My understanding of the metabolic sequence during an apnea:
1. You hold your breath
2. You start moving
3. Muscles initially burn the ATP stored in them
4. Within a fraction of a second the depleted ATP is regenerated by buring creatine phosphate (CP)
5. Within a fraction of a second after that, the creatine phosphate begins to regenerate from O2 stolen from the myoglobin in the muscles, but this process is somewhat slow and can't keep up
6. Several seconds later, the muscle begins to burn glycogen (stored in the muscle) and glucose (mainly from the blood). This reaction always produces lactic acid, but assuming that enough O2 is available from the blood & myoglobin, the lactic acid itself is converted to energy; this reaction produces CO2, water and energy as its byproducts (this 'completed' reaction is called aerobic metabolism, as opposed to the incomplete reaction which is called 'anaerobic').
7. Eventually, the O2 supply declines, replaced by CO2.
8. As the O2 supply declines, both the lung O2 decreases and the blood O2 decreases.
9. The myoglobin relies on the blood to replenish its oxygen, but as the blood becomes less oxygenated there is less and less O2 fed to the myoglobin.
10. The total O2 in the muscle from myglobin & blood now decreases, and not all the lactic acid from the [glyocogen+glucose] reaction can be converted into CO2, water and energy, so the lactic acid begins to acculumate in the muscle.
11. Because the O2 flow to the muscle is slowing down, if the athlete slows his effort to a crawl, the lactic acid can indeed be converted to energy, CO2 and water, despite the slow flow of O2.
12. If the athlete accelerates his energy production, then there is no hope to clear the lactic acid (due to the slowed O2 flow), and the lactic acid accumulates rapidly (in both the muscle AND the blood).
13. As the lactic acid accumulates, the muscle tissue itself becomes more acidic (i.e. pH decreases), which inhibits many enzymes responsible in the reactions which generate energy.
14. Due to the inhibition of these enzymes, the reaction rate of [glycogen+glucose] is further slowed, and the muscle begins to fatigue rapidly (the definition of fatigue is reduced power output which is what happens when the enzymes are inhibited)
15. Additionally, there is some sort of sensor mechanism which converts the pH in the muscle to a sensation of burning.
16. If effort continues, and O2 supply does not improve, eventually the pH in the muscle drops so low that the muscle fails entirely.
17. When O2 supply is resumed, the lactic acid will be cleared, then the creatine phosphate, ATP and myoglobin will be regenerated, and function returns to normal.
18. The more lactic acid which waits to be cleared, the greater the 'oxygen debt' of the athlete, and the longer it will take to recover after the apnea.
Some creatures can escape this cycle. The crucian carp can live on entirely anaerobic metabolism for three months, without a single molecule of oxygen. Although the crucian carp would like to clear out the lactic acid with oxygen, if oxygen is not available then it can convert the lactic acid to ethanol (alcohol), which it then excretes through its skin. Researchers often put crucian carps in small bowls, in deoxygenated water, and sure enough the water itself starts to get saturated with alcohol as the carp eliminates its lactic acid by converting it to ethanol and then 'sweating out the alcohol.'
After about three months, the crucian carp simply runs out of fuel (glycogen = carbohydrate), and it eventually dies.
Eric Fattah
BC, Canada
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