3 6 4 TEXT-BOOK OF PHYSIOLOGY. 



these three gases is known as its partial pressure, and depends on the 

 percentage volume of the gas present. If atmospheric air contains 

 at standard pressure and temperature 79.15 volumes per cent, of 

 nitrogen, its partial pressure will' be ^ of 760, or 601.54 mm. Hg; 

 if the air contains 0.04 volume per cent, of carbon dioxid and 20.85 

 volumes per cent, of oxygen, the partial pressure of each will be 0.30 

 mm. Hg and 158.46 mm. Hg respectively. The absorption of each 

 gas is independent of all the rest, and is the same for nitrogen, for 

 example, as if it alone were present at a pressure of 601.54 mm. Hg. 

 Again, if water holding in solution a certain volume of a gas 

 carbon dioxid, for example be exposed to an atmosphere containing 

 but 0.04 volume per cent, of carbon dioxid, and having therefore a 

 pressure of but 0.3 mm. Hg, the gas will at once begin to leave the 

 water, and continue to do so until the pressure of the carbon dioxid 

 in the atmosphere balances the tension of the gas in the water, at which 

 moment the escape of the gas ceases. The tension of a gas in a liquid 

 is equal to that pressure in millimeters of mercury of the same gas in 

 the atmosphere which is required to keep it in solution. Pressure 

 and tension are therefore in this case convertible terms. What is 

 true for the carbon dioxid is true for any other gas that may be in 

 solution. It will be recalled that the blood yields up its gases when 

 subjected to the vacuum of the mercurial pump; that is, to a diminu- 

 tion or complete removal of the atmospheric pressure. From this 

 it might be inferred that the gases are merely held in solution by 

 pressure, and at once escape the moment they are exposed to a 

 space in which there is a very slight or a total absence of pressure. 

 It is therefore necessary to test this supposed condition of the gases 

 in the blood by subjecting the latter to gradually diminishing pres- 

 sures, with a view of determining in how far the evolution of the gases 

 follows the law of partial pressures. For convenience the conditions 

 of each gas will be considered separately. 



Oxygen. If blood is subjected to a succession of pressures pro- 

 gressively less than the standard, it is found that though oxygen is 

 evolved, its evolution is not in accordance with the law of partial 

 pressures ; that is, in proportion to the diminution of pressure. Within 

 wide limits e. g., from 760 to 238 mm. atmospheric pressure, to 

 which correspond oxygen pressures of 160 and 50 mm. respec- 

 tivelythere is but a slight increase in the amount of oxygen evolved; 

 and it is not until the pressure of the oxygen falls to about 40 to 30 

 mm. that it begins to be liberated in large amounts From this on, 

 the oxygen continues to be liberated with decreasing pressures, until 

 the zero point is reached, when all gaseous discharge ceases. Co- 

 mcidently the blood changes in color from a bright red to a deep bluish- 

 red. It is evident from the results of this procedure that the con- 

 ition of the oxygen in the blood is but to a slight extent one of 



