

RESPIRATION 275 



To sum up, the differences of pressure of carbon dioxide and of 

 oxygen found by the micro tonometer in the blood and at the bifurca- 

 tion of the trachea support the view of diffusion (Fig. 145). It is 

 estimated also that the process of diffusion can carry oxygen in 

 amounts sufficient for hard work through the pulmonary endothelium. 

 The rate of diffusion in the lung is probably accelerated by the 

 chemical affinity of haemoglobin with oxygen : we need not therefore 

 ascribe a secreting power to the pulmonary endothelium. 



But it is still a question whether diffusion can cover the oxygen 

 needs at great altitudes e.g., 24,000 feet, to which a few climbers 

 have attained. & 



Carbon dioxide is about twenty-five times as soluble in water as 

 oxygen is, hence it passes through the alveolar wall far more easily 

 than oxygen with a given difference of partial pressure. A compara- 

 tively slight increase in breathing, by ventilating the lungs, enormously 

 increases the small difference in diffusion pressure on which the passage 

 of C0 2 depends, but only produces a slight propoitional increase in 

 the diffusion pressure which drives oxygen inwards. Hence, under 

 certain conditions, greyness of the face, faintness, and danger from 

 heart failure, the signs of oxygen -want, may arise when hyperpncea 

 and venous congestion, the signs of C0 2 excess, are absent. 



