What Happens to
Your Body Underwater
From the moment your face hits the water, a cascade of ancient reflexes transforms your body into a diving machine. Understanding this physiology makes you a safer, deeper, and more aware freediver.
1. The Mammalian Dive Reflex
The mammalian dive reflex (MDR) is evolution's gift to aquatic breath-holders. It is triggered by cold water contact on the face and apnea (breath-hold), and activates three protective mechanisms simultaneously.
Bradycardia
Heart rate slows by 10–25% in recreational divers and up to 50% in elite athletes. Slower heart rate means the heart consumes less oxygen, extending the dive window.
Peripheral Vasoconstriction
Blood vessels in the limbs and skin constrict, redirecting oxygen-rich blood to the brain, heart, and lungs — the vital organs that need it most.
Spleen Contraction
The spleen stores up to 200mL of oxygen-saturated red blood cells. During a dive it contracts and releases them into circulation, increasing haematocrit by 10–15%.
Training tip: Splashing cold water on your face before a pool dive activates the reflex faster. Regular training deepens all three responses — most divers see measurable bradycardia improvement within 3 months.
2. Blood Shift
Below approximately 10–15m, lung volume compresses below the body's functional residual capacity. Without a protective mechanism, the lung walls would tear. Instead, blood plasma migrates from peripheral blood vessels into the dense network of pulmonary capillaries, filling the remaining space.
How it works
- ▸Increased thoracic pressure drives fluid from systemic circulation into pulmonary vessels
- ▸Up to 1 litre of plasma fills the lung space that air once occupied
- ▸Alveoli remain intact — no tearing or barotrauma
- ▸Thorax becomes rigid, protecting against external pressure
Limits
- ▸Blood shift has a physiological ceiling — beyond it, thoracic squeeze occurs
- ▸Poorly conditioned divers face squeeze risk above 40m
- ▸Mouthfill technique introduces air from the mouth into the lungs to supplement blood shift at extreme depths
- ▸Training increases the depth at which blood shift activates and the volume it can supply
3. Lung Compression & Boyle's Law
Boyle's Law states that at constant temperature, the volume of a gas is inversely proportional to pressure. For a freediver, every 10m of depth adds one atmosphere of pressure — cutting lung volume roughly in half with each doubling of absolute pressure.
Squeeze Risk: When lung volume approaches residual volume (RV ≈ 1.5L) and blood shift cannot compensate, thoracic squeeze occurs — alveolar capillaries rupture, causing coughing of blood. Never push depth limits rapidly; physiological adaptation takes months of consistent training.
4. Hypoxia, LMC & Blackout
Hypoxia — insufficient oxygen to the brain — is the central safety challenge in freediving. It is silent and fast: a freediver can be conscious at depth and unconscious within seconds of surfacing. Understanding the mechanism is the first step to preventing it.
Why blackout happens on the way UP, not down
High pressure raises PaO₂ artificially — diver feels fine even with depleted O₂ reserves. At 30m (4 ATM), a PaO₂ of 30 mmHg at surface appears as 120 mmHg at depth.
Pressure drops sharply. PaO₂ falls below 35 mmHg — the blackout threshold. The diver loses consciousness with no warning. This zone is the most dangerous in all of freediving.
An unconscious freediver face-down in water will drown in minutes. An attentive buddy who performs immediate rescue is the only line of defence.
Loss of Motor Control (LMC)
- ▸Semi-conscious — eyes may be open but unfocused
- ▸Rhythmic convulsions of arms, face, or whole body
- ▸Usually within 7m of surface
- ▸Requires immediate buddy rescue — airway above water, stimulate breathing
Blackout (BO)
- ▸Full unconsciousness — no voluntary response
- ▸Can occur at depth or on the surface
- ▸Body may be limp or convulsing
- ▸Buddy must bring diver to surface, support airway, call for help if no recovery in 30s
5. Depth Zones — What Happens When
A depth-by-depth breakdown of the physiological events occurring during a freedive. Times are approximate and vary by diver conditioning and water temperature.
- ▸Dive reflex activates: heart rate begins to slow (5–10 bpm drop)
- ▸Peripheral vasoconstriction begins in extremities
- ▸Buoyancy is positive — you must fin or pull down
- ▸Lung volume near total lung capacity (TLC ~6L adult)
- ▸Heart rate drops 10–25% of resting rate
- ▸Spleen contracts, releasing stored red blood cells (+10–15% haematocrit)
- ▸Buoyancy neutral — no effort to maintain depth
- ▸Lung volume compresses to ~50% of TLC
- ▸Heart rate reaches 50–60% of resting in trained divers
- ▸Blood shift begins: plasma migrates from periphery into pulmonary vessels
- ▸Lungs compress below functional residual capacity — diaphragm lifts
- ▸Buoyancy becomes negative — you free-fall; freefall is effortless
- ▸Blood shift fully established — thorax fills with ~1L of blood plasma
- ▸Lung volume ~25% of TLC; alveoli remain intact due to blood fill
- ▸Oxygen partial pressure still adequate for consciousness
- ▸CO₂ urge to breathe present but manageable; diaphragm contractions begin
- ▸Lung volume approaches residual volume (RV ~1.5L) — squeeze risk zone
- ▸PaO₂ around 80–90 mmHg — adequate, but descending curve is steep
- ▸Heart rate may reach 30–40 bpm (extreme bradycardia in elite divers)
- ▸CNS narcosis is absent (unlike scuba) — mind stays clear
- ▸Lungs near or at RV; thoracic squeeze possible without mouthfill technique
- ▸PaO₂ begins dropping below 60 mmHg — hypoxia window opens on ascent
- ▸Blood shift at maximum; thorax rigid with engorged pulmonary vessels
- ▸Equalisation requires Frenzel or mouthfill — Valsalva impossible
- ▸PaO₂ at depth may be 120+ mmHg (hyperoxic) — masks hypoxia until ascent
- ▸Critical blackout zone: final 10–15m of ascent as PaO₂ plummets
- ▸Thoracic squeeze requires years of conditioning; RV effectively zero
- ▸Record dives (CWT −253m) require full physiological adaptation over years
6. How Training Changes Your Body
Consistent freediving training produces measurable physiological adaptations. Most improvements are noticeable within 3–6 months of regular practice.
