Question:
How many units (liters/sec.) of oxygen do humans require to
remain alive under:
1- relaxed conditions ?
2- and during heavy exertion, such as swimming?
Replies:
The amount of oxygen your cells require is the same whether you
are watching TV or swimming a 500 Free. But you breathe faster while swimming
because you need to repay the "oxygen debt" which accumulated during the
exercise. Here is how it happens: Normally glucose is converted into
pyruvate by glycolysis. Then in the mitochondria the pyruvate is oxidized
completely to CO2 and H2O using oxygen and resulting in lots of energy in the
form of ATP. At rest normal breathing delivers oxygen at just the right rate
for these reactions to take place unnoticed by you. During extreme exercise
the oxygen cannot be carried to the muscles fast enough to oxidize pyruvate to
produce all the ATP needed just then. So the muscles use stored glucose in
the form of glycogen to generate ATP. But glycogen cannot be oxidized in the
same way as glucose; it must be fermented. This results in much less ATP (but
still some) and lactic acid (not CO2 and H2O). All this lactic acid must be
converted back into glucose in a set of reactions that require oxygen. (These
reaction-called gluconeogenesis take place in the liver.) So while you are
exercising and making lactic acid in your muscles, you are breathing heavily
so your liver can convert lactic acid back to glucose. The excess oxygen
consumed in the recovery period represents the repayment of the "oxygen debt"
- the amount of oxygen required to supple ATP for gluconeogenesis in order to
regenerate the glycogen used during the intense exercise.
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