THE LUNGS AND RESPIRATION. 223 



by the liquid of the blood are necessarily cold-blooded, and 

 frequently very sluggish, much like a fire which, devoid of 

 a good draft, would have to burn slowly and smoulder. But 

 in the case of the blood of all higher animals a new factor 

 is added. This new factor is the haemoglobin contained in 

 the red corpuscles. This haemoglobin has the chemical 

 property of combining with oxygen whenever the pressure 

 of the oxygen is a half pound or more, and of giving up the 

 oxygen just as soon as the pressure of oxygen surrounding 

 it sinks below a half pound. This property of haemoglobin 

 is by no means a unique one. There are many chemicals 

 which, under high pressure, will form combinations with 

 other substances, and at lower pressure will again disunite. 

 We have here to do not with a mysterious physiological 

 problem, but with a simple every-day chemical fact. 



THE EOLE OF THE BED CORPUSCLES. 



I/et us now see just what role the red corpuscle with its 

 haemoglobin plays in the pulmonary capillaries. In a way 

 described above the oxygen is absorbed by the plasma and 

 the oxygen pressure in the same will at once begin to 

 rise. When, however, this pressure in the plasma rises 

 above a half pound the haemoglobin will at once seize the 

 oxygen absorbed in the plasma and unite chemically with 

 it. This oxygen so taken out of the plasma is, of course, 

 replaced at once by fresh oxygen which streams in from the 

 outside. As the pressure of the oxygen in the air is three 

 pounds to the square inch, it will continue to stream into 

 the plasma until the pressure there will be three pounds. 

 But before the pressure reaches three pounds, in fact, just 

 as soon as it passes the half-pound limit, the haemoglobin in 

 the red corpuscles will pick out the oxygen from the 

 plasma, unite chemically with it, and so prevent the pres- 

 sure in the plasma from reaching three pounds. This will 

 go on until finally all of the haemoglobin has united with 

 oxygen; that is, until all of the haemoglobin has been 

 changed into oxyhaemoglobin. After this point is reached 



