382 RESPIRATION [CH. XXIV. 



the experiment consists in examining the composition of the gases left in the tube. 

 The principle upon which the instrument rests is this : If blood is brought into 

 contact with a mixture of oxygen, nitrogen, and carbonic acid gas, it gives off some 

 of its gases if their partial pressure is greater in the blood than the respective 

 tensions of the gases in the mixture ; if, on the other hand, the tension is lower in 

 the blood than in the mixture for any gas or gases, they pass from the mixture to 

 the blood. These interchanges go on until equilibrium is established. For example, 

 suppose the aerotonometer contains at the start 10 per cent, of oxygen, 5 of carbonic 

 acid, and 85 of nitrogen. Blood is then passed through it for an hour, and the 

 gases are again analysed, and the mixture then contains 14 per cent, of oxygen, 2*8 

 of carbon dioxide, and the rest nitrogen ; the tension of oxygen and carbon dioxide 

 in the blood will then respectively be 14 and 2 '8 per cent of an atmosphere. 



The Carbonic Oxide Method of Estimating the Oxygen Tension of 

 Arterial Blood. This method was devised by Haldane, and is considered by him 

 and Lorrain Smith to give more trustworthy results than those obtained by the 

 aerotonometer. If blood is exposed to a mixture of carbonic oxide and oxygen, the 

 haemoglobin will become saturated by these gases according to their relative 

 tensions. If a number of experiments are performed using different percentages of 

 carbonic oxide, the results may be expressed graphically as the curve of dissociation 

 of carboxyhaemoglobin in air. When in place of such experiments in vitro, an 

 animal is made to breathe air containing a known percentage of carbonic oxide, 

 the comparison of the saturation of its blood with the saturation of its blood in 

 vitro exposed to the same percentage of carbonic oxide in air (which has an oxygen 

 tension of 20*9 per cent.) gives us the means of discovering the oxygen tension in 

 the arterial blood of the lung capillaries ; this will be higher or lower than that of 

 the air according as the saturation by carbonic oxide is correspondingly lower or 

 higher. A small animal like a mouse is made to breathe air containing a known 

 percentage of carbonic oxide. After a sufficient time the animal is killed and the 

 amount of carboxyhaemoglobin is determined colorimetrically in a drop of its blood. 

 The data thus obtained are compared with the data previously expressed in the 

 curve of dissociation of carboxyhaemoglobin in air ; it is then easy to calculate 

 whether the oxygen tension in the blood is higher or lower than that of air. The 

 results of the method show generally that the tension of oxygen in the arterial 

 blood as it leaves the lungs is higher than could result from simple diffusion of the 

 oxygen through the alveolar epithelium; in other words the epithelial cells are 

 capable of secreting oxygen into the blood until an oxygen-pressure is reached 

 considerably above that in the alveolar air. 



The results expressed in percentages of an atmosphere are as follows : 

 Oxygen tension of arterial blood in man, 38 - 5 ; in mouse, 22*6 ; in dog, 21 ; in 

 cat, 35 '3 ; in rabbit, 27 '6, and in birds, 30 to 50 per cent. The results in the case 

 of man and larger animals probably require revision, as it is not certain that the 

 time allowed for the establishment of the balance of carbonic oxide and oxygen 

 has been sufficient in any of the experiments. 



Tissue Respiration. Before the processes of respiration were 

 fully understood the lungs were looked upon as the seat of combus- 

 tion ; they were regarded as the stove for the rest of the body to 

 which effete material was brought by the venous blood to be burnt 

 up. When it was shown that the venous blood going to the lungs 

 already contained carbonic acid, and that the temperature of the 

 lungs is not higher than that of the rest of the body, this explanation 

 had of necessity to be dropped. 



Physiologists next transferred the seat of combustion to the 

 blood ; but since then numerous facts and experiments have demon- 

 strated that it is in the tissues themselves, and not in the blood, that 

 combustion occurs. The methylene-blue experiments already described 



