372 RESPIRATION [CH. XXVI. 



The Mechanism of Gaseous Exchange in the Lung. 



1. Oxygen. 



The simplest explanation of the passage of oxygen from the alveolar 

 air into the blood is that the process is one of diffusion. This view 

 (which is generally regarded as adequate in the case of normal 

 respiration) can be maintained if it can be proved that the pressure 

 of oxygen in the alveolar air is as great or greater than the tension 

 of oxygen in the arterial blood, and therefore a fortiori greater than 

 that of oxygen in the venous blood. 



The conception of respiration based upon this view would be that 

 the pressure of oxygen in the air of the alveoli though less than that 

 in the atmosphere, is greater than that in venous blood ; hence oxygen 

 passes from the alveolar air into the blood plasma; the oxygen 

 immediately combines with the haemoglobin, and thus leaves the 

 plasma free to absorb more oxygen; and this goes on until the 

 haemoglobin is entirely, or almost entirely, saturated with oxygen. 

 The reverse change occurs in the tissues when the partial pressure 

 of oxygen is lower than in the plasma, or in the lymph that bathes 

 the tissue elements ; the plasma parts with its oxygen to the lymph, 

 the lymph to the tissues; the oxyhaemoglobin then undergoes dis- 

 ifeciation to supply more oxygen to the plasma and lymph, and thus 

 in turn to the tissues once more. This goes on until the oxyhaemo- 

 globin loses on the average about half of its store of oxygen; 1 

 c.c. of arterial blood contains 0-2 c.c. oxygen; 1 c.c. of venous 

 blood contains 0-1 c.c. oxygen. 



Haldane and Priestley introduced a very simple method of 

 collecting alveolar air which has the advantage of being applicable to 

 man. A piece of rubber tubing is taken about 1 inch in diameter 

 and about 4 feet long. A mouthpiece is fitted into one end. About 

 2 inches from the mouthpiece a small hole is made into which is 

 inserted the tube of a gas-receiver, or sampling-tube, as in the 

 figure (fig. 298). The gas-receiver is fitted at the upper end with 

 a three-way tap, and the lower end is also closed by a tap. Before 

 it is used, it is filled with mercury. The subject of the experi- 

 ment breathes normally through the tube for a time, and then, 

 at the end of a normal inspiration, he expires quickly and very 

 deeply through the mouthpiece and instantly closes it with his 

 tongue. The lower tap of the receiver is then turned, and as 

 the mercury runs out, a sample of the air takes its place and 

 fills the receiver; this sample is then analysed. A second experi- 

 ment is then done, in which the subject expires deeply at the end 

 of a normal expiration, and another sample obtained. The mean 



