"Hemoglobin is the key to a healthy heartbeat."
- Placebo, Haemoglobin
Blood - vertebrate* blood, at any rate - is red because the erythrocytes that float in plasma are red. The erythrocytes, in turn, are red because of all the hemoglobin in their cytoplasm. It's chock-full of the stuff. In the diagram below, the four green wire-frame looking bits (as opposed to the red and blue ribbon-looking bits) are the ferrous heme groups that bind up the oxygen you breathe into your lungs:
Oxygen diffuses into your red blood cells through the alveoli of the lungs, where each of those heme groups grabs an oxygen molecule. It's easy to do, since oxygen is fairly plentiful in the air we breathe. That may not seem like a whole lot, but it increases the amount of oxygen your blood can absorb by about seventy times, since otherwise oxygen could only dissolve into your plasma as a gas. Then, as your blood courses through your body, the lack of oxygen causes the iron atom in each heme group to lose its grip on the oxygen molecule it's holding, and the oxygen diffuses into nearby cells where it fuels the chemical reactions that drive you. That's... really all it is! Just pressure. Lots of chemical reactions, it turns out, are more like making soup than they are like building a machine, and the body's urgent need for oxygen in every cell means that a quick and dirty solution like this is all it takes (no fancy-pants calcium ion pumps or anything).
Of course, hemoglobin isn't the only solution to this problem. Molluscs and arthropods use cuprous hemocyanin, which does the same thing but has way cooler color-change action. See, deoxygenated blood is dark burgundy in color, whereas oxygenated blood is a more vibrant red. When hemocyanin is deoxygenated, it's colorless - but when oxygenated, it's blue. Look at this crab:
To be fair, hemocyanin bonds a little stronger to oxygen, which is what makes it so good for the invertebrates that use it, because they often inhabit oxygen-poor environments. On a related note, carbon monoxide bonds much stronger to the heme groups, rendering them useless because it never leaves, and this is why CO is such a deadly poison. Other solutions to the "Gee, I need oxygen in my blood" problem include hemerythrin, which is pink when oxygenated and colorless when deoxygenated, and the Christmas-themed chlorocruorin, which is red when oxygenated but green when deoxygenated.
OK, enough about other kinds of blood, back to your blood! After your erythrocytes have dumped their truckload of oxygen into your hungry hungry cells, they pick up the carbon dioxide to carry it to the lungs for exhalation. This is done in three ways: about 7% of your waste CO2 is dissolved directly into plasma, 23% combines with hemoglobin, and a whopping 70% is transformed by carbonic anhydrase (which is in your erythrocytes' cytoplasm) into carbonic acid. The Alert Reader who is passing familiar with chemistry will notice that carbonic acid dissolves in water by separating into a negatively charged bicarbonate ion and a positively charged hydrogen ion (or, as physicists are wont to call it, a proton). The Alert Reader who is passing familiar with chemistry will also notice that free-floating protons in water tend to result in hydronium ions, and the negative log of the hydronium ion content is what is measured when we refer to "pH". Here is a chart showing what happens when your blood pH gets outside the narrow range of 7.35-7.45 that I mentioned when we spoke last:
The Alert Reader who is passing familiar with Greek will notice that everything outside of Normal ends in "-osis", which means problem (loosely translated). So how does your body fastidiously avoid such problems? Well, in the first place, it's not like all your cells take in oxygen at once and then pass off carbon dioxide all at once. That would just be silly. But your body is always metabolizing, all the time always until you're dead, and so it needs to keep a tight rein on your blood pH as you go through your varying levels of activity.
Blood is slightly alkaline, and your bones are basically load-bearing mineral deposits, so that helps at least a little bit (Fun Fact: consuming too much animal protein in relation to vegetable protein has been implicated in bone mass loss in females!). Short term pH imbalance can be corrected by altering respiration: expelling more CO2 will increase the proportion of CO2 dissolved in blood (less carbonic acid means more alkaline blood), and holding on to more CO2 will increase the proportion held as carbonic acid (or H+ and HCO3-) and increase acidity. In the long term, your kidneys respond by excreting the leftover acid or base that builds up in your bloodstream, and also regulating the amount of buffering ammonia in your blood.
Your blood also helps regulate your body temperature through the clever application of plumbing. Heat is generated in various organs such as the liver and the brain (even thinking is exothermic!), and blood acts as a coolant to help you avoid overheating by absorbing some of the heat and bringing it to the heat sink that is your skin. In addition to sweating, your body sheds excess heat by expanding its arterial walls, increasing blood flow to the capillaries near the surface of the skin where the heat escapes into the atmosphere, or at least into your sweat (if the surrounding air temperature is higher than your body temperature). When you need to conserve heat, your arteries constrict, reducing blood flow to the skin and extremities to conserve heat and thus maintain core body temperature. This is why the cold will often make you numb and pale: your thoracic cavity needs all the heat it can hold on to, and the rest of you is somewhat more expendable. Note that this is distinct from frostbite, which results from ice crystals puncturing cellular membranes and causing cell death (this is why frostbitten tissues are kinda gelatinous when thawed, and one of the key hurdles for cryonics to overcome in the quest for legitimacy).
OK, oxygen transport, acid-base homeostasis, and thermoregulation - check! Tune in next Wednesday when I chatter on about the army of your immune system!
* - There just had to be an exception, didn't there?! The crocodile icefish does not use hemoglobin, and is the only vertebrate known not to do so. It lives in sub-zero seawater where it can absorb all the oxygen it needs right through it's goddamned skin. Fuckin' icefishes have it so stupid easy.