RESPIRATION BEYOND THE LUNGS 389 



y Kx n 



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



hemoglobin with 2 , x 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 



y 



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



x n (100-y) 



of K by measuring the relative saturation of the blood with 2 at one pres- 

 sure of this gas, then by changing the value of x to correspond to other 

 2 pressures, we can find all positions of the curve for a given sample of 

 blood. 



An important practical application of this method is found in the 

 determination of the (7 H of blood, for, as we have seen, the dissociation 

 curve becomes lowered in proportion to the concentration of hydrogen 

 ions. The acidity of a sample of blood can therefore be found by com- 

 parison of its dissociation curve, as plotted from the values found for 

 K, with that of normal blood to which known quantities of acid have 

 been added. When the curves correspond, the bloods must contain the 

 same amounts of acid, other things being equal. In brief, then, the re- 

 action of the blood is proportional to the value of K. When this is low, 

 it indicates that the blood is taking up an abnormally low percentage 

 of its possible load of 2 at a given pressure of 2 , and that the acidity 

 is greater than normal ; when K is high, for the same reason the acidity 

 must be low. 



In determining K for the blood as it exists in the body, it is necessary 

 that it should be subjected to the same tension of C0 2 as obtains in the 

 blood vessels. K will then be proportional to the C H of the living blood. 

 This condition would be impossible to fulfil in drawn samples were it 

 not for the fact that we can place in the tonometer an atmosphere con- 

 taining the same partial pressure of C0 2 as is found in the alveolar air. 

 Since this value varies in different individuals, it must be separately 

 ascertained in each case (see page 344). As determined with these 

 modifications, K has been found to vary in healthy men between 

 0.000212 and 0.000363 (ten individuals). When acid substances appear 

 in the blood, as in acidosis, K becomes extremely low; thus, in one case 

 suffering from acidosis with dyspnea, it was found a few hours before 

 death to be only from 0.000082 to 0.00011. Similarly K becomes low 

 in the acidosis associated with mountain sickness, and it is said to be 

 raised after taking food that is rich in alkali.* 



*When K is found to be normal, the blood is said to be mesectlc; where K is low, it is said to 

 be myonectic; and when K is high and the acidity is therefore small, it is said to be pleonectic. 



