Floating motionless atop a tank of water in 2009, French diver Stéphane Mifsud claimed a world record for static apnea (stationary breath holding) of 11 minutes and 35 seconds. In 2010, another record setter, Ricardo da Gama Bahia of Brazil, flooded his body with oxygen for more than 20 minutes and then held his breath underwater for 20 minutes and 21 seconds. Both those achievements and many earlier records put to shame the breath-holding efforts of most people on dry land, who may nonetheless find that they, too, can hold out much longer than usual while swimming. The explanation, many say, is partly rooted in an evolved physiological response that helps seals, whales, otters and other aquatic mammals stay underwater for half an hour or more at a time: the diving reflex.

When a mammal’s face submerges in cold water and its airway snaps shut, other changes triggered in the cardiovascular system collectively help the animal make the most of the oxygen in its blood and lungs. First, the heart rate slows significantly—by roughly 90 percent in some marine mammals. In humans, the reduction is relatively paltry: a 1978 study by Dexter F. Speck, now at the University of Kentucky College of Medicine, and David S. Bruce of Wheaton College found that the diving reflex can lower people’s heart rates by 10 percent or more. Still, that change is enough to extend break point: the moment one feels forced to take a breath.

Second, the diving reflex makes capillaries in the skin and limbs constrict, redirecting blood away from the body’s surface and toward the vital organs. This shift saves more oxygen for the brain and heart, but to a degree it also seems to fortify the torso against the crushing effects of water pressure at great depth. Moreover, it helps to preserve the body’s core temperature in icy water. A disadvantageous consequence, however, is that the muscles in the limbs must then rely more on anaerobic energy metabolism to keep working, so they build up lactic acid and tire more rapidly than they would from comparable exercise at the surface.

Although the diving reflex is involuntary, a 2000 study by Erika Schagatay, then at Lund University in Sweden, and her colleagues found that with experience, human divers could significantly increase the magnitude of some of these changes and delay the point at which they have to take a breath. Such training no doubt contributed to the amazing breath-holding feats of the underwater record holders.