THE INDIVIDUAL GROUPS OF PROTEINS 497 



THE FUNCTION OF HEMOGLOBIN AS CARRIER OF 



OXYGEN. 



The solubility of oxygen at N.T.P. in water is about -4 c.c. in 

 100 c.c. ; in blood it is about 7 c.c. in 100 c.c., excluding the amount 

 in combination with haemoglobin. This quantity of oxygen in the 

 blood does not suffice to supply the needs of the body for oxygen. 

 Haemoglobin has the function of being a special solvent for oxygen and 

 of acting as the carrier of oxygen to the tissues. I gm. of haemo- 

 globin combines with I '34 c.c. of oxygen at N.T.P. This quantity of 

 oxygen is so definite that it is generally believed that the oxygen is in 

 actual chemical combination, but it is thought also that it is simply in 

 solution in the haemoglobin (by adsorption). 



Haemoglobin readily combines with oxygen and readily gives it 

 up again, especially under the conditions in blood where the tem- 

 perature is relatively high (37), salts and acids, especially carbonic 

 acid, are present. Under these conditions the dissociation of oxygen 

 from the haemoglobin is as rapid as the association of oxygen by 

 the haemoglobin ; they occur at about the same rate, in less than 

 one second, i.e. in about the same time as the flow of blood through 

 the capillaries. Pure haemoglobin solutions behave differently, the 

 association is rapid, the dissociation is slow in comparison. 



The association and dissociation of the haemoglobin follow the sam"e 

 laws that hold for the partial pressure of gases, whether they are in a 

 gaseous state or dissolved in a liquid in contact with gas. Transference 

 takes place from the region where the partial pressure (or tension if dis- 

 solved) or relative amount of the gas is high to the region where the 

 partial pressure or the relative amount of gas is low. Association 

 occurs in the lungs where the partial pressure of oxygen in the alveolar 

 air is high ; dissociation occurs in the tissues where the partial pressure 

 or tension of the oxygen in the haemoglobin is higher than it is in the 

 tissues. 



Determinations have been made to ascertain the relative amounts 

 of haemoglobin and oxyhaemoglobin which are formed when a haemo- 

 globin (or oxyhaemoglobin) solution is exposed in a closed vessel to 

 different partial pressures of oxygen. These varying partial pressures 

 of oxygen are prepared by mixing oxygen or air with nitrogen in the 

 required proportions in the vessels. Small volumes of blood are intro- 

 duced and exposed to the large volumes of the known gaseous 

 mixtures at a definite temperature until equilibrium is reached. The 

 amount of oxygen in the sample of blood is then determined by the 

 ferricyanide method (p. 498) and expressed as a percentage of the 

 total oxygen capacity of the blood. The following table is typical of 

 the results obtained ; the percentage of haemoglobin is found by deduct' 

 ing the percentage of oxyhaemoglobin from 100, 



