CHANGES IN AIR AND BLOOD IN RESPIRATION. 669 



liquid and become dissolved. Some of these dissolved molecules 

 escape from the water from time to time, again becoming 

 gaseous. It is evident, however, that if a liquid, water, is brought 

 into contact with a gas under definite pressure, — that is, containing 

 a definite number of molecules to a unit volume, — an equilibrium 

 wall be established. As many molecules will penetrate the liquid 

 in a given time as escape from it, and the liquid will hold a definite 

 number of the gas molecules in solution; it will be saturated for 

 that pressure of gas. If the pressure of the gas is increased, how- 

 ever, an equilibrium will be established at a higher level and more 

 molecules of gas will be dissolved in the liquid. Experiments have 

 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 in- 

 creases and decreases proportionally with the rise and fall of the 

 gas pressure. This is the law of Henry. 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 according to its "partial pressure," 

 —that is, under a pressure of ^ of an atmosphere (152 mms. Hg). 

 The water will contain only -g- 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 hquid 

 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; CO., 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 CO 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 



* As given by Bohr, the absorption coefficients of these three gases 

 at 40° C. are as follows: Oxygen, 0.0231; nitrogen, 0.0118; carbon dioxid, 

 0.530. 



