THE CHEMISTRY OF RESPIRATION 1123 



air amounts to 90 square metres. This is a minimal figure, since no account 

 in the calculation is taken of the augmentation of surface caused by the 

 fact that the capillaries project into the lumen of the alveolus ; and by 

 Htifner the total surface exposed is estimated at 140 square metres. The 

 former figure however amounts to about 1000 square feet and is equivalent 

 to the floor-space of a room 50 feet long by 20 feet wide. It is important 

 to realise that the blood passing through the pulmonary artery suddenly 

 spreads out into a layer which is not more than one blood corpuscle thick, 

 and is exposed to the air over this huge area, whence it is picked up again 

 and collected into the pulmonary veins. Such a means of facilitating rapid 

 interchange of gases between the blood and a given volume of air we cannot 

 possibly imitate artificially. The thickness of the tissue separating this 

 layer of air from the alveolar air is on the average -004 mm. Loewy a,nd 

 Zuntz have directly determined the velocity of diffusion of carbon dioxide 

 and nitrous oxide through the frog's lung, and have calculated therefrom 

 the rate at which oxygen would diffuse through a similar layer of tissue, 

 taking into account the much greater solubility of carbon dioxide as com- 

 pared with oxygen. They estimate that, under a constant difference of 

 pressure of 35 mm. Hg., 6-7 c.c. of oxygen would pass in a minute through 

 each square centimetre of the alveolar wall. Through the whole surface 

 of the lung this would amount to an absorption of 6083 c.c. oxygen. The 

 oxygen actually absorbed by a man at rest amounts to about 300 c.c. per 

 minute, so that the physical conditions allow an ample margin for any increase 

 in the consumption of oxygen ; in fact, a difference of pressure of a couple 

 of millimetres would suffice to cause a passage of the 250 c.c. per minute 

 which is required by the resting man. In the same way it is easy to account 

 for the passage of carbon dioxide in the reverse direction. This gas diffuses 

 through a wet membrane about twenty-five times as rapidly as oxygen, 

 so that a difference of pressure between the blood and the alveolar air 

 amounting to only -03 mm. Hg. would suffice to cause a passage outwards 

 of the 250 c.c. normally expired per minute. 



It is evident that the only limitation for the absorption of oxygen is 

 given by the power of the haemoglobin to combine with the oxygen which 

 passes through the alveolar wall into the blood plasma. 



If we look at the dissociation curve of the oxyhaemoglobin in mammalian 

 blood given on p. 1110, we see that the-amount of oxygen which can be taken 

 up by haemoglobin in the presence of the normal tension of carbon dioxide, 

 i. e. 40 mm. Hg., begins to diminish very rapidly when the pressure of the 

 oxygen falls below 50 mm. Hg. Thus at 40 mm. oxygen pressure and a 

 carbon dioxide tension of 40 mm., oxy haemoglobin is about 65 per cent, 

 saturated, and at 30 mm. it is only 50 per cent, saturated. Under normal 

 circumstances the blood leaves the lungs over 90 per cent, saturated with 

 oxygen. If the saturation falls to 60 per cent, we should expect to obtain 

 evidence of failure of oxygen supply to the tissues. According to Loew}'' 

 the oxygen tension in the alveoli can sink to between 30 and 35 mm. Hg. 

 before any signs of oxygen lack make their appearance. These results were 



