FIGURE 1.

Homeostatic control of cardiovascular hemodynamics and respiration are disrupted during underwater submersion

A: note the ∼80% drop in HR when a laboratory rat (top left photograph) volunteers to dive underwater (down arrow), which persists until it surfaces (up arrow). The transitory increase in arterial blood pressure due to sympathetic activation can also be seen. B: respirations also cease during diving, inducing radical changes in blood chemistry. Po₂ falls while underwater, whereas Pco₂ rises dramatically. Despite this, respiratory chemoreceptors that normally would increase ventilation are muted. The hypoxia in tissues deprived of blood after the selective peripheral vasoconstriction induces anerobic metabolism, with an increase in lactic acid as by-product. Note, however, that its release into the bloodstream does not occur until after the animal surfaces, when the stringent vasoconstriction of muscular, splanchnic, and cutaneous circulations is released. B is adapted from Ref. 215 (and is used with permission) showing such changes in a seal (bottom left photomicrograph) during a dive.