184 ESSENTIALS OF CHEMICAL PITYSrOLOOY 



saturation corresponds with an oxygen tension of about 14 mm. of 

 mercury, or 1*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 thr 

 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 contain- 

 ing a known percentage of carbonic oxide, the comparison of the saturation 

 of its blood w r ith 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 per- 

 centage 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 carboxyhaenioglobin 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 experiment s. 



Tissue-Respiration. Before the processes of respiration were fully 

 understood the lungs were looked upon as the seat of combustion ; 

 they were regarded as the stove for the rest of the body where 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 greater 

 than that of the rest of the body, this explanation had of necessity to 

 be dropped. 



Physiologists next transferred the seat of the combustion to the 

 blood ; but since then innumerable facts and experiments have 

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

 combustion occurs. The methylene-blue experiment already described 

 (p. 128) shows this ; and the following experiment is also quite con- 

 clusive. A frog can be kept alive for some time after salt solution is 



