RESPIRATION. 197 



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 oxy- 

 gen 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 difficult to un- 

 derstand 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 pro- 

 portional to the total quantity of the ga 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 grm. haemoglobin). Ojt 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 oxygen; 

 in the case of arterial blood of the dog, this is found to be when the oxy- 

 gen tension of the atmosphere is equal to 3*9 per cent, (or 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 temperature, 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 car- 

 t bonic acid of the air in the pulmonary alveoli should be less than the ten- 

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

 the alveolar air probably does not exceed in the dog 3 or 4 per cent., 

 while that of the venous blood is 5 '4 per cent., or equal to 41 mm. of 

 mercury. 



B. Respiratory Changes in the Blood. * 



Circulation of Blood in the Respiratory Organs. To be ex- )( 



posed to the air thus alternately moved into and out of the air cells and 

 minute bronchial tubes, the blood is propelled from the right ventricle 



