The oxygen stores of the body are relatively small in comparison to the consumption (around 250 ml/min for an adult). Total body oxygen is about 1.5 litres, which is held as:
• 50% in combination with Hb
• 20% in combination with myoglobin
Not all stored oxygen is available for use since severe hypoxaemia occurs before even half of the oxygen stored in combination with Hb and myoglobin is released. These available stores will last only for 3-4 min of apnoea, assuming air breathing and normal oxygen consumption. Breathing 100% O2 increases the oxygen stores to about 4.25 litres, mainly by increasing the O2 contained in the lungs.
In contrast, total body stores of CO2 are about 120 litres. During apnoea, PACO2 increases by about 1 kPa in the first min. This initial rise in PACO2 then decreases to a rate of 0.4 kPa/min, as alveolar PCO2 levels build up and CO2 elimination by diffusion through the airways increases.
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