, THE CHEMISTRY OF RESPIRATION 1107 



absorption of the gas on a curve of which the ordinates represent the amount 

 of gas dissolved and the abscissa the different pressures of the gas, we shall 

 find that the curve is a straight line. The relation between the amount 

 absorbed is not altered by the presence of other gases at the same time. 

 The pressure of the whole atmosphere is 760 mm. Since the atmosphere 

 consists roughly of four parts of nitrogen with one part of oxygen, the 

 atmospheric pressure is due as to one-fifth to the oxygen and as to four- 

 fifths to the nitrogen. If we shake up water at C. with the atmospheric 

 air at the ordinary pressure, 100 c.c. of water will absorb 4-89 c.c. X x of 

 oxygen, and of nitrogen 2-39 c.c. x i. We may therefore extend our 

 statement as to the solubility of gases in fluids and say that the amount of 

 gas dissolved in a fluid is proportional to the partial pressure of the gas. 



When water is shaken up with a gas until it will take up no more, i. e. 

 until it is saturated for that pressure, a state of equilibrium exists between 

 the gas dissolved in the fluid and the gas in contact with the fluid. In 

 this state of equilibrium the number of molecules of the gas entering the 

 fluid is exactly equal to the number of molecules of the gas leaving the 

 fluid. If we remove the liquid after saturation, say, at one atmosphere, to a 

 vessel where it is in contact with gas at a pressure of half an atmosphere, the 

 liquid will give off gas until the amount left in solution is diminished to one- 

 half. The gas dissolved in a liquid thus has a pressure or tension which 

 tends to make it escape from the liquid. The only way in which we can 

 " measure this tension is by rinding what pressure of gas is in exact equilibrium 

 with the liquid. Thus if we take some water containing carbon dioxide in 

 solution, divide it into two parts, and shake up one part with a gaseous 

 mixture containing 4 per cent, of carbon dioxide and the other part with a 

 mixture containing 5 per cent, of carbon dioxide, and find that the solution 

 loses gas to the former and takes up carbon dioxide from the latter, we 

 may conclude that the tension of carbon dioxide in the original fluid was 

 something between 4 and 5 per cent, of an atmosphere. It is by some such 

 means that the tensions of gases in the blood are measured, the instruments 

 for this purpose receiving the name of aerotonometers. 



The solvent power of water for gases is diminished if the water contains 

 other solid substances in solution. Blood plasma or blood corpuscles will 

 therefore have a smaller solvent power for gases than has pure water. It 

 has been shown by Bohr that the depression of solubility caused by the 

 'presence of proteins or salts in solution is the same for all gases. The absorp- 

 tion coefficient of blood plasma for gases is reduced to 97-5 per cent, of pure 

 water, and of blood to 92 per cent., that of the blood corpuscles being as low 

 as 81 per cent. We may thus reckon the absorption coefficient of blood 

 plasma, blood, and blood corpuscles for oxygen, nitrogen, and carbon 

 dioxide. 



From the following Table we see that 100 volumes of blood at 38 C. might 

 contain 2-2 c.c. of oxygen in solution if the blood had been exposed to 

 oxygen at a pressure of one atmosphere. The blood in the lungs is however 

 exposed to air which contains only about one-sixth of its volume of oxygen, 



