THE RESPIRATION 301 



This may be determined by exposing the material in a series 

 of chambers to air containing different proportions of the gas, 

 and ascertaining by analyses of the air whether the gas has 

 been given off or taken up or has remained unaltered. 



We know that the pressure of gas in an atmosphere 

 depends upon the proportion present. Suppose an atmos- 

 phere contains 20 per cent, of oxygen, then the pressure, or 

 partial pressure, of the oxygen is got by multiplying the 

 percentage amount of the gas by 760 i.e. a whole atmos- 

 phere's pressure and dividing by 100 



20x760 



100 



= 152 mm. 



By ascertaining at what partial pressures of a gas, that gas 

 is neither given off nor taken up by a fluid, we may determine 

 the " tension " of the gas in the fluid whether it be in solution 

 or in loose chemical combination. 



The presence of a moist membrane between the fluid and 

 the air makes no difference to these interchanges. 



These facts may now be applied to the question as to 

 whether the gaseous interchange between the air in the 

 air vesicles and the blood is due simply to diffusion. 



The questions to be decided are 



1st. What is the partial pressure of these gases in the blood 

 going to and coming from the lungs ? 



2nd. What is their partial pressure in the air vesicles ? 



Partial pressure of Oxygen and Carbon Dioxide in the 

 Blood. The percentage amount of these gases tells us nothing 

 of their partial pressure since they are in chemical combina- 

 tion and not in solution. Experiments made upon the 

 subject have given very varying results, but the most recent 

 and perfect series of observations go to show that the partial 

 pressure of oxygen in blood coming from the lungs i.e. in 

 arterial blood is about 100 mm., while the C0 2 pressure in 

 the same blood varies enormously, but on an average is about 

 20 mm. of Hg. 



In venous blood the oxygen pressure must be much lower 

 some have put it as low as 21 mm. Hg ; while the pressure 

 of C0 2 is higher, something over 40 mm. Hg. 



Partial pressure of Gases in the Air Vesicle. The air in 



