Blood Shift: How Your Body Protects Your Lungs at Depth
In the 1960s, physiologists were certain that freediving beyond 30-40 metres would crush human lungs. The math seemed simple: as pressure increases with depth, lung volume decreases according to Boyle's Law. Dive deep enough, and your lungs should compress to the point of damage. Yet today, elite freedivers routinely descend past 100 metres—and the deepest recorded dive exceeds 300 metres.
What the early scientists didn't account for was blood shift—one of the most extraordinary physiological adaptations in human biology.
What Is Blood Shift?
Blood shift (also called thoracic blood pooling) is the massive redistribution of blood from your extremities and abdomen into your chest cavity during deep breath-hold diving. This flood of blood into the thorax takes up space that would otherwise require air, protecting your lungs from the crushing effects of pressure.
First studied and described by American Navy physiologist Karl Schaefer in 1968, blood shift represents your body's elegant solution to an apparently impossible problem: how to survive at depths where lung physics should fail. This is one component of the mammalian dive reflex.
Why Blood Shift Is Necessary
To understand why blood shift matters, you need to understand what happens to your lungs as you descend.
The Pressure Problem
For every 10 metres you descend, ambient pressure increases by one atmosphere. According to Boyle's Law, gas volumes compress inversely with pressure:
At 10 metres (2 ATA): lungs compress to 1/2 surface volume
At 20 metres (3 ATA): lungs compress to 1/3 surface volume
At 30 metres (4 ATA): lungs compress to 1/4 surface volume
The Residual Volume Limit
Your lungs have a minimum volume they can safely compress to, called the residual volume (RV)—typically around 1.5-2 litres in adults. Early scientists assumed that once lung compression reached this residual volume, further descent would cause damage.
For a diver with a total lung capacity of 6 litres and a residual volume of 1.5 litres, simple math predicted a maximum safe depth of around 30 metres. Beyond this, the negative pressure differential would cause fluid and blood to be sucked into the alveoli—a condition called pulmonary barotrauma or "lung squeeze."
How Blood Shift Solves the Problem
Blood shift effectively reduces your residual volume by filling the space around your compressed lungs with incompressible fluid—blood. Research by Schaefer and colleagues measured blood shifts of over 1,000 mL at just 30 metres depth. This blood engorges the capillaries surrounding the alveoli, taking up the space that would otherwise create damaging negative pressure.
Consider the case of legendary freediver Jacques Mayol, who reached 70 metres. His residual volume of 1.88 L and total lung capacity of 7.22 L should have limited him to approximately 28 metres. Calculations showed that a blood shift of nearly 1,000 mL into his thorax was necessary to prevent lung squeeze at his record depth.
How Blood Shift Works
Blood shift is triggered as part of the mammalian dive reflex, but it intensifies dramatically as depth increases and lung compression becomes more significant.
The Mechanism
Peripheral vasoconstriction begins at the surface as part of the dive reflex, reducing blood flow to your limbs. Central blood pooling concentrates blood in your core organs and thoracic cavity. Pulmonary engorgement occurs as blood floods into the capillary beds surrounding your alveoli. Splenic contraction releases additional red blood cells into circulation, further increasing thoracic blood volume.
Learn about all these adaptations in our comprehensive physiology guide.
The Numbers
Research shows that 750-1,200 mL of blood can shift into the thoracic cavity during deep dives. This is a remarkable volume—equivalent to about 15-25% of your total blood volume moving into your chest.
This blood shift effectively reduces residual volume, allowing compression far beyond what lung physics alone would permit. An elite freediver with an RV of 1.7 L, TLC of 10 L, and a blood shift of 1,000 mL could theoretically reach depths of over 130 metres—compared to just 49 metres without blood shift.
The Role of the Alveoli
Your alveoli are the tiny air sacs where gas exchange occurs—roughly 300 million of them in each lung. During deep dives, blood shift causes these structures to become engorged with blood rather than collapsed.
This serves multiple purposes: structural support as blood-filled capillaries help maintain alveolar architecture; volume compensation as blood takes up space that compressed air no longer occupies; and pressure equalisation as incompressible blood prevents the negative pressure that would otherwise damage tissue.
Developing Blood Shift Safely
Blood shift is an innate response, but like other aspects of the dive reflex, it can be enhanced through proper training. The key word is "gradually."
Why Gradual Progression Matters
Your body's blood shift capacity develops over months and years of regular depth exposure. Attempting depths before your cardiovascular system has adapted is the primary cause of lung squeeze in freedivers.
Think of it like building flexibility: you wouldn't attempt the splits on your first day of stretching. Similarly, your thoracic blood shift capacity must be developed progressively.
Training Recommendations
Increase depth gradually: Add no more than a few metres at a time, with adequate adaptation periods between depth progressions.
Develop thoracic flexibility: Chest and diaphragm stretches (like uddiyana bandha) improve your chest's ability to accommodate compressed lungs and shifted blood. These become particularly important once you reach depths around 30 metres.
Prioritise relaxation: Tension in your core reduces your thoracic flexibility and inhibits the blood shift response. The more relaxed you are, the more effectively blood can pool in your chest. Learn relaxation techniques in our breathing guide.
Warm up properly: Your first dives of a session should be shallower, allowing your dive reflex and blood shift to activate progressively.
Listen to your body: Any unusual sensations in your chest, coughing, or blood-tinged sputum are warning signs that should never be ignored.
When Blood Shift Fails: Lung Squeeze
Despite our remarkable adaptations, blood shift has limits. Lung squeeze (pulmonary barotrauma) occurs when the protective mechanisms are overwhelmed.
Risk Factors
Progressing too deep too fast: Attempting depths before adequate adaptation
Cold water: Reduces thoracic flexibility and dive reflex efficiency
Tension and anxiety: Inhibits relaxation and blood shift
Poor technique at depth: Jerky movements or improper turns can stress compressed lungs
Inadequate warm-up: Diving deep without activating the dive reflex progressively
Illness or fatigue: Compromises physiological responses
Warning Signs
Coughing during or after a dive
Blood-tinged mucus or sputum
Chest tightness or pain
Unusual fatigue
Difficulty breathing
Understand all the safety risks in our guide to freediving deaths and prevention.
If You Experience Symptoms
Any signs of lung squeeze require immediate cessation of diving, rest and monitoring, medical evaluation if symptoms persist or are severe, and extended recovery before returning to depth (typically 6-8 weeks minimum).
Most lung squeeze incidents are mild and self-limiting, but approximately 20% of affected divers require medical attention, and some cases result in hospitalisation.
Blood Shift and Depth Records
The existence of blood shift explains how humans have shattered every predicted depth limit. Modern research and diving practice demonstrate that people can dive to depths exceeding 200 metres while maintaining lung function.
Herbert Nitsch's record-breaking 253-metre dive (before his accident) and Alexey Molchanov's current records in the 130+ metre range are only possible because of robust blood shift responses developed through years of progressive training.
Blood shift represents one of the most elegant examples of human physiological adaptation. Your body possesses the innate capacity to protect itself at depths that should be impossible—but this protection must be developed gradually and respected absolutely. Rush the process, and you risk serious injury. Respect it, and you unlock access to a world that scientists once believed was forever closed to breath-hold divers.
This article is for educational purposes and does not constitute medical advice. Always train with certified instructors and progress depths gradually under proper supervision. Find a qualified instructor with our guide to evaluating freediving instructors.