Truly aquatic animals have no need for a specialized diving response, because they are able to obtain oxygen directly from the water and thus sustain themselves indefinitely in the aquatic medium. Terrestrial or land animals, on the other hand, have in many cases gone back to the aquatic mode of living in varying degrees. In all of these has arisen the problem of obtaining the necessary gas exchange in an aquatic environment with only the physical attributes of a terrestrial animal. Many of these terrestrial animals have adapted so well as to achieve almost perfect freedom in the water.
All classes of terrestrial vertebrates (birds, mammals, and reptiles) have species which spend part of their lives under water or seek food or shelter by becoming submerged for a period of time. One example, a fresh-water mammalian species, is the North American beaver. One of the first changes noted in these and other animals that normally dive beneath the surface of the water is a reduction in heart rate, called bradycardia. However, those animals which traditionally utilize an aquatic habitat show a more pronounced reduction in heart rate in a much shorter time. In the other two classes of animals which contain divers, the reptiles and birds, some excellent examples reflect reductions in heart rate similar to the rates seen in mammals.
Warm-blooded animals show a more rapid response to submergence than do reptilian species. Those mammals, such as seals, which are traditionally found in open-ocean environments show the most dramatic responses of all. The reptiles in general utilize less oxygen than do warm-blooded animals and have slower resting heart rates, so that their response would be expected to be less spectacular. Also, reptilian reaction times are generally slower than those seen in mammals, and are usually modified by the temperature of the water in which they dive, so that a slower response to the diving stimulus is expected.
Specifically, the diving response enables the animal to remain underwater for longer periods of time. Not only does the heart rate slow down, but blood flow to specific muscles is decreased or stopped altogether. Lactic acid produced as a result of anaerobic glycolysis in muscles thus isolated is less likely to enter the systemic circulation and alter the functions of vital organs such as the brain and heart. This response also reduces the likelihood that hydrogen ions will build up in the blood and stimulate chemoreceptors (receptors which respond to changes in blood carbon dioxide levels and hydrogen ion concentration) which control breathing. Blood flow thus remains constant and unrestricted to the most essential organs, such as brain, heart, and lungs, during the dive, and arterial pressure to these organs remains at the normal level, so that perfusion of the capillaries with blood is not altered.
Other changes in the bodies of divers are also apparent, but are seen as changes in total body chemistry. For instance, glycogen and adenosinetriphosphate, which are the principal energy sources of muscles, are in more than ample supply in divers, and appear to have been stored for that time when they are most needed. Oxygen stores are essential to the length of time that a diver may stay submerged, and diving animals have concentrations of myoglobin in their muscles which gives them a remarkable ability to store oxygen. See also Diving.
