254 ESSENTIALS OF PHYSIOLOGY. 



reading of the manometer remains unchanged ; but if it is not fully saturated, 

 it takes up some oxygen, and the level of the clove oil rises in that limb of the 

 manometer. The manometer is replaced in the water bath till the reading is 

 constant. If the difference in the level of oil in the two limbs is 20 mm., the 

 blood must have taken up 20 mm. x 3 '0 = 60 c.mm. of oxygen ; and if its total 

 oxygen capacity is ascertained, the degree to which it was previously 

 saturated can be calculated. 



Thus, if the oxygen capacity of 1 c.c. blood was O18 c.c., and it took up in 

 the foregoing experiment 0'06 c.c. oxygen, it must have previously contained 



0-12 c.c., and its percentage saturation was 100 x : =66 per cent. 



0'18 



THE CONDITION IN WHICH THE GASES AEE HELD 

 IN THE BLOOD. 



Theoretically, the gases in the blood might be either simply 

 dissolved in it or chemically combined with some constituent of the 

 blood. In order to decide which of these possibilities is the correct one, 

 it is necessary to consider first the conditions which modify the amount 

 of any gas present in a fluid such as water. On exposing water to the 

 air, a certain amount of oxygen and nitrogen is dissolved in it. Con- 

 fining our attention to oxygen, the amount dissolved in a known 

 volume of water depends upon (1) the pressure exerted by the oxygen 

 upon the surface of the water, (2) the temperature of the water, and 

 (3) the capacity of water to dissolve oxygen. The capacity of water to 

 dissolve any gas is constant for the same gas, provided the pressure of 

 the gas and the temperature of the water remain unchanged. Some 

 gases, such as carbonic acid, are very soluble in water, others such 

 as oxygen and nitrogen are only slightly soluble. The coefficient of 

 solubility of a gas is defined as the amount of that gas which is dissolved 

 at a given temperature in 1 c.c. of the liquid, when the pressure of the 

 gas on the liquid is 760 mm. Hg. 



Since the capacity of water to dissolve any gas is constant, the 

 amount of the gas dissolved at a given temperature (which will be 

 assumed to be constant) varies directly with the pressure of the gas 

 on the surface of the liquid. When water is exposed to pure oxygen at 

 atmospheric pressure, the pressure of oxygen on the water is 760 mm. 

 Hg ; if it is exposed to atmospheric air, both the oxygen and nitrogen 

 exert a pressure, which is proportional to their percentage in the air. 

 In such a mixture of gases, the proportion of the total pressure exerted 

 by the oxygen on the walls of the vessel containing it, or on the surface 

 of a fluid, is called the partial pressure or tension of oxygen, and it is 

 measured by determining (1) the percentage of oxygen in the gaseous 

 mixture, and (2) the total pressure of the mixture. The partial pressure 



