My Freediving Glossary

Probably I have a thing with glossaries. :) I posted one for Android, and here's my go for Freediving. Well, guess it just has something to do with my term understanding obsession. These are compiled from various sources, sorry for not being a good netizen, I'm too lazy to list them all. :( This is by no means exhaustive and may not constitute of what a good glossary should, but I think it will help nonetheless.

blackout

the sudden loss of consciousness caused by oxygen (O₂) starvation during a dive on breath-hold

• Shallow water blackout happens towards the end of a breath-hold dive in water typically shallower than 5 metres. The primary mechanism for shallow water blackout is hypocapnia (too little carbon dioxide in the blood stream) brought about by hyperventilation (rapid and/or deep breathing) prior to the dive.

• Deep water blackout happens on ascending from a deep freedive or breath-hold dive, typically of 10 metres or more. Consciousness depends on a minimum partial pressure of O₂ (ppO₂) in the lungs not on the absolute quantity of the gas in the system, and the immediate cause of deep water blackout is the rapid drop in ppO₂ on ascent. During descent, the lung volume decreases due to chest compression under increasing water pressure, resulting in increased ppO₂. The brain and tissues use O₂ during descent, but ppO₂ levels remain over the critical limit as the water pressure keeps increasing. The problem is in the ascent: the re-expanding lungs increase in volume and this results in a rapid decrease of ppO₂ in the lungs to critical levels. The last 10 meters during ascent are the most dangerous, as it is where the greatest relative lung expansion occurs, from 50% to 100% of their volume.

Bohr effect

The Bohr effect is a physiological phenomenon first described in 1904 by the Danish physiologist Christian Bohr, stating that hemoglobin's oxygen (O₂) binding affinity is inversely related both to acidity and to the concentration of carbon dioxide (CO₂).

Hyperventilation (deep and/or rapid breathing) will induce a decrease of CO₂ levels in arterial blood. A decrease in CO₂ provokes an increase in acidity (lowering pH), which results in hemoglobin picking up more O₂. Therefore, with low pH, O₂ tends to remain strongly bound to hemoglobin. So freedivers hyperventilate hoping to prolong their breath-hold but in the end, hyperventilation is preventing the release of O₂ to the tissues, which therefore actually shortens the breath-holding ability.

breathe up

a specific pre-dive breathing and relaxation technique

buoyancy

The term buoyancy is used to describe not only an object's ability to float in the water, but its tendency to sink or to do neither. Positive and negative buoyancy means that the object or person floats upwards or sinks downwards in the water, respectively. Neutral buoyancy means that the object or person neither sinks downwards nor floats upwards, but remains suspended in the water at a single depth.

equalization

ear clearing, any of various maneuvers to equalize the pressure in the middle ear with the outside pressure.

hyperventilation

In freediving, hyperventilation generally refers to the action of breathing larger amounts of air than normal, either by taking bigger breaths or by breathing more rapidly, or both. When you breathe, you breathe in oxygen (O₂) and breathe out carbon dioxide (CO₂). CO₂ builds up in the bloodstream when O₂ is metabolized and it needs to be expelled as a waste product. Excessive breathing causes the concentration of CO₂ in the blood stream to fall and produces a state known as hypocapnia (too little CO₂ in the blood stream), while the increase of the blood saturation on O₂ is insignificant, because:

• Deeper or quicker breaths as in hyperventilation exchange more of the gases in the lungs with ambient air and have the net effect of expelling more CO₂ from the body, since the CO₂ concentration in normal air is very low. 
• Under normal circumstances the breathing rate dictated by the body alone already leads to 96%-99% O₂ saturation of the blood, hyperventilation can only increase this saturation weakly, and the effect is minor.

The primary urge to breathe is triggered by rising CO₂ levels in the bloodstream--low O₂ stimulus is weak and easily overridden--and the body relies on this to control breathing. On the other hand, blackouts (loss of consciousness) are caused by hypoxia (reduced supply of O₂) instead of rising CO₂ levels. Put together, hyperventilation leads to hypocapnia which suppresses the urge to breathe, allowing divers to extend their dive by closing down the body's natural breathing mechanism, and leaves the diver susceptible to loss of consciousness from hypoxia (i.e. the O₂ level drops into the diver's blackout zone before the CO₂ can rise enough to force the diver to resurface to breathe).

hypocapnia

a state of reduced carbon dioxide in the blood. Hypocapnia usually results from hyperventilation (deep and/or rapid breathing). Hyperventilation and the subsequent hypocapnia are a cause of shallow water blackout. Excessive hypocapnia is readily identifiable as it causes dizziness and tingling of the fingers.

hypoxia

In freediving, generally refers to cerebral hypoxia, a form of hypoxia (reduced supply of oxygen) specifically involving the brain. Cerebral hypoxia is the cause of blackouts.

purge

forceful exhale

samba

Loss of Motion Control (LMC), you shake (minor to heavily) like an epileptic stroke when your oxygen level is too low. That's why it's called a samba; you look like dancing a samba.

tidal breathing

normal inhalation and exhalation when extra effort is not applied