368 RESPIRATION [CH. XXVI. 



The figures for a pure solution of haemoglobin would be : 



No. 1. No. 2. No. 3. No. 4. No. 5. No. 6. 



Percentage of haemoglobin . 100 63 45 28 13 6 



Percentage of oxyhaemoglobin . 37 55 72 87 94 



100 100 100 100 100 100 



The same answer may be expressed graphically ; if the pressures 

 of oxygen are plotted horizontally, and the percentages of oxy- 

 and reduced hsemoglobin in the solution are plotted vertically, we 

 get the curve shown in the accompanying diagram (fig. 295), which is 

 called the dissociation curve of licemoglobin. 



In a reversible reaction such as the one under consideration, the 

 molecules are continually forming and breaking up again, and the 

 rate at which this occurs is influenced by various conditions. 

 Among the conditions of importance in the body are (1) tempera- 

 ture; (2) the presence of salts; and (3) the presence of acids, 

 especially carbonic acid. These factors tend to make the oxyhaemo- 

 globin molecules break down more rapidly and form more rapidly. 

 From the point of view of the body's needs, it is clearly necessary, 

 not only that hsemoglobin should acquire oxygen at the pressure of 

 that gas in the lungs, and part with it at the diminished oxygen 

 pressure in the tissues, but that the two processes should occur at 

 about the same rate, that is, within one second, which is about the 

 time occupied by any given portion of blood in travelling along the 

 capillaries (see p. 282). 



It would be futile to have an oxygen carrier in the blood which 

 took a fraction of a second to acquire its oxygen, and a fraction of 

 an hour to release it. Yet a solution of pure haemoglobin is just 

 such a substance, for its power of acquiring oxygen is very great, 

 and its power of releasing it is very small. Happily, however, a 

 haemoglobin solution and blood are two very different things. In 

 the red corpuscles, the haemoglobin is dissolved in a medium con- 

 taining various salts of which those of potassium are most prominent ; 

 these salts confer on haemoglobin the property of giving up its 

 oxygen much more readily when exposed to low concentrations of 

 oxygen such as are present in the capillaries of the tissues, while at 

 the same time it acquires oxygen more readily at higher oxygen 

 pressures such as blood is exposed to in the lungs. But the mere 

 presence of potassium salts is not in itself capable of rendering the 

 haemoglobin an efficient oxygen carrier ; two other factors come into 

 play in the body one is the high temperature, and the second is the 

 presence of carbonic acid. This is illustrated in the curves depicted in 

 fig. 296. To obtain the rate of oxidation, oxygen at alveolar pressure, 

 mixed with nitrogen, was passed through reduced blood ; to obtain the 

 rate of reduction, nitrogen, free from oxygen, was passed through 



