21.13: Oxygen is transported while bound to hemoglobin.

Among vertebrates, oxygen is transported to body tissues within red blood cells, bound to a molecule called hemoglobin. Hemoglobin is like an oxygen “shuttle bus,” transporting oxygen around the body. In the lungs, it picks up O2 and hangs on to it as the blood cell returns to the heart and is pumped to the body. Only when it reaches tissues such as organs or muscles that are in need of oxygen but are far from sources of the vital gas does the hemoglobin release its O2 “passengers.” Having released some (or all) of its oxygen, the hemoglobin returns to the lungs, where it can load up on oxygen again. It is the oxygen-carrying hemoglobin that gives our blood its red color. When it gives up its O2, hemoglobin turns more of a purplish-maroon color. That’s why books often show oxygenated blood in arteries as red and deoxygenated blood in veins as blue or purple.

Figure 21.27: Hemoglobin: the oxygen transporter.

Hemoglobin is a tiny molecule—so tiny that there are about 250 million copies of it in every red blood cell. Built right inside the blood cell as it is being formed, the hemoglobin remains there for the cell’s entire life. Each molecule of hemoglobin is a tangled mass of four polypeptide chains. Nestled within the molecule are four cozy compartments, each of which can carry one molecule of oxygen gas on a seat of iron. This iron attaches to the O2 that diffuses into the red blood cell, temporarily making it part of the hemoglobin molecule (FIGURE 21-27). As we saw earlier, a shortage of iron in your diet can lead to anemia. This is because when iron is in short supply, less oxygen can be bound by hemoglobin and transported by each red blood cell, causing muscles and organs to be starved of oxygen and leading to feelings of fatigue and weakness.

Although it is banned by the Olympics and most sports organizations, “blood doping” is used by some athletes to improve their performance. One method involves withdrawing red blood cells during the weeks and months leading up to a big competition, storing them, and re-injecting them in the few days just before the competition. Because red blood cells are filled with hemoglobin, the oxygen-carrying pigment, blood doping can increase the athlete’s capacity for delivering O2 to his or her tissues. In addition to being against the rules in most competitions, blood doping also carries some health risks, because it increases the viscosity of the blood. In the 1990s, dozens of apparently healthy, elite cyclists died inexplicably from heart failure. It was suspected that their blood had become so thick with red blood cells from blood doping that the burden on the heart to pump their sludge-like blood became too great.

Oxygen binds to hemoglobin, but hemoglobin doesn’t hold on to the oxygen so tightly that it never lets go. Like Post-it notes, which are useful because they are sticky enough to attach to surfaces but not so sticky as to become permanently affixed, hemoglobin functions as if it “knows” when to bind to O2 and when to release it. This hinges on something called the partial pressure of oxygen (denoted as ), the force of oxygen particles in the air pressing against the body (FIGURE 21-28). The partial pressure of oxygen can be thought of as a measure of the amount of oxygen present.

Figure 21.28: The force of particles on you. Air pressure and the partial pressure of oxygen are reduced at higher altitudes.
Figure 21.29: Hemoglobin binds and releases oxygen, depending on the partial pressure of oxygen in the vicinity.

Where in a body might hemoglobin encounter relatively high or low partial pressures of oxygen? When you breathe in air that has a large amount of oxygen, all four O2 compartments in hemoglobin promptly bind oxygen molecules. Deep in the tissues of your body, though, oxygen is not in great supply—especially if you are exerting yourself and your muscles have been consuming oxygen as they contract. When these tissues become depleted of oxygen, any hemoglobin in the vicinity encounters a reduced amount (a low partial pressure) of oxygen ( ). And what does hemoglobin do when it encounters a small amount of oxygen? Because the oxygen it carries is not held too tightly, some of it is released (FIGURE 21-29). This oxygen is quickly soaked up by the tissue, which can then continue to generate ATP.

