1072 PHYSIOLOGY 



right bronchus in man by introducing a catheter through the larynx and 

 trachea, so that the renewal of air in the right half of the lung is entirely 

 stopped for some time. A sample of air in the blocked lung can be taken 

 at any time by means of the catheter. The interchange of gases between 

 alveolar air and blood will go on until the tension of gases in the air is the 

 same as that coming to the blocked portion of lung. By this means the 

 tension of the oxygen in the venous blood was found to be 5-3 per cent. = 37 

 mm. Hg., and that of the carbon dioxide 6 per cent. = 46 mm. Hg. 

 The tensions in the alveolar air of man may be taken as follows : 



Oxygen . . . . . . 107 mm. Hg. 



Carbon dioxide. . . . . 40 



As the venous blood enters the lungs there is thus a difference of pressure 

 of 107 37 = 70 mm. Hg., which will tend to cause a flow of oxygen from 

 alveolar air to blood and a difference of 46 - 40 = 6 mm. Hg., tending 

 to cause a flow of carbon dioxide from blood to alveolar air. Is this difference 

 sufficient to account for the amount 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 capil- 

 laries 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 

 calculated 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 



