The Cardiovascular System Underwater
The Haenyeo, or "sea women," of Jeju Island are physiological marvels, adapted to a life of breath-hold diving in often frigid waters. Their bodies exhibit a suite of traits fine-tuned for aquatic work, particularly in the cardiovascular system. One adaptation is a pronounced diving reflex, where the heart rate slows dramatically upon submersion to conserve oxygen. In simulated dives, Haenyeo show a mean heart rate decrease of 18.8 beats per minute (bpm), significantly more than the 12.6 bpm drop in non-diving Jeju women. Some experienced Haenyeo have even showed a heart rate decrease of more than 40 bpm in just 15 seconds. This bradycardia reduces the heart's workload and oxygen demand, allowing for longer time spent underwater.
Studies of elderly Haenyeo (average age 68) working in winter seas between 10°C and 13°C recorded a heart rate drop of about 20% from surface swimming (125 bpm) to bottom time (101 bpm). Their training also appears to affect their respiratory system. The lung capacity of a Haenyeo averages 3.4 liters, about 12% greater than the 3.1 liters of non-diving women, which is considered an adaptation of the respiratory muscles to water pressure. While the Bajau people of Indonesia, another diving population, have genetically larger spleens that are a biological scuba tank by storing oxygenated red blood cells, studies show Haenyeo spleens are not significantly different in size from non-diving Jeju residents. Their adaptations are more related to training and other genetic factors.