304 RESPIRATION 



shaken vigorously for about one minute. Make a preliminary reading of the liberated 

 gas and repeat the evacuation until the readings check. For final reading draw the 

 fluids into the chamberd below the lower stopcock, using care not to trap any gas, and 

 run the mercury around the side tube c, level the mercury bulb against the mercury 

 meniscus in the graduated limb as in Fig. 2376 and read. The gas of the 2 c.c. sample 

 of blood consists of the oxygen bound by the hemoglobin and of the air in solution at the 

 temperature and barometer of the analysis. The corrections for dissolved air including 

 nitrogen are readily made from tables of solubility. (See Van Slyke in Journal of Bio- 

 logical Chemistry, Vol. 33, p. 126; also Vol. 49, p. i.) 



The large quantity of oxygen found in arterial and in venous blood is 

 the more striking when the facts of absorption of gases by liquids are 

 reviewed. A liquid such as water will, when exposed to a gas, take up the 

 gas by absorption according to definite physical laws. Under constant 

 temperature the amount of gas absorbed, oxygen for example, varies 

 directly as the pressure of the gas, or partial pressure if the gas is in a mix- 

 ture. The oxygen absorbed by water from pure air is in direct pro- 

 portion to the partial pressure of oxygen in the air, which is 159 mm. 

 mercury. 



The amount of gas absorbed by i c.c. of water under standard pressure 

 (one atmosphere at o c C.) is termed the absorption coefficient. The 

 absorption of oxygen by water for one atmosphere of oxygen is .048 c.c. 

 For blood plasma the coefficient is a little less than for water. The 

 amount of oxygen in simple solution in 100 c.c. of blood at the partial 

 pressure of oxygen in alveolar air is therefore only about 0.32 c.c. The 

 actual amount of oxygen in solution in any particular specimen of plasma 

 is rather less and is determined by the oxygen tension. 



The saturation of oxygen in arterial whole blood is measured by the 

 method of subjecting the blood to an atmosphere in which the oxygen 

 tension is accurately known. The instrument is called a tonometer. 

 The procedure depends upon the fact that a thin film of blood exposed to 

 mixtures of gases in air gives up gases to or absorbs them from the air 

 until an equilibrium is established. When a sample of whole blood is 

 exposed to atmospheric air in a tonometer the blood becomes fully satur- 

 ated with oxygen and the volume it contains is spoken of as the capacity. 

 When such blood has its gases extracted by the Van Slyke apparatus and 

 the results computed to standard, the volumes per cent, contained are 

 such as indicated in the table, page 302. When alveolar airs are used 

 the degree of saturation is of course proportionately less than the satu- 

 ration against pure air because of the diminished per cent, of alveolar 

 oxygen. The volumes per cent, of oxygen absorbed is found to vary also 

 according to the per cent, of oxygen in the sample of air and the content 

 of hemoglobin in the blood. 



By means of the tonometer observers have measured the tension of 



