328 RESPIRATORY FUNCTION OF THE BLOOD 



dissociation curve of blood, but to retain the same general shape 

 as the curve for pure haemoglobin. 



Influence of H * concentration. 



An increase, however, in the carbon-dioxide tension of the 

 haemoglobin solution produces a curve of the sigmoid shape, 

 typical of the blood dissociation curve, and, in fact, the dissociation 

 curve of haemoglobin in a solution containing the solutes of blood, 

 including COc>, in the projjortion in ichich theij occur in blood, lies 

 point for point on the dissociation curve of that blood. The presence 

 of any other acid in the blood, tending to increase the H+ con- 

 centration, as will be shown later, liberates COg from bicarbonates 

 present, and, therefore, has the same effect on the oxygen dis- 

 sociation curve as an equivalent increase in CO2 tension. It might 

 be thought that this influence of carbon-dioxide on the union 

 between haemoglobin and oxygen was a direct one, the oxygen in 

 the oxy-haemoglobin being simply replaced by carbon-dioxide, but 

 there is no molecular equivalence between the amounts of the two 

 gases involved in the reaction, and, therefore, this simple replace- 

 ment cannot be the explanation of the influence. 



Examination of the curves of Fig. 82 reveals the importance to 

 the body of the part played by the solutes of the blood in the 

 transport of oxygen. At high oxygen tensions, the presence of 

 salts and COg in the blood does not materially decrease the per- 

 centage saturation of the haemoglobin, but at low oxygen tensions, 

 such as are found in the tissues, it enables much more oxygen to be 

 given off by the blood than would be the case in a solution of 

 pure haemoglobin. 



In the tissues the tension of oxygen is low, say 15 nun. Hg. At this tension 

 whole blood can only be 15 per cent, saturated. Therefore the oxygen 

 carried by 77 per cent. (92 — 15) of the haemoglobin is discharged (where the 

 blood in the lungs is supposed to be 92 per cent, saturated). On the other 

 hand, pure haemoglobin is still 65 per cent, saturated at 15 mm. Hg. It will 

 only be able to discharge the oxygen borne by 27 per cent, of its haemoglobin. 

 In other words, because of the presence of solutes, whole blood is able to set 

 free in the tissues the full amount of oxygen that could be obtained from pure 

 haemoglobin, with, in addition, the amount that would be carried by the 

 haemoglobin re})resented by the space between the heavy curve and the 

 dotted curve in Fig. 82. That is, solutes so aid in the unloading of oxygen that 

 50 j)er cent, of the haemoglobin that tcould otherwise have retained its oxygen is 

 induced to give it up to the tissiies. Because of the solutes, whole blood 

 becomes an effective carrier of oxygen, and the total volume of fluid (and mass 

 of haemoglobin) is kept within reasonable limits. 



The effect of the carbon-dioxide tension is particularly important 

 at very low oxygen tensions, as with increasing carbon-dioxide 



