1182 PHYSIOLOGY 



or salt solution would do. Although carbon dioxide combines with 

 haemoglobin, it does not displace oxygen from the oxyhaemoglobin 

 molecule. Thus we may have haemoglobin saturated at the same time 

 with oxygen and with carbon dioxide. The presence of carbon dioxide 

 does, however, alter the ease with which oxyhaemoglobin dissociates. 



The relation between the partial pressure of oxygen and the 

 amount of oxy haemoglobin formed under varying conditions can be 

 investigated in the following way (Barcroft) : 



A large glass globe with a stop-cock at one or both ends (Fig. 493) is filled with a 

 gaseous mixture of known composition containing oxygen. Into it are introduced 

 2 or 3 c.c. of blood or of haemoglobin solution. It is then tightly stoppered and 



FIG. 493-. Barcroft's apparatus for determining the curve of absorption of 

 oxygen by haemoglobin. 



immersed in a horizontal position in a pail of water kept at a constant temperature. 

 In the pail it is suspended between its two ends, so that it can be slowly revolved 

 by means of a piece of string passing round its neck. In this way the blood 

 is continually spread in a thin layer over the sides of the vessel. At the end of 

 a quarter to half an hour it will have attained equilibrium with the gaseous 

 mixture. It is then turned into an erect position so that the fluid can run down 

 into the neck closed by a stop-cock, whence 1 c.c. may be drawn off for analysis 

 in a Barcroft apparatus. A further portion of the same blood may be shaken 

 up with air so as to saturate it completely, and the saturation of the two samples 

 may be compared in the differential gas apparatus. 



Barcroft has shown that the dissociation curve of haemoglobin is 

 largely altered by slight variations in the fluid in which the haemo- 

 globin is dissolved. The most important of these conditions are (1 ) the 

 saline content of the fluid, (2) the reaction of the fluid. Under this 

 latter heading must be classed the amount of carbon dioxide present, 

 since its action on the dissociation curve is similar to that produced 

 by the presence of weak acids such as lactic acid. The influence of 

 dissolved salts on the dissociation curve is shown in Fig. 494. 



It is interesting to note that the differences between the dissocia- 

 tion curve of blood and of haemoglobin solution, as well as between 



