The goal of the diving reflex is to conserve oxygen. It does this primarily by redistributing blood flow, depressing metabolism, and decreasing respiratory exchange. Although interconnected, the two independent mechanisms by which this is accomplished are bradycardia and peripheral vasoconstriction (Speck and Bruce, 1978).

Bradycardia

Bradycardia is a reduction in heart rate, and it is controlled via the parasympathetic nervous system. This lessens the cadiac output, and thus the amount of oxygen distributed throughout the body (Fagius and Sundlöf, 1986; Butler, 1982; Panneton et al., 2010). The bradycardia response can differ widely between species. In humans heart rate usually decreases by 10-25%, although it has been recorded to drop up to 77% in extremely cold temperatures (Dahms et al., 2010). A more dramatic average is recorded in grey seals, whose bpm drops from 125 to 10 (Butler, 1982).

Fig 1. from (Rosengren et al., 2002) with permission from the author. Hear rate of a human subject when submerged at dotted vertical line. Note the drastic and immediete drop, and the delayed recovery after a 2 sec dive as marked by the black bar.

Peripheral vasoconstriction

Peripheral vasoconstriction is the closing off of capillaries to the organism’s extremities and particular organs to reduce blood flow. This mechanism is monitored via the sympathetic nervous system (Dahms et al., 2010). Selective constriction reduces or entirely eliminates perfusion to some skeletal muscles and nonessential organs such as the gut, kidneys, liver, and skeletal muscles. This ensurse that the now limited cardiac output is distributed to the most oxygen sensitive tissue, such as the brain and heart (Fagius and Sundlöf, 1986). Even with this modification, the brain still undergoes as 50% reduction in available oxygen. Brain function is essential to most diving animals as they are often either escaping a predator or hunting for prey (Ramirez et al, 2007).

Image courtesy of (Ramirez et al., 2007) with the author's permission. Aniographs of peripheral abdominal arteries of a grey seal before and directly after dive.