ABNORMALrTY OF LUNGS OR THORACIC CAGE

The lungs normally are ventilated with enough air to dilute the carbon dioxide produced by metabolism, so that the partial pressure of carbon dioxide (Pco,) in the alveoli is kept remarkably close to 40 mm. Hg. It is the alveolar ventilation and not the total amount of air going in or out of the mouth (or 'minute-volume') that determines the Pco2 because a varying proportion of the total ventilation goes to the dead space and takes no part in gas exchange. Alveolar ventilation will be inadequate when total ventilation is reduced but will also be inadequate when there is normal or even increased total ventilation if dead space ventilation is increased. Such an increase in dead space ventilation will occur if breathing is rapid and shallow and mainly into the upper anatomical dead space, or if there is ventilation of alveoli poorly supplied with blood. Little gas exchange takes place in poorly perfused alveoli, so that these alveoli can be regarded as additional 'functional' dead space. In all cases inadequacy of alveolar ventilation will result in a rise in alveolar Pco2; and because of the high diffusibility of carbon dioxide the arterial and alveolar Pco2 are virtually identical so that the arterial Pco2 will also be increased.