600 PHYSIOLOGY OF RESPIRATION. 



shown, in accordance with this mechanical conception, that the 

 amount of a given gas dissolved by a given liquid varies, the temper- 

 ature remaining the same, directly with the pressure, that is, 

 it increases and decreases proportionally with the rise and fall of 

 the gas pressure. This is the law of Henry-Dai ton. On the other 

 hand, the amount of gas dissolved by a liquid varies inversely with 

 the temperature. It follows, also, from the same mechanical views 

 that in a mixture of gases each gas is dissolved in proportion 

 to the pressure that it exerts, and not in proportion to the pressure 

 of the mixture. Air consists, in round numbers, of 4 parts of N and 

 1 part of O. Consequently, when a volume of water is exposed to 

 the air the oxygen is dissolved a according to its " partial pressure," 

 that is, under a pressure of -J- of an atmosphere (152 mms. Hg). 

 The water will contain only ^ as much oxygen as it would if exposed 

 to a full atmosphere of oxygen that is, to pure oxygen. And, on 

 the other hand, if water has been saturated with oxygen at one 

 atmosphere (760 mms.) of pressure and is then exposed to air, 

 four-fifths of the dissolved oxygen will be given off, since the pressure 

 of the surrounding oxygen has been diminished that much. Ab- 

 sorption coefficient. By this term is meant the number that ex- 

 presses the proportion of gas dissolved in a unit volume of the liquid 

 under one atmosphere of pressure. The absorption coefficient will 

 vary, of course, with the temperature. The gases that interest us 

 in this connection are oxygen, nitrogen, and carbon dioxid. The 

 absorption coefficients of these gases for the blood at the tempera- 

 ture of the body are as follows : 0,0.0262; N, 0.0130; C0 2 , 0.5283. 

 That is, 1 c.c. of blood at body temperature dissolves 0.0262 of 

 1 c.c. of oxygen if exposed to an atmosphere of pure oxygen, and 

 so on. The solubility of the C0 2 is therefore twenty times as great 

 as that of oxygen. Accepting these figures, we may calculate how 

 much of these three gases can be held in the arterial blood in physical 

 solution, provided we know the pressure of the gases in the alveoli 

 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. 



