522 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



same relations of partial pressure are observed in connection with CO 2 , except 

 that the air in the alveoli is incessantly acquiring this gas from the blood, causing 

 the per cent, composition of CO 2 to be much in excess of that found in the 

 atmosphere. The partial pressure of CO 2 in the alveolar air is about 38.00 

 millimeters, while in inspired air it is only 0.30 millimeter ; hence CO 2 must 

 diffuse from the alveoli outward. 



There are, therefore, three important factors concerned in the admixture 

 and purification of the air in the lungs: (1) The tidal movements caused 

 by inspiration and expiration, which movements by the mere force of air-cur- 

 rents cause a partial mixture of the air ; (2) the smaller wave-movements (car- 

 dio-pneumatic) produced by the heart-beats, and similar in effect to, but much 

 less effective than, the first ; (3) the diffusion of O and CO 2 , depending upon dif- 

 ferences in their partial pressures in the various parts of the respiratory tract. 



The Forces Concerned in the Interchange of O and CO 2 between 

 the Alveoli and the Blood. The gases in the lungs are in the form of 

 a mechanical mixture, while in the blood they are in solution or in chemical 

 combination ; hence we now have to deal with conditions quite different, involv- 

 ing the consideration of the relations of gases to liquids a relationship of 

 twofold nature, inasmuch as the gas may be found not only in solution, but 

 in chemical association. 



When an atmosphere consisting of O, CO 2 , and N is brought in contact 

 with water, each gas is absorbed independently not only of the others, but 

 of the nature and quantity of all other gases which may happen to be in 

 solution. The quantity of each gas dissolved depends upon its relative solu- 

 bility as well as upon the temperature and the barometric pressure. The 

 coefficient of absorption of any fluid is the quantity of gas dissolved at a given 

 temperature and pressure, and is in inverse relation to temperature and in direct 

 relation to pressure. The following absorption-coefficients of water for O, CO 2 , 

 and N at 760 millimeters of Hg have been obtained by Winkler : l 



Temperature. O. CO 2 . N. 



0.04890 1.7967 0.02348 



15 0.03415 1.0020 0.01682 



40 , 0.02306 . . 0.01183 



Thus, at C and 760 millimeters pressure each volume of water absorbs 0.0489 

 volume of O; at 15, 0.03415 volume; and at 40, 0.02306 volume. The 

 absorption-coefficient falls, it will be observed, with the increase of temperature. 

 Comparing the solubilities of the three gases, it will be seen that at the same 

 temperature and pressure a considerably larger quantity of CO 2 is absorbed 

 than of O nearly four times more whereas the quantity of N absorbed is 

 less than one-half as much as that of O. 



The quantity of a gas absorbed by a given liquid at a given temperature is 

 proportionate to its coefficient of solubility and to the pressure, and is the same 

 whether the gas exist free or as a constituent of a complex atmosphere, pro- 

 1 Zeitschrift fur physikalische Chemie, 1892, vol. 9, p. 173. 



