496 RESPIRATION 



conditions, however, when the oxygen tension in the alveolar air is very low, the 

 lining cells may acquire a secretory power. 



The Interchange of the Gases Between the Blood and the Tissues. 

 The Absorption of Gases by Liquids. — If a gas is brought into contact 

 with water, a certain number of its molecules enter the latter and be- 

 come dissolved, the amount absorbed being dependent upon the nature 

 of the gas, the temperature and the pressure under which it exists. 

 Provided that these factors remain unchanged, an equilibrium is 

 eventually established, during which the water retains a definite 

 quantity of the gas. But this condition of saturation does not signify 

 that the gaseous molecules remain absolutely stationary, because 

 in accordance with the kinetic theory of matter, it is commonly believed 

 that the molecular constituents of any entity are in constant motion. 

 In many cases, they pursue a definite course and collide with one another 

 so that they are deflected from their paths. It should be emphasized, 

 however, that molecular motion does not consist in incessant collisions, 

 because the distances which molecules actually traverse without 

 striking one another are relatively great. Furthermore, it cannot 

 be denied that these mechanical interferences seriously impede the 

 general progress of the molecules. But, while some of them may be 

 momentarily brought to a standstill, others are forced onward with 

 a certain momentum which makes them exceed their average velocity. 

 In the outer layers of the water, large numbers of these molecules 

 strike the walls of the receptacle and rebound, while elsewhere many 

 of them escape into the overlying mass of gas only to reenter the water 

 later on. In the state of saturation just as many molecules leave the 

 water as enter it. 



If the preceding experiment is now repeated with a mixture of 

 gases, it will be found that practically the same interchange takes place, 

 the absorption of each constituent being proportional to the pressure 

 exerted by it, i.e., to its partial pressure. Thus, if the pressure of one 

 of the gases is greater in the atmosphere than in the water, it will 

 pass into the water, and vice versa. Moreover, it is to be noted that 

 the flow of this particular gas is independent of that of any other of 

 the constituents of this mixture and may be increased or decreased by 

 simply altering its partial pressure in one of these regions.^ 



The absorption behaves toward changes in temperature in an 

 inverse manner. Furthermore, inasmuch as these changes are not 

 proportional to one another, it becomes necessary to determine the 

 absorption for every degree of change in temperature. Thus, it has 

 been found that the volume of oxygen absorbed by one volume of 

 water at 0° C. amounts to 0.0489 c.c, that of carbon dioxid to 1.713 

 c.c, and that of nitrogen to 0.0234 c.c. At 15° C. the volume of these 

 gases absorbed equals 0.0310 c.c, 1.0025 c.c. and 0.0168 c.c, 

 respectively. As a means for comparison we have the so-called coeffi- 



iLaw of Henry, Philos. Transact., 1803. 



