Abstract
Alveolar‐arterial oxygen tension difference and physiological dead space was measured in 80 normal subjects in resting conditions. A significant rise in both parameters with rising age was demonstrated. The composition of the alveolar‐arterial oxygen difference was studied by two independent methods of shunt measurement: Determination of alveolar‐arterial oxygen tension difference during oxygen breathing was used for measurement of the total venoarterial shunt, whereas analysis of the concentration of tritium (hydrogen) in arterial blood after intravenous injection of this gas in physical solution was employed as a measure of right‐to‐left shunt. It is concluded, that of the normal alveolar‐arterial oxygen difference only a small fraction is caused by right‐to‐left “anatomical” shunt in the lungs and less than half is caused by bronchial and thebesian vein drainage to the left side of the heart. The remaining part, which increases with age, must be due to inequalities in the distribution of ventilation/perfusion ratio in the lungs.

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