404 TEXT-BOOK OF PHYSIOLOGY. 



The changes produced in the blood by respiration, both external and in- 

 ternal, become apparent from a comparison of these analyses. The venous 

 blood while passing through the lungs gains from eight to eleven volumes 

 per cent, of oxygen and loses five volumes per cent, of carbon dioxid. The 

 arterial blood while passing through the tissues loses oxygen and gains 

 carbon dioxid in corresponding amounts. The volume of nitrogen is not 

 appreciably changed. 



The Relation of the Gases in the Blood. The mechanism by which 

 the gases become associated with the blood at the moment of their entrance 

 into it, and again become dissociated just prior to their exit from it, as well 

 as their relation to the blood while in transit, will be more readily understood 

 after reference to a few elementary facts relative to the absorption of gases 

 by liquids in general and the conditions of temperature and pressure by 

 which it is influenced. 



It is well known that liquids will absorb or dissolve at any constant 

 pressure unequal volumes of different gases in accordance with their solubili- 

 ties and with variations in temperature. Water, for example, will absorb, 

 in accordance with the foregoing conditions, oxygen, carbon dioxid, and 

 nitrogen, as well as many other gases. The volume of any gas thus absorbed 

 is known as the coefficient of absorption, and may be defined as the number 

 of cubic centimeters of the gas which one cubic centimeter of water will 

 absorb when the gas, in contact with the water, stands under a pressure of 

 one atmosphere or 760 mm. of mercury and at a temperature of o C. The 

 volume absorbed, however, varies inversely as the temperature. Thus at 

 o C. the volume of oxygen absorbed by one volume of water is 0.0489 c.c.; 

 of carbon dioxid 1.713 c.c.; of nitrogen 0.0234 c.c. With a rise of tempera- 

 ture, the pressure remaining constant, the absorptive power of water for each 

 of these gases diminishes. Thus at 15 C., the volumes of oxygen, carbon 

 dioxid and nitrogen absorbed are 0.0310 c.c., 1.0025 c.c. and 0.0168 c.c. 

 respectively. Though the volume of the gas absorbed diminishes as the 

 temperature rises, it is independent of pressure, for no matter to what extent 

 the pressure may vary the volume absorbed is always the same. (Law of 

 Henry.) 



If the weight of the gas absorbed be considered rather than the volume 

 (that is the product of the volume and the density or the number of molecules 

 in the volume), then the temperature remaining constant, the weight of the 

 volume absorbed increases and decreases proportionately as the pressure 

 rises and falls. Thus at a pressure of 760 mm. of mercury and at a tempera- 

 ture of o C., the volume of oxygen absorbed by one volume of water is 

 0.0489 c.c.; at 1520 mm. of mercury, the same volume is absorbed but its 

 weight is doubled. If the pressure falls below 760 mm. of mercury the 

 same volume is absorbed but its weight is diminished. (Law of Dalton.) 

 Because of the foregoing facts, it is necessary in all gaseous determina- 

 tions to reduce for purposes of comparison the obtained volumes to standard 

 temperature (o C.) and pressure (760 mm. of mercury). 



When the liquid is once saturated with a gas at a constant pressure 

 and temperature, there is coincidently with the entrance of the gas into the 

 liquid, an equivalent exit of the gas from it, though the volume retained in the 

 liquid remains constant. The reason for this fact is, that under the condi- 



