THE RESPIRATORY SYSTEM. 255 



of oxygen in atmospheric air is, therefore, 20 per cent, of 760 mm. Hg, 

 namely 152 mm. Hg. 



The Tension of G-as in a Fluid. When water is exposed to a 

 gaseous mixture containing oxygen, the molecules of oxygen tend to 

 pass into the liquid and be dissolved ; at the same time, the molecules 

 of oxygen already in solution tend to pass from the water into the 

 gaseous mixture. This tendency of the molecules of oxygen to leave 

 the fluid is called the tension of oxygen in the fluid. When these two 

 opposing processes are equal, the gaseous mixture and the fluid are in 

 equilibrium, and the amount of oxygen dissolved in the fluid remains 

 constant. In these circumstances, the tension of oxygen in the fluid is 

 equal to the partial pressure of the oxygen in the gaseous mixture to 

 which the fluid is exposed. 



The tension of oxygen in a fluid cannot be measured directly, but 

 is determined by placing samples of the fluid in a series of closed vessels, 

 containing oxygen at various known partial pressures, and finding in 

 which vessel the fluid neither gives off" nor takes up oxygen. The tension 

 of oxygen in the fluid in this vessel is equal to the partial pressure of 

 the oxygen in the gaseous mixture to which the fluid is exposed ; and 

 if the amount of oxygen in the gaseous mixture is 5 per cent, and the 

 total pressure of the gaseous mixture is 760 mm. Hg, the partial 

 pressure of oxygen is 38 mm. Hg; this is equal to the tension of 

 oxygen in the fluid. The forms of apparatus which have been devised 

 for measuring the tension of a gas in a fluid are called aerotonometers. 



The Tension of Oxygen in Blood. In the case of water, oxygen is 

 held in simple solution, and, if the temperature is constant, the amount of 

 oxygen in the water varies directly with the partial pressure of oxygen 

 in the air to which it is exposed. If the pressure of oxygen is doubled, 

 twice as much is dissolved in the water. When similar experiments 

 are carried out with blood, the amount of oxygen present in the blood, 

 as determined by exposing it to a vacuum in a gas pump or by 

 Barcroft's apparatus, is not proportional to the partial pressure of 

 oxygen in the gaseous mixture to which the blood is exposed. For 

 example, when the blood is in equilibrium with air containing oxygen 

 at a partial pressure of 100 mm. Hg, it will contain about 18 volumes 

 of oxygen per 100 volumes of blood. If the partial pressure of oxygen 

 is reduced to 50 mm. Hg, the blood will contain 14 '5 volumes of 

 oxygen per 100 volumes of blood. 



Since the amount of oxygen in blood is not directly proportional to 

 the partial pressure of oxygen in the air to which the blood is exposed, 

 it is evident that oxygen is not held in^ blood simply in solution. 

 Further, a given volume of blood can take up many times as much 



