Immediate bradycardia 10-25%. Peripheral vasoconstriction. Apnoeic reflex. Blood shift at greater depth protects the lungs. Trained freedivers: 50%+ heart rate reduction. Bajau: significantly larger spleens than average.
The human body has ancestral physiological responses to submersion: bradycardia, peripheral vasoconstriction, and redistribution of blood volume. Understanding these mechanisms is key for the modern freediver. The community debates how to train to enhance the reflex, the lactic acid limits in anaerobic effort, the risks of prior hyperventilation, and diaphragm management during deep descents.
Its three main components are activated when the face comes into contact with cold water: bradycardia (immediate 10–25% reduction), peripheral vasoconstriction (blood redirected toward the heart and brain), and reflex apnea. In trained freedivers, bradycardia can reach reductions of 50% or more relative to resting levels. At greater depth, pulmonary compression activates the "blood shift," redirecting blood toward the thorax and protecting the lungs from pressure-induced collapse. Recent studies investigate whether populations with a freediving tradition such as the Bajau of Southeast Asia have significantly larger spleens than average.