Human Breath-Hold Diving Ability and the Underlying Physiology.

Humans freedive for recreation, food, and for records, displaying an ability superior to any other known terrestrial mammal. Harvest divers may spend up to 5 h daily submerged, and competitive deep divers may swim down to depths beyond 100 m - and up again - on one breath. This is paralleled only by aquatic and semi-aquatic mammals. In all air breathers, the ability to breath-hold dive is set by a number of physiological factors. Some of these have been extensively studied, like the 'diving response,' which diverts blood to the brain, heart and working muscles, while less sensitive organs can rely on anaerobic metabolism. The diving response is effectively triggered in trained human divers and may reduce the heart rate by 50% - similar to the response in semi-aquatic mammals, and it has been shown to conserve oxygen and prolong apneic duration. More recently, a spleen response during diving was described in humans resembling that found in seals. The spleen ejects its stored reserve supply of red blood cells into the circulation, elevating hematocrit, and thereby the oxygen carrying capacity, which also increases apneic duration. Other features not typical of terrestrial mammals, such as: naked skin with a continuous layer of subcutaneous fat; under water vision; effective locomotion in water; large flexible lungs; an ability to equalize ears and sinuses; efficient voluntary respiratory control and vocal communication; a long lifespan; and a large brain, are shared by many species with a semi-aquatic evolutionary history and are also shared by Man. This would seem to suggest a period of evolutionary pressure from life involving swimming and diving in human prehistory. [ABSTRACT FROM AUTHOR]