Interactions between hypoxia and hypothermia.

Hypoxic animals have an impressive arsenal of defense mechanisms, many of which are common to other kind of stresses. Physiologic defense mechanisms, e.g. increasing cardiac output or ventilation, are effective but also energy demanding. An alternative to improving O2 supply is reduction of O2 demand. This is easily accomplished by ectothermic vertebrates by seeking a cooler environment and augmenting the rate of cooling by increased perfusion of the skin. More surprising is that endotherms respond to hypoxia in the same way. Unlike near drowning, where the hypothermia is forced (but still effective), the behavioral hypothermia appears to be regulated and analogous to fever. Much more work is needed in this formative area of research to establish mechanisms, functional significance, and pathways common to stresses other than hypoxia.