The see-saw of carbon dioxide and oxygen

This is a useful concept which applies to subjects breathing air or any other gas mixture free of carbon dioxide. It relates the partial pressure of oxygen in alveolar gas to that of carbon dioxide. Oxygen and carbon dioxide are continually moving between the alveolar gas and the blood in the pulmonary capillaries. 

In any gas mixture, the partial pressure exerted by one component of the gas is directly proportional to the concentration of that component. 

In alveolar gas, the P02 is lower than in inspired air by the same amount as the PCO2 is higher. 

IN HYPERVENTILATION, THE BODY IS DEPLETED OF CARBON DIOXIDE BUT IS NOT LOADED WITH EXTRA OXYGEN 

In considering hypoventilation earlier in this book, attention was focussed on the rise in carbon dioxide partial pressure since this is the factor of importance in acid-base considerations. In the present context, it is clear that a rise in the blood carbon dioxide partial pressure is necessarily accompanied by a fall in oxygen partial pressure. Of the two, the fall in oxygen partial pressure is much more damaging to the organism because cells cannot survive without oxygen. 

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