Stronger dive reflex
4–8 weeksBradycardia onset becomes faster and deeper. Peripheral vasoconstriction activates earlier in the dive.
Enlarged spleen
3–6 monthsSpleen volume increases with regular training, releasing more stored RBCs per dive — measurable by ultrasound.
Higher CO₂ tolerance
2–6 weeksThe urge-to-breathe threshold rises with CO₂ table training. Diaphragm contractions start later and feel less urgent.
Blood buffering capacity
3–6 monthsThe body becomes better at managing lactic acid and CO₂ accumulation, allowing more efficient oxygen use.
Diaphragm flexibility
2–4 weeksYoga-based stretching and packing exercises increase diaphragm range of motion, delaying the mechanical urge to breathe.
Improved lung elasticity
3–12 monthsRegular full packs and stretches increase total lung capacity and the RV/TLC ratio, extending the safe depth range.
Physiology FAQ
What is the mammalian dive reflex?
The mammalian dive reflex (MDR) is an involuntary physiological response triggered by cold water on the face and breath-holding. It causes bradycardia (heart rate slowing), peripheral vasoconstriction (blood redirected from limbs to core), and — in trained divers — spleen contraction that releases stored red blood cells. This reflex is present in all mammals and allows the body to conserve oxygen during a dive.
What is blood shift in freediving?
Blood shift is the movement of blood plasma from peripheral blood vessels into the pulmonary (lung) blood vessels as a freediver descends. As pressure compresses the lungs below their residual volume, the engorgement of blood vessels in the thorax prevents lung tissue from collapsing or tearing. This protective mechanism allows freedivers to reach depths that would otherwise cause fatal thoracic squeeze.
Why do freedivers blackout on the way up, not on the way down?
At depth, the increased pressure raises the partial pressure of oxygen (PaO₂) in the blood, keeping the diver conscious even with reduced O₂ reserves. As the diver ascends, pressure drops sharply — and PaO₂ drops with it. In the final 10–15 metres of ascent, O₂ partial pressure can fall below the threshold for consciousness (≈35 mmHg) very rapidly, causing sudden blackout without warning. This is called shallow-water blackout.
What is residual volume and why does it matter?
Residual volume (RV) is the amount of air remaining in the lungs after a maximal exhalation — typically 1–1.5L in adults. In freediving, this is the theoretical depth limit beyond which the lungs cannot compress further. Without blood shift, diving past the RV depth would cause thoracic squeeze. Blood shift effectively extends functional capacity by filling lung space with fluid, allowing elite freedivers to exceed what Boyle's law predicts.
What is a Loss of Motor Control (LMC)?
LMC is a partial blackout — the diver loses voluntary muscle control, often showing rhythmic arm or facial convulsions, but remains semi-conscious. It typically occurs within 7 metres of the surface as oxygen levels drop. LMC requires immediate buddy rescue: support the airway above water, remove the mask, stimulate and command the diver to breathe. LMC left unsupported can progress to full blackout.
Does freediving cause nitrogen narcosis?
No — nitrogen narcosis is a scuba phenomenon caused by breathing compressed gas at high partial pressure. Freedivers breathe no compressed gas and therefore cannot suffer nitrogen narcosis. However, freedivers can experience hypoxia (oxygen depletion) and oxygen toxicity at extreme depths — entirely different conditions with different causes.
How does training improve freediving physiology?
Regular freediving develops several physiological adaptations: a stronger dive reflex (lower resting heart rate, faster bradycardia onset), a larger spleen (more stored red blood cells available), increased tolerance to CO₂ (delayed urge to breathe), improved blood buffering, and greater diaphragm flexibility. Most improvements are measurable within 3–6 months of consistent training.
What is thoracic squeeze?
Thoracic squeeze (pulmonary barotrauma) occurs when the lungs compress below residual volume and blood shift is insufficient to fill the remaining space. The result is tearing of alveolar capillaries, causing coughing blood, pain, and in severe cases pulmonary oedema. It is rare in recreational freediving but becomes a risk below 40m without proper technique and conditioning.
Put the Science Into Practice
Understanding your body is step one. Learn how to rescue a buddy, practise with our breathing tools, and find a certified instructor to guide your training.