Oxygen and the diving seal.

Diving seals are extraordinary animals. They are able to avoid hypoxia and the effects of oxygen deprivation far more efficiently than the vast majority of mammals. One of the kings of the diving world is the elephant seal (Mirounga angustirostris). These animals are capable of performing dives of up to two hours in duration (1) and have been recorded diving to depths of 1.5 kilometers (2). Perhaps more impressive are their routine diving behaviors exhibited during the 5 to 8 month migrations to the sea. During the biannual migrations between foraging grounds and the beaches where they moult and breed, these animals spend 80-95% of their time submerged (3). They follow a pattern of long, deep, continuous dives interspersed with brief surface intervals of 1-3 minutes (4). It was probably in the early '30s and '40s that we really began to understand the physiology behind the impressive breath hold ability of these animals. Per Scholander, Lawrence Irving and their colleagues investigated the physiology of diving in a wide variety of organisms, subjecting them to forced diving protocols and facial immersion (5, 6). Their findings revealed that across species, there are three main physiological responses to facial immersion: 1) apnea; 2) bradycardia; and 3) peripheral vasoconstriction and hyperperfusion of the peripheral tissues. Over time, this triad of physiological events became known collectively as the mammalian diving response. The events that occur during diving are under the control of multiple reflexes, rather than the result of one single reflexive action. Experimentally, these physiological responses can be elicited through facial immersion. In marine mammals, the use of a diving helmet has been as effective as total body immersion in producing diving bradycardia (Figure 1).