192 HANDBOOK OF PHYSIOLOGY. 



blood absorbs oxygen from, and gives up carbonic acid to, the air of the 

 alveoli. In the first place, it must be remembered that the tidal air 

 only amounts to about 25-30 cubic inches at each inspiration, and that 

 this is of course insufficient to fill the lungs, but it mixes with the sta- 

 tionary air by diffusion, and so supplies to it new oxygen. The amount 

 of oxygen in expired air, which may be taken as the average composi- 

 tion of the mixed air in the lungs, is about 16 to 17 per cent ; in the 

 pulmonary alveoli it may be rather less than this. From this air the 

 venous blood has to take up oxygen in the proportion of 8 to 12 vols. in 

 every hundred volumes of blood, as the difference between the amount 

 of oxygen in arterial and venous blood is no less than that. It seems 

 therefore somewhat diffcult to understand how this can be accomplished 

 at the low oxygen tension of the pulmonary air. But as was pointed 

 out in a previous Chapter (IV. ), the oxygen is not simply dissolved in 

 the blood, but is to a great extent chemically combined with the 

 haemoglobin of the red corpuscles ; and when a fluid contains a body 

 which enters into loose chemical combination in this Way with a gas, 

 the tension of the gas in the fluid is not directly proportional to the 

 total quantity of the gas taken up by the fluid, but to the excess 

 above the total quantity which the substance dissolved in the fluid is 

 capable of taking up (a known quantity in the case of haemoglobin, viz., 

 1.59 cm. for one gm. haemoglobin). On the other hand, if the sub- 

 stance be not saturated, i. e., if it be not combined with as much of the 

 gas as it is capable of taking up, further combination leads to no increase 

 of its tension. However, there is a point at which the haemoglobin 

 gives up its oxygen when it is exposed to a low partial pressure of oxygen, 

 and there is also a point at which it neither takes up nor gives out oxy- 

 gen ; in the case of arterial blood of the dog, this is found to be when 

 the oxygen tension of the atmosphere is equal to 3.9 per cent (29.6 mm. 

 of mercury), which is equivalent to saying that the oxygen tension of 

 arterial blood is 3. 9 per cent; venous blood, in a similar manner, has 

 been found to have an oxygen tension of 2.8 per cent. At a higher tem- 

 perature, the tension is raised, as there is a greater tendency at a high 

 temperature for the chemical compound to undergo dissociation. It is 

 therefore easy to see that the oxygen tension of the air of the pulmonary 

 alveoli is quite sufficient, even supposing it much less than that of the 

 expired air, to enable the venous blood to take up oxygen, and what is 

 more, it will take it up until the haemoglobin is very nearly saturated 

 with the gas. 



As regards the elimination of carbonic acid from the blood, there is 

 evidence to show that it is given up by a process of simple diffusion, the 

 only condition necessary for the process being that the tension of 

 the carbonic acid of the air in the pulmonary alveoli should be less than 

 the tension of the carbonic acid in venous blood. The carbonic acid 



