games
- New Characters are introduced
- New world explored and obstacle is introduced
- Stakes raise and conflict is furthered
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Scene 4: Rising Action/ Fun and Games
- New obstacles are introduced
- New world is explored
- Hero becomes aware of potential
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Scene 5: Midpoint
- An obstacle is overcome or almost overcome
- Hero unlocks potential
- Dark turn happens that raises the stakes for the hero
Intermission
Act 2
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Scene 1: Bad Guys Close In or Tensions Rise
- Villain becomes aware of Hero and begins to set additional obstacles
- Relationships begin to fracture
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Scene 2: All is Lost
- Hero receives crushing defeat and relationships are torn apart
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Scene 3: Dark night of the Soul
- Hero contemplates giving up the journey or ending their life
- Side character counsels Hero and reminds them of the lessons learned throughout the journey and tremendous cost of failure.
- Hero finds strength in the interaction and from within and decides to confront the villain directly
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Scene 4: Finale
- Hero and Villain meet in final conflict
- Climax is reached
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Scene 5: Denouement or Resolution
- Conflict is resolved and things return to as normal as possible
- A new normal is established
- Choose a conflict, setting, and characters for your play.
- Draw a line on a sheet of paper that runs from one end of the page to the other. This will be your timeline.
- Choose an inciting incident and climax for your plot and place them at opposite ends of your timeline.
- Follow the examples above and chart a logical progression that leads you from inciting incident to climax maximizing the growth or transformation of your hero character.
- Evaluate whether any events need to shift.
- Separate your events into scenes.
- Begin writing.


7.1: Female Reproductive System Introduction
Learning Objectives
- Apply the rules of medical language to build, analyze, spell, pronounce, abbreviate, and define terms as they relate to the female reproductive system
- Identify meanings of keyword components of the female reproductive system
- Categorize diagnostic, therapeutic, procedural, or anatomic terms related to the female reproductive system
- Use terms related to the female reproductive system
- Use terms related to the diseases and disorders of the female reproductive system
Introduction to the Female Reproductive System
The female reproductive system produces hormones and matures eggs (called ova) during the monthly menstrual cycle. Every month, an egg (called an ovum) is released from one of the female’s ovaries with the potential to become fertilized by male sperm. If fertilization occurs, female hormones develop the fertilized egg into a fetus and, ultimately, a baby ready for delivery in about 40 weeks.
This chapter will review common word components related to the female reproductive system to assist learning how to analyze, build, and define medical terms. Other female reproductive terms, whose definitions cannot be easily built from word components, will be described in context based on the anatomy and physiology of the female reproductive system and common diseases and disorders. Medical specialists, diagnostic tests, and procedures related to the female reproductive system will also be discussed. Additional information about pregnancy will be discussed in the “ Obstetrics Terminology ” chapter.
View the following YouTube video [1] on the anatomy and physiology of the female reproductive system: Reproductive System, Part 1 – Female Reproductive System: Crash Course A&P #40
- CrashCourse. (2015, October 26). Reproductive system, Part 1 - Female reproductive system: Crash Course Anatomy & Physiology #40 [Video]. YouTube. https://youtu.be/RFDatCchpus?si=1h92ZbG3QUjVHw6b ↵


6: Thermochemistry
Useful forms of energy are also available from a variety of chemical reactions other than combustion. For example, the energy produced by the batteries in a cell phone, car, or flashlight results from chemical reactions. This chapter introduces many of the basic ideas necessary to explore the relationships between chemical changes and energy, with a focus on thermal energy.
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- 6.1: Energy Basics
- Energy is the capacity to do work (applying a force to move matter). Heat is energy that is transferred between objects at different temperatures; it flows from a high to a low temperature. Chemical and physical processes can absorb heat (endothermic) or release heat (exothermic). The SI unit of energy, heat, and work is the joule (J). Specific heat and heat capacity are measures of the energy needed to change the temperature of a substance or object.
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- 6.2: Calorimetry
- Calorimetry is used to measure the amount of thermal energy transferred in a chemical or physical process. This requires careful measurement of the temperature change that occurs during the process and the masses of the system and surroundings. These measured quantities are then used to compute the amount of heat produced or consumed in the process using known mathematical relations. Calorimeters are designed to minimize energy exchange between the system and its surroundings.
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- 6.3: Enthalpy- Introduction
- If a chemical change is carried out at constant pressure and the only work done is caused by expansion or contraction, q for the change is called the enthalpy change with the symbol ΔH. Examples of enthalpy changes include enthalpy of combustion, enthalpy of fusion, enthalpy of vaporization, and standard enthalpy of formation. If the enthalpies of formation are available for the reactants and products of a reaction, the enthalpy change can be calculated using Hess’s law.
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- 6.4: Enthalpy- Heat of Combustion
- If a chemical change is carried out at constant pressure and the only work done is caused by expansion or contraction, q for the change is called the enthalpy change with the symbol ΔH. Examples of enthalpy changes include enthalpy of combustion, enthalpy of fusion, enthalpy of vaporization, and standard enthalpy of formation. If the enthalpies of formation are available for the reactants and products of a reaction, the enthalpy change can be calculated using Hess’s law.
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- 6.5: Enthalpy- Heat of Formation
- If a chemical change is carried out at constant pressure and the only work done is caused by expansion or contraction, q for the change is called the enthalpy change with the symbol ΔH. Examples of enthalpy changes include enthalpy of combustion, enthalpy of fusion, enthalpy of vaporization, and standard enthalpy of formation. If the enthalpies of formation are available for the reactants and products of a reaction, the enthalpy change can be calculated using Hess’s law.
Contributors and Attributions
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Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at<EMAIL_ADDRESS>).