670 PHYSIOLOGY OF RESPIRATION. 



of the lungs. The composition of the alveolar air will be discussed 

 farther on, but we may assume at present that it contains 80 per 

 cent, of nitrogen, 15 per cent, of oxygen, and 5 per cent, of carbon 

 dioxid. In 100 c.c. of blood, therefore, the following amounts of 

 these gases should be held in solution: 



Nitrogen 100 X 0.013 X 0.80 = 1.04 c.c. 



Oxygen 100 X 0.0262 X 0.15 = 0.393 " 



Carbon dioxid 100 X 0.5283 X 0.05 = 2.64 " 



As will be seen from the analyses given above of the actual amounts 

 of these gases obtained from the blood, the nitrogen alone is present 

 in quantities corresponding to what would be expected if it is 

 held in simple physical solution. 



The Tension or Pressure of Gases in Solution or Combi- 

 nation. — When a gas is held in solution the equilibrium is de- 

 stroyed if the pressure of this gas in the surrounding medium or 

 atmosphere is changed. If this pressure is increased the liquid 

 takes up more of the gas, and an equilibrium is established at a 

 higher level. If the pressure is decreased the liquid gives off 

 some of the gas. The pressure of the gas in the surrounding at- 

 mosphere at which equilibrium is established measures the tension 

 of the gas in the liquid at that time. Thus, when a bowl of water is 

 exposed to the air the tension of the oxygen in solution is 152 mms. 

 Hg; that of the nitrogen is 608 mms. Hg. If the same water is 

 exposed to pure oxygen the tension of the oxygen in solution is 

 equal to 760 mms. Hg, while that of the nitrogen sinks to zero if 

 the gas that is given off from the water is removed. With com- 

 pounds such as oxyhemoglobin the tension under which the oxy- 

 gen is held is measured by the pressure of the gas in the sur- 

 rounding atmosphere at which the compound neither takes up 

 nor gives off oxygen. If, therefore, it is necessary to determine 

 the tension of any gas held in solution or in dissociable combi- 

 nation it is sufficient to determine the percentage of that gas in 

 the surrounding atmosphere and thus ascertain the partial pres- 

 sure that it exerts. If the atmosphere contains 5 per cent, of a 

 given gas the partial pressure exerted by it, leaving out of account 

 the aqueous tension, is equal to 38 mms. Hg. (760 X 0.05), and 

 this figure expresses the tension under which the gas is held in 

 solution or combination in a liquid exposed to such an atmos- 

 phere. As regards the tension of the gases in arterial and venous 

 blood, this procedure is, of course, not possible, since the blood 

 is surrounded not by an atmosphere whose composition can be 

 analyzed, but by the Uquids of the body, the lymph and cell juices. 

 To determine the tension of the gases in the blood it is necessary 



