1196 PHYSIOLOGY 



of gas given off or taken up by the blood in its passage through the 

 lungs ? In a state of medium distension the 3000 c.c. of air contained 

 by the lungs have been estimated to occupy seven hundred million 

 alveoli, each of which has a diameter of 0*2 mm., so that the total 

 surface over which the blood is exposed to the alveolar air amounts to 

 90 square metres. This is a minimal figure, since no account is taken 

 in the calculation of the augmentation of surface caused by the fact 

 that the capillaries project into the lumen of the alveolus, and by 

 Hiifner the total surface exposed is calculated 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 and Zuntz have directly calcu- 

 lated 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 compared 

 with oxygen. They calculate 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 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 oxyhsemoglobin in mamma- 

 lian blood given on p. 1 184 we see that the amount of oxygen which can be 

 taken up by haemoglobin in the presence of the normal tension of carbon 



