## Abstract

The influence of gravity on the distributions of ventilation and blood flow, as demonstrated by the effects of posture, is discussed and explained on the theoretical basis of gravitational gradients of pressure within the lung tissue and blood vessels. Positive acceleration steepens these gradients so that measurement of regional ventilation and perfusion in subjects riding on a human centrifuge allows the theory to be extended. Ventilation and blood flow were measured using radioactive xenon and scanning the lung. In addition, pulmonary arterial pressures were monitored during acceleration, and a hydrostatic indifference plane was demonstrated lying 5 cm below the hilum. At this level, the pulmonary arterial pressure averaged 22$\cdot $2 cm water (16$\cdot $3 mmHg) systolic, 9$\cdot $3 cm water (6$\cdot $8 mmHg) diastolic, and had a mean pressure of 14$\cdot $5 cm water (10$\cdot $7 mmHg). It was unaffected by accelerations of up to three times normal gravity (3 g). A gradient in intraoesophageal pressure was demonstrated by the use of a double balloon lying in the lower oesophagus, and this gradient, believed to be related to the density of the lung, was found to be proportional to the applied acceleration. It averaged 0$\cdot $37 cm water/cm per g. At 3 g with the subjects seated erect, the base of the lung was better ventilated than the apex (in terms of ventilation per unit alveolar volume) in the ratio of 2$\cdot $6 to 1; the corresponding ratio at 1 g was 1$\cdot $8 to 1. Ventilation increased linearly with distance down the lung at all levels of acceleration investigated. At 3 g, the upper 14 cm of the vertical height of the lung were without perfusion, and perfusion increased linearly with distance down the remaining lung three times as fast as it did at 1 g. At 1 g, only the uppermost 4$\cdot $5 cm of the lung were without perfusion. The unperfused lung represented 45% of the total ventilated volume at 3 g and 13% of the total at 1 g. At 3 g, the pulmonary blood flow at the lung base was 3$\cdot $3 times the average value for the whole lung, whereas at 1 g this excess was only 1$\cdot $8 times. These results are discussed in terms of an interstitial pressure gradient in the lung of the order of 0$\cdot $3 cm water/cm per g along the gravitational axis, and an intravascular (hydrostatic) pressure gradient of 1 cm water/cm per g.