Historical introduction

Abstract

It could be argued that, of all the organs in the body, the lung is the most vulnerable to gravity, increased acceleration, and weightlessness. One of the reasons is that the blood in the pulmonary capillaries is separated from the air in the alveoli by an extremely thin blood-gas barrier over a vertical height of some 30 em. Because blood has a much greater density than air, substantial pressure differences across the capillary walls therefore exist at different levels in the lung, and consequently there is a striking topographical inequality of blood flow. Another reason why gravity affects the lung is that the lung is very distensible and therefore it distorts under its own weight. Consequently, there are regional differences of alveolar expansion, mechanical stresses, intrapleural pressures, and ventilation. Finally, the fact that ventilation and blood flow do not match each other at different levels in the upright lung means that there are topographical differences of pulmonary gas exchange, and these can have important effects on overall gas exchange.