RESPIRATION 539 



the gas extracted from blood, but the amount of each gas per 

 100 parts of blood : — 



Amount in 100 /irtr^*' of Blood [Human). 



Venous . 

 Arterial . 



Oxygen is taken by tJte blood from the air, carbon dioxide 

 is given by the blood to the air. 



III. The Causes of the Respiratory Exchange- — How is this 

 effected ? The extensive capillary network in the walls of the 

 air vesicles in man, if spread out in a continuous sheet, would 

 present a surface of about 75 square metres. Between the blood 

 in the capillaries and the air in the air vesicles are two layers of 

 living cells — 



Ist. The endothelium lining the capillaries. 



^nd. The flattened cells lining the air vesicles. 



Through these cells the interchange of gases must take 

 place. 



The interchange might take place in either or both of two 

 ways — 



\st. By simple diffusion. 



2nd. By some special action of the cells. 



If the process follows strictly the laws of diffusion, it is 

 unnecessary to invoke the activity of the cells as playing 

 a part. But, if the gaseous interchange does not strictly 

 follow these laws, we must conclude that the cells do play 

 a part. 



Diffusion tal:es place from the point of higher partial 

 pressure to the point of lower pressure till equilibrium is 

 established. 



To determine if the process can be accounted for by 

 diffusion, it is therefore necessary to know — 



1. The partial pressure of the gases in the air in the vesicles 

 of the lungs. 



2. The partial pressure or tension of the gases in the blood 

 going to and coming from the lungs. 



1. Partial Pressure of Gases in the Air Vesicles. — The method 



