21.18–21.19: Evolutionary adaptations maximize oxygen delivery.

A rare white yak near Goyko Lake, in the Himalayas, almost 5,000 m above sea level.

Breathing is more difficult in some places than others. Because the diffusion of gases between the outside and inside of an animal depends on several physical factors, animals differ widely in their respiratory efficiency. One of the most important constraints on the rate at which oxygen diffuses into the blood is air pressure.

Figure 21.38: Gas exchange is more difficult in some environments than others.

As we noted when discussing birds’ respiratory systems, at high altitudes much less oxygen is available. You don’t even need to go climbing in the Himalayas to experience this. A relatively short walk up a much smaller mountain will convince you. At 15,000 feet above sea level, for example, the air pressure is half of what it is at sea level. This is because there is less atmosphere above the air, pushing down on it (see Figure 21-28). With lower pressure from above, less oxygen is squeezed into a given volume. The lower air pressure makes it hard to push air into lungs and to drive the oxygen across the alveolar and capillary membranes and into the blood. We struggle at high elevations, because our hemoglobin isn’t designed to pick up oxygen at such a low pressure; the hemoglobin isn’t sticky enough.

Mountain climbers often carry canisters of pressurized oxygen with them. In the lungs, the high-pressure oxygen can more easily diffuse into the bloodstream. But llamas and other animals living at high elevations don’t have cans of oxygen. How do they survive with less available oxygen (FIGURE 21-38)? Their hemoglobin has a difficult task: it must pick up oxygen at very low pressure (a condition under which hemoglobin typically has a low affinity for oxygen) so that it can release it to muscles.

Living at elevations of about 3.1 miles (5,000 m) above sea level, llamas solve this problem by producing a slightly different form of hemoglobin. Under the low-oxygen conditions of high elevations, llama hemoglobin has a higher affinity for oxygen than human hemoglobin. It’s “stickier.” This stickiness enables the hemoglobin to become saturated with four molecules of oxygen even when the llama breathes in relatively “oxygen-poor” air. Because oxygen is being consumed by cellular respiration, the oxygen bound to the hemoglobin is still released to muscles to allow normal activity.