4 i4 TEXT-BOOK OF PHYSIOLOGY 



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- 

 tions, the volume of the gas dissolved by the liquid though small in amount 

 exerts a pressure in the opposite direction equivalent to the pressure acting 

 upon the liquid. If one cubic centimeter of water absorbs 0.0489 c.c. of 

 oxygen at 760 mm. and oC., this volume will exert a pressure opposite 

 in direction of 760 mm. of mercury. For this reason the entrance and exit of 

 the gas are equal and opposite. 



If water be exposed to atmospheric air consisting of oxygen, carbon 

 dioxid, and nitrogen in the ordinary proportions, at any given temperature 

 and pressure, the water will absorb unequal volumes of each of the three 

 gases. The pressure under which each gas is absorbed is a part only, 

 however, of the total atmospheric pressure at the time. The pressure 

 exerted by any one of these 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 percent, 

 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 

 pressure of the gas in the water, at which moment the escape of the gas 

 ceases. The pressure of a gas in a liquid is equal to that pressure in milli- 

 meters of mercury of the same gas in the atmosphere which is required to keep 

 it in solution. What is true for the carbon dioxid is true for any other gas 

 that may be in solution. If a liquid has a greater density than water, as 

 from the presence of inorganic salts, the absorptive power under standard 

 conditions of temperature and pressure becomes less. It is for this reason 

 that blood-plasma contains less oxygen, nitrogen, and carbon dioxid than 

 water. 



It will be recalled that the blood yields up its gases when subjected to the 

 vacuum of the mercurial pump; that is, to a diminution 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. In other words, that the absorption of gases by the blood and 

 their escape from it follow the law of pressure as stated in foregoing para- 

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

 in the blood by subjecting the latter to gradually diminishing pressures, 

 with a view of determining in how far the discharge of the gases follows the 

 law of falling pressures. For convenience the conditions of each gas will 

 be considered separately. 



