Apnea Diving – Part 2 of 2 Blood Shift and Spleen Effect


The mammalian dive reflex is a physiological response to diving. Mammalian Dive Reflex Basics describes two important aspects of the reflex observed in freedivers: bradycardia, the slowing of the heart rate; and vasoconstriction, the narrowing of the arteries to reduce blood flow. These responses are triggered by submersion in water.

The mammalian dive reflex includes two other adaptations, blood shift and the spleen effect.

Unlike bradycardia and vasoconstriction, these reflexes occur in response to the increase of water pressure around a diver, and not simply to submersion in water. Without blood shift and the spleen effect, freedivers would be unable to dive very deep.

 Why Doesn’t Water Pressure Crush a Freediver’s Chest on Deep Dives?:

Water pressure increases with depth according to Boyle’s Law. The increase in pressure compresses the air in a freediver’s lungs as he descends. The freediver’s lungs are compressed as well. For example, at 100 meters below the surface, a freediver’s lungs will occupy 1/11th of their original volume.

Until the 1960’s, physiologists predicted that humans would be unable to freediver deeper than 50 meters 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.

Freediver Enzo Maiorca disproved this theory in 1961 by freediving deeper than 50 meters.

Scientists realized that some unknown aspect of human physiology prevented the chest cavity from compressing and causing injury. During a study in 1974 on freediver Jacques Mayol, scientists finally discovered the reason.

 Blood Shift Allows a Freediver to Descend Without Crushing His Chest:

The blood shunted from a diver’s extremities by vasoconstriction travels to 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 (for our intents and purposes) 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.

 The Spleen Effect Supports the Blood Shift by Manufacturing Blood Cells:

Physiologists long believed that the spleen was a redundant organ, sharing the liver’s function of destroying old red blood cells with the liver. In fact, the spleen may be removed from the body without interfering with the vital processes of the body.

However, the spleen has a secondary function that makes it an important organ for freedivers. Because huge volumes of blood circulate through the spleen, it acts as a reservoir of blood. When extra volumes of blood are required for the blood shift, the spleen releases blood into the diver’s system. The spleen itself shrinks as it empties blood into circulation.

The spleen effect may increase the length of breath-holds and the time at depth during freedives by properly distributing red blood cells throughout the body.

 Side Effects of the Blood Shift and Spleen Effect:

The blood shift and spleen effect adaptations observed during freediving are fascinating and essential for freedivers who plan to descend below the surface (as opposed to static apnea). However, these adaptations do have a few side effects: immersion diuresis and accelerated accumulation of lactic acid.

  1. Immersion Diuresis:
    As the amount of blood in the diver’s chest cavity increases, the diver’s body senses the increase in blood volume, and attempts to normalize it by removing water from the blood through the synthesis of urine. This is one reason that scuba diving and freediving make divers need to pee underwater. It is also one of the reasons that divers become dehydrated rapidly.2. Lactic Acid :
    Lactic acid also accumulates in the limbs more quickly due to the reduction of blood flow and volume in the extremities from vasoconstriction. Lactic acid may cause cramps or soreness.

Strengthening the Mammalian Diving Reflex Improves Freediving Abilities:

All divers will experience the mammalian diving reflex as it is a natural response to submersion and descent in the water. With training and stretching, the mammalian diving reflex may be strengthened which can improve an individual’s freediving abilities. Suggestions for strengthening the mammalian diving reflex include:

  • Stretch intercostal muscles before every freedive to increase strength of the diaphragm and thoracic elasticity.
    • Practice and warm up in shallow water by freediving after exhaling to reduce lung volume without descending very deep. This will induce the dive reflex and prepare a freediver to go deep.
    • Practice freediving at depth regularly.
    • Increase freediving depths gradually and to improve your mammalian diving reflex.

 The Take-Home Message About Pressure, Depth and the Mammalian Diving Reflex:

The mammalian diving reflex includes a variety of physiological reactions. Vasoconstriction and bradycardia are induced by simple submersion in water (even without a significant increase in depth). Blood shift and the spleen effect are triggered as the diver experiences an increase in water pressure with depth. The mammalian diving reflex enables humans to freedive to significant depths and spend relatively long periods of time underwater. By strengthening the mammalian dive reflex, a diver can improve his freediving performance.

Julien Borde Updated May 22, 2016

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