RESPIRATION .BEYOND THE LUNGS 383 



conditions readjusted, the manometer closed off from the ouside air, 

 and the apparatus again shaken so that the ferricyanide mixes with the 

 hemoglobin solution. This drives off all the 2 from the oxyhemoglobin 

 solution, and, therefore, raises the pressure in the atmosphere of that 

 bottle so that the clove oil moves to the opposite side of the manom- 

 eter, the degree of displacement being proportional to the amount of 

 oxyhemoglobin. 



We have now all the necessary data for estimating the relative amounts 

 of reduced hemoglobin in the hemoglobin solution as removed from the 

 tonometers, for it is plain that the second estimation, as described above, 

 tells us how much oxyhemoglobin might have been formed had all the 

 hemoglobin been saturated and the first one, how much 2 had yet to be 

 taken up by the original hemoglobin solution to produce saturation. 



The Dissociation Curve. The next step is to plot the results obtained 

 from the various hemoglobin solutions in the form of -a curve. This is 

 known as the dissociation curve of hemoglobin. It is plotted with the 

 relative percentages of reduced and oxyhemoglobin in each of the solu- 

 tions along the ordinates, and the partial pressures of 2 in millimeters 

 of mercury. to which they were exposed along the abscissae. The curve 

 thus drawn is exactly of the same shape as that which would be pro- 

 duced if we were to place the tonometers in a row at distances from one 

 another corresponding to the partial pressure of 2 which each con- 

 tained, and then to mark on each tonometer the relative amounts of 

 reduced and oxyhemoglobin found in the solutions after shaking. A 

 line joining these marks on the tonometers would then exactly corre- 

 spond to the curve drawn by the method described above. This will be 

 clear from the accompanying figure from Barcroft's book (Fig. 137). 



In such a chart the space below the curve can be taken to represent 

 the percentage of oxyhemoglobin (red in chart), and that above it of 

 reduced hemoglobin (blue in chart), at the varying partial pressures of 

 2 which are indicated along the abscissae as being contained in the at- 

 mosphere of the tonometers, and which must be proportional to the 

 partial pressure of 2 in the solution in which the hemoglobin is dis- 

 solved. 



Difference between Curves of Blood and Hemoglobin Solutions. The 

 curve obtained from pure hemoglobin solutions is very far, however, 

 from clearing up the problem as to how. the blood absorbs and 

 discharges 2 . On the contrary, it makes this problem appear 

 all the more difficult, for, according to the curve (Fig. 137) the hemo- 

 globin is already more than half combined with 2 at a partial pressure 

 of this gas of no more than 10 mm. Hg, which means that in the low 

 partial pressure of 2 existing in the capillaries the oxyhemoglobin, in- 



