GASEOUS EXCHANGE IN LUNGS. 217 



is due to the oxidation of substances which do not appear in the 

 expired gases. This subject is fully described under the head of 

 respiratory quotient in the chapter on metabolism (p. 91). 



These observations do not enable us to decide whether the laws 

 of diffusion of gases apply to the gaseous exchange of the lungs. 

 To do this we must know the actual pressures of the respiratory 

 gases in the venous blood coming to the lungs and in the air of 

 the alveoli. Many types of experiments have been devised to ob- 

 tain these values, and although the actual figures vary somewhat 

 in the hands of different investigators, the results as a whole in- 

 dicate that tha gaseous. e_xcJbau.ge_Df-h.e_.lungs is dependent solely 

 on the presence of a higher pressure of oxygen and a lower pres- 

 sure of carbon dioxide in the alveolar air than are present in. the 

 blood coming to the lungs. The ability of haemoglobin to take up 

 oyygen with great readiness at oxygen pressures which exceed 

 50 or 60 mm. mercury pressure indicates that the blood can still 

 obtain oxygen from air which contains only one-half of the nor- 

 mal pressure of oxygen. In whatever way we estimate it, the 

 oxygen pressure in the alveoli is always greater than this. 



We will not go into details regarding the methods which have 

 been employed in solution of these problems ; suffice it to say that 

 a very fair sample of alveolar air can be secured by collecting a 

 sample of air from a tube through which a forced expiration has 

 been made. The last portions of such expired air must obviously 

 be alveolar air. 



Mechanism of Gaseous Exchange in Lungs. We have seen 

 that in the blood the pressure or tension of the oxygen is greater, 

 whereas that of the CO, is less than in the tissues. These rela- 

 tions will account for the gas exchange which occurs between the 

 blood and tissues if we apply the physical law of the diffusion of 

 gases, which states that two gases under different pressures ;iml 

 separated by a membrane through which they may pass free- 

 ly, will mix with each other until the tensions on both sides 

 of the membrane are equal. Before this law can be applied to 

 explain the exchange of gases between the blood and air within 

 the lungs, we must prove that the tension of the oxygen is less, 

 and of the C0 2 greater in the venous blood than in the alveolar 



