ABSTRACT
Understanding the physiological and metabolic elements of mammalian hypoxia tolerance might be found in the successful strategies of innately hypoxia-tolerant mammals. Adult marine mammals are elite divers that can withstand intense exposure to hypoxia during underwater swimming and hunting. In contrast to humans, the physiological specializations that facilitate hypoxia tolerance of marine mammals improve as these species develop from fetus to neonate to adult. This includes a strong cardiovascular dive response (bradycardia, peripheral vasoconstriction) that conserves oxygen by reduced cardiac output and oxygen delivery to many tissues, along with reduced metabolic demand on the heart. A number of anatomical and biochemical features in the heart, brain, and lung of marine mammals support their diving capabilities and their ability to survive profound hypoxemia. The fetal marine mammal receives continuous blood supply through the uterine artery when the pregnant female dives; however, the depletion of on-board maternal oxygen stores during submergence indicates that the marine mammal fetus is also exposed to hypoxic episodes, even late in development. The fetus also experiences declines in arterial oxygen tensions and blood pH in parallel with the diving female, and a slow-developing dive bradycardia.
