Freediving and the Mammalian Dive Reflex: How Humans Can Dive to 214 Meters

Herbert Nitsch Dove to 214 Meters on a Single Breath — Below the Depth Claimed to Cause Fatal Lung Squeeze

In 2007, Austrian freediver Herbert Nitsch descended to 214 meters (702 feet) on a single breath in the "No Limits" discipline, using a weighted sled to descend and an inflatable balloon to ascend. At that depth, water pressure is 22.4 atmospheres — 22 times the pressure at the surface. Standard physiological calculations predict that at these pressures, the chest cavity should be crushed to the size of a fist as lung volume compresses according to Boyle's Law. That it is not — that Nitsch and other elite freedivers regularly descend beyond 100 meters without pulmonary hemorrhage — is explained by a set of physiological adaptations collectively called the mammalian dive reflex, and by a remarkable blood redistribution mechanism called the blood shift. These are not unique to Nitsch; every human being possesses these mechanisms. Elite freedivers have simply trained to maximize and tolerate them.

The Mammalian Dive Reflex: A Survival System Written Into Vertebrate DNA

The mammalian dive reflex is a coordinated autonomic response triggered by facial immersion in cold water. It is present in all air-breathing mammals, from seals to humans, and is most pronounced in marine mammals. In humans, the reflex activates within seconds of cold water contact with the face — specifically through trigeminal nerve receptors around the nose and cheeks — and produces three primary physiological changes:

• Bradycardia (heart rate slowing): Heart rate drops immediately upon facial immersion, typically 10–25% in untrained individuals; trained freedivers achieve heart rates of 15–20 bpm during deep dives — less than half their resting rate; this dramatic reduction in cardiac work reduces myocardial oxygen consumption, extending available dive time
• Peripheral vasoconstriction: Blood vessels in the extremities (hands, feet, limbs) constrict, shunting blood toward the core; this preserves core oxygen delivery to the heart and brain while sacrificing peripheral tissues; the physiological logic is triage — vital organs are protected first
• Spleen contraction: The spleen — which stores approximately 240 mL of highly oxygenated red blood cells in reserve — contracts and injects this stored blood into circulation; a 2009 study by Schagatay et al. found spleen volume decreased by 20% during repeated apnea dives, releasing an estimated 0.3–0.5 L of RBC-rich blood, increasing hematocrit and oxygen-carrying capacity

The Blood Shift: Engineering Against Boyle's Law

Boyle's Law (PV = constant) predicts that at 30 meters depth (4 atmospheres absolute), lung volume compresses to 25% of its surface value. At 10 meters — approximately the depth where lung volume equals residual volume (the minimum air volume that remains after a full exhalation) — further compression would collapse lung tissue. Yet freedivers routinely go far below 10 meters. The blood shift is the mechanism that makes this possible. As depth increases beyond residual volume depth, plasma from the pulmonary circulation is drawn into the lung vasculature, filling the alveolar airspaces with blood plasma. This fluid, being essentially incompressible, resists the pressure-driven volume reduction and prevents lung collapse. The lung essentially fills with blood from the inside, creating a liquid-filled state rather than a collapsed air-filled state. This mechanism — documented in humans and known to be far more extensive in diving mammals — allows lung volumes to reach as low as 10–15% of their surface value in extreme deep freedivers.

Shallow Water Blackout: The Physiology of Freediving's Primary Killer

Shallow water blackout (more precisely called hypoxic blackout) is responsible for most freediving fatalities and occurs due to a counterintuitive interaction between oxygen consumption and carbon dioxide accumulation:

• The urge to breathe during breath holding is driven primarily by rising CO₂ levels (hypercapnia), not falling O₂ levels; this creates a dangerous asymmetry
• Hyperventilation before a dive — a common but dangerous practice — purges CO₂ from the blood, delaying the breathing urge while providing no additional O₂ storage (hemoglobin is already ~98% saturated at rest)
• During ascent from depth, partial pressure of oxygen in the lungs drops as pressure decreases (Henry's Law); O₂ that was dissolved under pressure at depth becomes unavailable as pressure falls
• The diver can lose consciousness due to hypoxia (O₂ levels too low to maintain brain function) while the CO₂ drive has not yet reached the threshold that would trigger the urge to breathe
• This explains why hyperventilating before a dive is banned in competitive freediving and why a "buddy system" is considered non-negotiable safety protocol

Training Adaptations in Elite Freedivers

• Elite freedivers develop spleen hypertrophy — the spleen grows larger with training, storing more oxygenated RBCs; the Bajau people of maritime Southeast Asia, who have practiced subsistence breath-hold diving for generations, have been shown in a 2018 Cell study to have genetically larger spleens and genetic variants (PDE10A) associated with enhanced diving ability
• Trained freedivers show stronger bradycardia responses (deeper heart rate slowing) and faster activation of the dive reflex compared to untrained controls — suggesting the reflex is trainable
• CO₂ tolerance training — deliberately practicing breath holds at uncomfortably high CO₂ levels — extends competitive breath hold durations by habituating the discomfort response rather than by changing physiology
• The current static apnea world record stands at 24 minutes and 3 seconds, set by Budimir Šobat in 2021, after breathing pure oxygen prior to the attempt