300 



HUMAN PHYSIOLOGY 





FIG. 140. Shows the Difference 

 in the Gases of Arterial and 

 Venous Blood. 



from the lungs (Fig. 137). 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. 



This 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. 



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 decreased the gas 

 will tend to come off from the fluid, if it is increased it will 

 be taken up by the fluid. This is well seen in the case of 

 soda water. But the same law applies to such chemical 

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

 air i.e. under a low pressure of carbon dioxide the gas is 

 given off; but if lime is in an atmosphere of C0 2 the gas does 

 not come off, but is taken up and the carbonate is formed. 



It will thus be seen that for every temperature there is a 

 certain pressure of the gas at which the solution or chemical 

 combination will neither give off nor take up more of the gas. 



