A seemingly general feature in the adjustment to diving in various mammals, birds, and reptiles is a slow-down and redistribution of the circulation, leaving muscles, periphery, and certain visceral organs with a markedly reduced circulation. This saves oxygen and causes widespread reduction of the energy metabolism. Active as well as quiet diving develops bradycardia without drop in central blood pressure, and lactic acid forming in the muscles during the dive floods the blood when breathing starts. In 31 native skin divers in Australia, diving rarely exceeded 1 min. During the dive, blood pressure stayed normal while heart rate dropped to half; blood lactate (five divers) remained normal but showed an acute rise in the recovery. Evidently humans utilize an adjustment similar to animals. Most of our divers developed various cardiac arrhythmias during the dive, persisting sometimes into the beginning of the recovery. This asphyxial defense seems developed at birth, for fetal bradycardia is common during delivery, and an acute rise of lactic acid after delivery has recently been described. Phylogenetically the mechanism is ancient, for it is found also in fishes when taken out of water.

Submitted on August 7, 1961