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When you are sitting at your desk, the amount of oxygen in your tissues isn’t very low—not many of your muscles are contracting, and your breathing rate isn’t especially high. Much like a car coasting downhill, you aren’t using much fuel. In these circumstances, hemoglobin typically gives up only one of its four molecules of bound oxygen gas before returning to the lungs to load up again. It cycles back and forth between getting packed with four oxygens in your lungs and being reduced to three oxygens in your tissues.

This seems wasteful. If your hemoglobin usually oscillates between picking up a single molecule of oxygen in the lungs and dropping off that O2 in the organs or muscles, what is the point of its carrying around the other three oxygen molecules? These are its emergency reserves for when you need a lot more oxygen to fuel energetic activity. If you are exercising vigorously, for instance, the amount of oxygen in your tissues can drop so low that hemoglobin gives up another of its oxygen molecules. In extreme cases of exertion, it might give up three or even all four of the oxygens it carries. FIGURE 21-30 shows an oxygen binding curve for hemoglobin, illustrating the relationship between the amount of oxygen (measured as its ) and the proportion of oxygen molecules that hemoglobin holds on to.

Figure 21.30: “Sticky, but not too sticky.” A graph showing hemoglobin’s affinity for oxygen.

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Figure 21.31: How a fetus gets oxygen.

When a woman is pregnant, the growing fetus does not breathe air. However, during development the fetus needs a great deal of oxygen. How does it get the O2 it needs? It has to scavenge oxygen molecules released by the mother’s hemoglobin. This is made all the more challenging because the fetus and mother have their own separate circulatory systems and there is no intermingling of blood between them. At the placenta, however, the fetal and maternal blood vessels become highly branched and come in extremely close contact. Here, the fetus is able to get access to oxygen. It does this by producing its own special type of hemoglobin that is a bit stickier than normal adult hemoglobin. At a that is low enough for the mother’s hemoglobin to release oxygen, the fetal hemoglobin—with its greater stickiness (or oxygen affinity)—binds to the oxygen. It can then deliver that oxygen to its own, fetal tissues (FIGURE 21-31).

If hemoglobin isn’t put together exactly right, the health consequences can be serious and painful, as is seen in sickle-cell disease. A single change in the genetic instructions for building hemoglobin causes a malfunction in the hemoglobin molecules (see Chapter 7). When they lose their bound oxygen molecules—such as when an individual is exercising—the hemoglobin molecules suddenly become misshapen and stick together, causing the entire red blood cell to collapse into a sharply pointed sickle. Once sickled, red blood cells cause numerous problems. Many break open, which can cause anemia if the cells aren’t replaced promptly. Others clump together, often blocking capillaries where, normally, they pass through one at a time. This leads to intense pain, especially in the joints and muscles, and can cause strokes if it occurs in the brain. About 70,000 people in the United States live with sickle-cell disease. They can minimize the effects by avoiding strenuous activity or other situations in which the amount of oxygen in their muscles and other tissues drops too low.

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Q

Question 21.7

What is carbon monoxide poisoning?

In an unfortunate coincidence, carbon monoxide (CO) also binds to hemoglobin, but with a higher affinity than oxygen. In areas with high carbon monoxide concentrations—such as around a faulty furnace or a kerosene heater without adequate ventilation—the carbon monoxide will outcompete oxygen for hemoglobin’s binding sites. And, to make this situation even worse, not only is carbon monoxide odorless and colorless, but hemoglobin doesn’t bind it loosely, like a Post-it note—instead, it binds CO very tightly. Thus, when the hemoglobin travels to the body tissues, it has no oxygen to release. In the absence of O2, cellular respiration cannot generate the ATP the tissue needs (see Chapter 4). Consequently, the tissue is suffocated even as the person takes deeper and deeper breaths.

TAKE-HOME MESSAGE 21.13

Red blood cells are filled with hemoglobin, a molecule that picks up oxygen in the lungs and transports it around the body. Hemoglobin releases its oxygen in organs and tissues, such as muscles, where it is needed for cellular respiration.

What is blood doping, and how can it affect performance?