RESPIRATION BEYOND THE LUNGS 401 



and reduction in the presence of a partial pressure of C0 2 of 40 mm., 

 which corresponds to that of the arterial blood, we shall find that oxida- 

 tion becomes a little slower, whereas reduction is further quickened. 

 Indeed the two curves, as seen in C in the figure, come practically to 

 correspond, indicating that the environmental conditions under which 

 hemoglobin combines and gives off 2 in the blood are exactly adjusted. 



One word more with regard to the influence of C H . Its effect in flat- 

 tening out the curve, especially at the lower partial pressures of 2 , 

 indicates that when a high C H is present, the blood will very readily part 

 with its 2 supply. Now, the most significant application of this fact 

 is that high concentrations of H ion will occur just exactly where it 

 will be of benefit namely, in the capillaries (because of the C0 2 and 

 lactic acid produced by the tissues). Some doubt has, however, recently 

 been thrown on the importance of this factor. 



Since, as we have seen, hemoglobin absorbs 2 according to chemical 

 laws, it will naturally be asked not only why the dissociation curve flat- 

 tens out while yet maintaining the shape of a right-angled hyperbola, 

 as by the action of acids or an increase in temperature, but also why it 

 should change its shape when salts are also present. The explanation 

 offered by Barcroft and his pupils is that the changes depend on the fact 

 that since hemoglobin is a colloidal substance, its molecules undergo 

 processes of aggregation under the conditions referred to above, and 

 therefore cause the reaction to become of a different type from that 

 represented by the equation Hb0 2 ^ Hb + 2 . As has been pointed out 

 by Bayliss, although such a process might suffice to explain the 

 flattening out of the curve, it fails to explain the change in its shape; 

 for, according to the laws of mass action, such a change could occur 

 only if molecules of a different type came to take part in the reaction. 



Dissociation Constant. Notwithstanding these criticisms, it is of con- 

 siderable practical importance to know that an equation exists from 

 which the entire dissociation curve can be plotted by making only one 

 determination of the relative amounts of oxy- and reduced hemoglobin 

 at a particular tension or partial pressure of oxygen. This equation is as 



y Kx n 



follows: -JOQ- = -= n , where y equals the percentage saturation of 



hemoglobin with 2 , + the 2 tension, and K and n are constants, K 

 being the equilibrium constant and n the average number of molecules 

 of hemoglobin supposed to exist in each aggregate. 



When this equation is applied to human blood, the value of n remains 

 unchanged and is given as 2.5, so that by transposition we are enabled 



to find the value of K as follows: K = -r^ . If we find the value 



x n (100-y) 



