RESPIRATION 315 



of the blood in passing through the lungs (p. 215). Analyses 

 show that the blood going to the lungs is poorer in oxygen 

 and richer in carbon dioxide than the blood coming from the 

 lungs (fig. 144). Oxygen is taken by the blood from the air, 

 carbon dioxide is given by the blood to the air. 



How is this effected ? The extensive capillary network in the 

 walls of the air vesicles, 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 



1st. The endothelium lining the capillaries. 



2nd. The flattened cells lining the air vesicles. Through 

 these cells the interchange of gases must take place. 



The interchange might take place in two different ways 



1st. By simple mechanical diffusion. 



"2nd. By some special action of the cells. 



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

 is then 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. 



To determine if the process can be accounted for by diffusion 

 it is therefore necessary to know 



1st. The partial pressure or tension of the gases in the 

 blood going to and coming from the lungs. 



2nd. The partial pressure of the gases in the air in the air 

 vesicles. 



I. The Partial Pressure of the Gases in the Blood. Whether 

 a gas is simply dissolved or whether it be held in loose 

 chemical combination, the amount held will depend upon 

 the temperature of the fluid and upon the pressure of the 

 gas over the fluid. If the temperature is raised the fluid 

 will hold less of the gas in solution, and any chemical 

 combination will tend to split, as is seen when carbonate of 

 lime is heated and the carbon dioxide is driven off. 



If the pressure of any gas over a fluid be increased it will 

 be taken up by the fluid, if it is decreased the gas will tend 

 to come off from the fluid, as occurs when a bottle of soda 

 water is opened. But the same law applies to such chemical 

 compounds as carbonate of lime. If it is heated in ordinary 



