I already discussed freediving injuries such as lungs and trachea squeezes due to lack of relaxation in the abdominal muscles in my article Squeezes - Freediving lung or trachea injuries and Postural Vanity

Let’s analyse the lung physiology at depth and freediving lung squeezes a bit more in detail and why they happen.

What is the pleura?

The pulmonary pleurae are the two opposing layers of serous membrane overlying the lungs and the inside of the surrounding chest walls: the inner pleura, called the visceral pleura, and the outer layer, called the parietal pleura, lines the inner surfaces of the thoracic cavity.

Between two pleurae is the pleural cavity, which is normally collapsed and filled with only a tiny amount of serous fluid secreted by the pleurae. The two lungs bounded by parietal pleura, almost fill the thoracic cavity.

What is blood shift?

Water pressure increases with depth according to Boyle’s Law. As he descends to depth, the increase in pressure compresses the air in a freediver’s lungs, compressing also his lungs as well. At 100 metres below the surface, a freediver’s lungs occupy 1/11th of their original volume.

Until the 1960’s, physiologists predicted that humans would be unable to freediver deeper than 50 metres due to compression of the lungs and chest cavity. It was thought that the rib cage would crush inwards into the empty space normally occupied by the lungs. As the lung volume is emptied down to the absolute minimum, the lowest volume is called the residual volume (RV).

It was thanks to freediver Enzo Maiorca that this theory was disproved in 1961 by freediving deeper than 50 metres.

Scientists realised that some unknown aspect of human physiology prevented the chest cavity from compressing and causing injury. It was during a study in 1974 on freediver Jacques Mayol, that scientists finally discovered the reason: blood shift, which allows a freediver to descend past Residual Volume without crushing his chest.

The blood displaced from a diver’s extremities by vasoconstriction travels to the organs in his chest cavity, occupying the space created when air in the lungs compresses.

Most importantly, blood travels to the alveoli, tiny sacs in a diver’s lungs where gas exchange occurs. The alveoli are engulfed in blood plasma from the surrounding tissues. As blood is an incompressible fluid, it maintains its volume no matter how deeply the diver descends. Because fluid replaces the empty space left behind when air in the diver’s lungs compresses, his chest and lungs are not crushed by the increased pressure of the water.

Essentially, the central pooling of blood in the chest equalises the pressure gradient when the Residual Volume is reached and thereby decreases the effective Residual Volume and allows for deeper depths to be attained safely. 

What is a lung squeeze and why does it happen?

A squeeze is essentially a damage or injury to the lungs as a result of the effects of increased pressure on the closed gas spaces of the lungs during breath-hold diving. Medically, it is known as lung or trachea barotrauma.

When a freediver dives past Residual Volume, his lungs will be partly collapsed and blood shift will compensate for the volumetric difference. 

In these conditions, lung squeeze happens at depth when the volume of the freediver’s rib cage is subject to a sudden increase. This will create a sudden intrapulmonary depression which will suck extra-alveolar blood in an intra-alveolar environment.

This happens in a few specific instances, for example:

• Lack of thoracic flexibility

• Lack of abdominal muscles relaxation - postural vanity - please read my article Squeezes - Freediving lung or trachea injuries and Postural Vanity for more insights on this matter

• Diaphragm contractions at depth

• Aggressive turn at the bottom plate

• Too wide movements in CNF or FIM at big depths

• Abdominal charge which is too deep (Valsalva past 20m or Mouthfill charge past 50m)

After an abdominal charge, the rib cage will want to restore their original volume, creating intrapulmonary depression, therefore a lung squeeze

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