RESPIRATION 129 



movements. In addition to the mechanical factors, the physical 

 process of diffusion is of great importance. The rapidity of 

 diffusion depends, among other things, upon the differences of 

 the partial pressures of the gases in various regions. If the 

 total atmospheric pressure is 760 mm. Hg, and 62 forms i 

 of the gaseous constituents of the air, then it will exert a press- 

 ure of its own equal to \ of 760, or 152 mm. Hg; and carbon 

 dioxide, forming 0.04 volume in 100 of the air, will exert a 



04 

 pressure of y n ~ of 760, or about 0.30 mm. Hg. It has been 



estimated that the partial pressures of 62 and CO2 in alveolar 

 air are equal to 100 and 35 mm. Hg respectively. The differ- 

 ences in partial pressures, therefore, will cause 62 to diffuse 

 toward the alveoli, and CO 2 from the alveoli to the outside 

 air. 



The gases in the blood are not only in solution, but also in 

 weak chemical combination, so that diffusion from the alveoli 

 into the blood and vice versa is somewhat complicated. The 

 amount of a gas that is absorbed when brought in contact 

 with water depends upon the relative solubility, the tempera- 

 ture, and the barometric pressure. Each, of a mixture of 

 gases, is absorbed independently of the others. The relative 

 solubility is expressed by the coefficient of absorption of the 

 fluid, which is experimentally determined, and is found to be 

 in inverse ratio to the temperature and in direct relation to 

 the pressure. The absorption coefficient of water for C>2, as 

 an example, at zero Centigrade and 760 mm. pressure, is equal 

 to 0.0489. This means that under the given conditions of 

 temperature and pressure 1 volume of water will take up 

 0.0489 volume of O 2 . Since, however, the O 2 forms but i of 

 the quantity of the air, water will absorb from the atmosphere 

 only -1- of 0.0489 volume, which is equal to 0.009+, or nearly 

 0.01 volume. As the partial pressure of O 2 is raised or lowered, 

 O 2 will leave or enter the water, so that the gas in solution is 

 said to be under tension. The absorption coefficient of blood 

 for O 2 is about that of water, but at the bodily temperature 

 is decreased to less than . Every volume of blood should, 

 therefore, contain -J- volume of oxygen in 100; but experiment 

 shows that there is much more present. Upon subjecting blood 

 to a vacuum, O 2 is given off according to the laws of partial 

 9 



