We began this chapter with the observation that when a seal begins underwater activity, its heart rate slows and blood flow to all of its tissues except its brain drops dramatically. This "diving reflex" of marine mammals is in stark contrast to the increase in heart rate and blood flow we experience when we begin exercise. The obvious difference between the situation of the seal and the human is that the human has access to atmospheric oxygen during exercise, but the diving seal does not.
The seal has several adaptations that enable it to remain under water for a long time. The seal's oxygen storage capacity is about twice that of human due to the seal's greater blood volume, the greater oxygen-carrying capacity of its blood, and the greater amount of myoglobin in its muscles.
These adaptations are not sufficient, however, to explain dives of half an hour or more. The seal's most important adaptation for diving is the diving reflex, which is a slowing of the heart (Figure 49.20) and a constriction of the major blood vessels going to all tissues except certain critical ones, such as the nervous system, the heart, and the eyes. The seal's central blood pressure remains high, but blood flow to its tissues decreases. This reduced blood flow has two effects: One is to switch the tissues to glycolytic (anaerobic) metabolism, and the other is to suppress the metabolism of the tissue.
While diving, the seal accumulates lactic acid in its muscles, which constitutes an "oxygen debt" to be paid back through elevated metabolism after the dive ends. But the total metabolic "debt" is much less than the metabolism that would have occurred over the same period of time had the seal not dived. The diving reflex causes the seal to be hy-pometabolic (to have a metabolic rate below its basal rate) during the dive. Hypometabolism, increased oxygen stores, and a high capacity for anaerobic metabolism make it possible for the seal to perform its amazing diving feats.
The seal's diving reflex may seem like a unique adaptation, but it provides yet another example of how natural selection shapes biological traits that are widely shared among related species. Humans also have a diving reflex. It is controlled by the vagus nerve and the parasympathetic nervous system. When our faces are submerged, we experience a mild slowing of our heart rate. This reflex probably serves as a protective response during the birth process, when pressure on the umbilical cord can deprive the fetus of maternal oxygen before breathing can begin. There are many cases, how-
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