When you hold your breath, the reduction in oxygen in the lungs is not the trigger that makes you want to take a breath. Rather it is the body’s “sensor” in the brain that reacts to levels of CO2.
You dive down to thirty-three feet. Now your lungs are half their size. No longer do you feel the discomfort of lung over expansion. Not only that, but the density of the air has doubled. Note - the size has decreased - but the number of air molecules hitting the lung was has doubled. You are relaxed.
You now dive to sixty-six feet. You are now barely negatively buoyant so you are no longer struggling from rising to the surface.
For really deep dives during breath holding it is important to understand the mechanism of oxygen transfer from the lungs to the bloodstream. The bronchial tubes that start from the trachea divide into smaller and smaller “pipes” (bronchiole) ending with microscopic sacs with very thin walls. These sacs are called alveoli. Surrounded by capillaries, they are so thin that gases (oxygen carbon dioxide, etc) can pass between these walls.without the blood moving into the lungs. The process for this transfer is called diffusion. Gases move from an area of high partial pressure to an area of low partial pressure. There is more oxygen along the outer wall of the alveoli than inside the moving capillaries, so oxygen diffuses through the alveoli wall. Conversely carbon dioxide has a higher concentration in the bloodstream so moves from the bloodstream to the lungs.
At extreme depths the process alters. Massive pressure forcing the lung to a fraction of its size blood can enter the alveoli sacs, filling them with blood. This prevents the sacs from collapsing while still offering the necessary gas exchange.
Divers who hold their breath for extended periods force back the natural urge through sheer act of will. Many go through a period of convulsion where the body contracts in an involuntary rehearsal for a real breath. Apnea divers call this the “samba.” The body - realizing a desperation for new air isn’t forthcoming will release oxygen rich blood from the spleen, giving the brain a dose of what it needs.
The heart slows to a rhythm not thought possible to sustain life. Many free divers’ heart rates get down to about fourteen beats a minute. Some have been recorded as low as seven. Scientists say this rate is too slow to support consciousness.1
You are in a state of euphoria and the urge to breathe has just started to tap you on the shoulder. You have consumed half your oxygen, but you barely notice because you have four times the oxygen hitting the walls of your - now tiny- lungs. Some blood and other flu has started invading the alveoli, those skas that help transmit the oxygen to the bloodstream. This is due to the pressure and the bodies natural mechanism to protect the lungs from collapsing.
You begin to surface. Your lungs are expanding and suddenly you feel oxygen starved. Why? When you were at depth the oxygen molecules bounced off your lungs, now they have farther to travel - those that are left. Those that hit the wall may not translate to the bloodstream because of the fluid that lines your cell wall of the lungs.
You are fighting the urge to breath as lactic acid courses through your muscles, causing them to constrict as they fight for air. Luckily your training has told you to relax through that pain. You see the surface, but your lungs cannot supply the oxygen to the brain - the largest and hungriest of oxygen organs in your body. It cannot sustain consciousness so - with only a few feet to the surface, you shut down.
This is shallow water blackout.
Still, with the dangers of this game, the free diving community grows every year. The sport has increased in popularity to the point that other agencies, specifically those in the SCUBA society (PADI, SSI, etc.) have joined the ranks of AIDA in providing free diving training. Nonetheless breath holding is crucial to understanding our watery evolution. One side of the story defines our uniqueness among primates in our capacity to temporarily tour the other seventy odd percent of our planet. Yet, apnea diving has uncovered the dangers and limits which separate us from the marine mammals who seem unencumbered by the physics of going deep.
There is a part of our evolution that calls us back to the depths. Perhaps it is the company I keep, but it is rare the person who hasn't felt some compelling unexplained magnet that draws them toward the ocean.
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I am working on this book after taking 2020 to isolate and hibernate. I hope to be done by spring. Hope you like it!