250 RESPIRATORY FUNCTION OF THE BLOOD 



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. 

 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 represented by the 

 space between the heavy curve and the dotted curve in Fig. 56. That 

 is, salts so aid in the unloading of oxygen that 50 per cent, of the haemoglobin 



DISSOCIATION CURVE. OF H/EMOGLOBIN AT 37 C. 



z 

 o 



L 



I 



10 20 



TENSION 



JO <*O SO 



OF OXYGEN IN 



60 



MM 



80 



90 JOO 



7O 



OF Hg 



FIG. 56. Dissociation curve representing the equilibrium between oxygen tension, 

 oxy-haemoglobin and reduced haemoglobin : 



=Curve from pure 14% haemoglobin solution. 



= 14% haemoglobin in plasma. 

 - = , ,, with increased 



tension of carbon-dioxide. 



The hatched portion represents the amount of haemoglobin which gives up its 

 oxygen when the oxygen tension is reduced from 100 to 15 mm. Hg. 



that would otherwise have retained its oxygen, is induced to give it up to the 

 tissues. 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. 



Nature of union. 



The average normal man has about 5 litres of blood, 2 -5 litres 

 of this is occupied by 25 xlO 12 red corpuscles with a total surface 

 of about 3000 sq. metres. When fully saturated with oxygen at 



