Freediving and lung protection

Blood shift, boyle's law and lung protection during diving

The human body is a marvel of adaptability, capable of remarkable feats when subjected to extreme conditions. One such example is the physiological response known as "blood shift" that occurs during deep diving. Together with the application of Boyle's law, this is key to understanding how the body protects the lungs from compression during diving.

Boyle's law and it's role in freediving

Boyle's law, named after physicist Robert Boyle, states that the pressure and volume of a gas are inversely proportional at a constant temperature. In the context of diving, the "gas" is the air in a freediver's lungs, and the "pressure" is the water pressure surrounding the freediver on descent.

As the freediver descends, water pressure increases, which correspondingly decreases the volume of air in their lungs. Without a protective mechanism, this could lead to a dangerous condition called lung barotrauma, where the lungs can compress and potentially suffer damage.

It was previously thought that the human lungs were incapable of withstanding the compression brought about by diving to great depths. However, modern research and diving practice show that people can dive to depths exceeding 200 meters and maintain lung function.

Blood shift: an adaptive response

This is where the physiological phenomenon known as "blood shift" comes into play. As a diver descends and the air in their lungs compresses, blood and other body fluids redistribute into the thoracic cavity and the blood vessels of the lungs. This effectively replaces the volume of compressed air, preserving lung structure and function and preventing their collapse.

Blood shift naturally occurs in deep-diving mammals like seals and dolphins, but it also occurs in humans during prolonged dives to significant depths without breath.
The role of blood shift in lung protection
By filling the lungs with fluid, blood shift protects against the risk of lung collapse and damage due to increased pressure at depth. This is an impressive demonstration of the body's ability to adapt to extreme conditions and effectively mitigates the potential dangers of high hydrostatic pressure from the outside.

Blood rush to the alveoli

Alveoli are small air sacs in the lungs where gas exchange occurs. During dives to great depths, blood shift leads to the alveoli filling with blood. This helps maintain their structure and prevents collapse as the air inside them compresses. This mechanism also provides additional volume to replace the compressed air and helps maintain normal pressure inside the lungs.


The complex interplay between blood shift and Boyle's law allows the body to remarkably protect the lungs during deep diving. Understanding these mechanisms not only satisfies scientific curiosity but may have potential applications in areas like medicine and sports science. Blood shift is a subject of active research, and scientists continue to work on understanding the finer details of this complex physiological process. Learn more about apnea at our Apnetica Freediving school.

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