Freediving Physiology

Freediving Physiology
Understanding how your body works underwater is very important for both the feeling of comfort and wellbeing underwater and to avoid barotrauma (pressure related injury). This article will examine the various effects of immersion in water on the freediver's body.

The Dive Reflex
Submerging the face into water causes the mammalian diving reflex, which is found in all mammals (including humans), but especially in marine mammals (as, whales and seals.) This reflex puts the body into oxygen saving modus to maximize the time that can be spent under water. The effect of this reflex is greater in cold water than in warm water, and includes three factors:

Bradycardia, a reduction in the heart rate (of up to 50% in humans).

Peripheral Vasoconstriction, a decrease in blood flow to the extremities, in order to increase the supply of blood and oxygen to the vital organs, especially the brain.

Blood shift, the shifting of blood to the thoracic cavity, i.e. the chest between the diaphragm and the neck to avoid the collapse of the lungs under higher pressure during deeper dives.

Thus, both a conscious and an unconscious person can survive longer without oxygen underwater then in a comparable situation on dry land.

Pressure
The underwater environment has very different physical and chemical characteristics to the world in which we live above the surface. Water is denser then air and acts on the organism to produce modifications that are of great consequence to the freediver. The aspect that requires the most attention is pressure. Each dive exposes the body to variations of pressure proportional to depth. These variations in pressure require specific behavioral strategies. Physics teaches us the liquids are practically incompressible while gases are compressible. Water constitutes about 70% of our body mass. The remaining 30% is either solid (also incompressible) or spaces containing gas, which are subjected to the same pressure variations as those that affect us during our diving. This explains why when we immerse the 'empty' spaces of our body and equipment they are subject to a squeeze. Hence the ears, lungs and mask must all be compensated (see Equalization).

In physics, water pressure is a result of the application of a force downwards from the surface, given by :

P = F/S = 1kg/1cm² = 1 ATM = 1.013 Bar = 1013 MILLIBAR = 760mmHG

Atmospheric, Hydrostatic & Ambient
With reference to diving it is necessary to define what is meant by:

(ATM) Atmospheric Pressure, The pressure exerted by the weight of a column of air with a height of 10,000m (the height of the atmosphere that circles the earth) on a square centimeter on sea level.

(ATU) Hydrostatic Pressure, The pressure exerted by the height of the column of water above each square centimeter of an immersed body. Every 10m of depth is equal to 1 ATM.

(ATA) Ambient Pressure, also called absolute pressure, this is the sum of the atmospheric pressure at sea level, which is always 1 ATM, and the hydrostatic pressure that varies by 1 ATM every 10m of depth. In other words :

ATA = ATM + ATU

Therefore :

Sea level	ATA = 1ATM + 0 ATU = 1 ATA
At -10m	ATA = 1ATM + 1 ATU = 2 ATA
At -20m	ATA = 1ATM + 2 ATU = 3 ATA
At -90m	ATA = 1ATM + 9 ATU = 10 ATA

Boyle's Law
Boyle's law states that:

The volume of a gas at constant temperature is inversely proportional to the pressure exerted on it.

This means that during a freedive descent, the freediver's lung volume is reduced in proportion to the pressure acting on it. At a depth of -50m a freediver will have a lung volume one sixth that of its volume on the surface. For many years it has been believed that the lungs will collapse after reaching a certain depth. However recent discoveries prove that blood plasma enters the thoracic cavity and compensates for the loss of air volume.

Hyperventilation And Black Out
For many years it has been thought that hyperventilating before a freedive increases the freediver's ability to stay longer underwater because a greater amount of oxygen is being inhaled. This, however, has been proven to be absolutely wrong. What really happens is as follows: Chemical receptors in our pulmonary system inform us about:

High levels of CO2 (carbon dioxide) which is being produced by the metabolization of oxygen.

Low levels O2 (oxygen) which are being reached because of the metabolization of the gas in order to sustain normal bodily function.

When the level of CO2 reaches a certain limit a stimulation begins which forces to body to resume breathing. When hyperventilating, what you are actually doing is reducing the level of CO2 while maintaining the same level of Oxygen (21% in volume), not to mention causing many muscles to contract in the process which also uses up your O2. When submerged, you do not feel the need to breath so rapidly because, as already said, CO2 level is low. By the time you feel the need to breath, the O2 level in your body is very close to the critical limit in which the body shuts down (blacks out) in order to save oxygen for the brain - you do not feel anything unusual, but as you ascend from the depth, especially in the last 10 meters (where the pressure variations are the greatest), your body might (without any prior warning):

Black Out (faint) - this is also referred to as 'SWB' or 'Shallow Water Blackout'.

Or enter a state of

Loss Of Motor Control - also referred to as 'LMC' or 'Samba' because of the involuntary movements of the body in this state which resemble a samba dance.

The Unknown
Forward progress has been made into the scientific understanding of man's responses to apnea, but there are still many uncertainties and physiological phenomena that await explanation.

"We are only at the beginning of a mysterious and fascinating road: we know neither its direction nor its destination, but we must walk it all the same. However it will take time, much time." - Dr. Luca Torcello, from The Manual of Freediving by Umberto Pelizarri.

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