256 ESSENTIALS OF PHYSIOLOGY. 



oxygen as an equal volume of water. Hence it is clear that oxygen 

 must form an unstable compound with some constituent of the blood ; 

 this constituent is haemoglobin, and a solution of pure haemoglobin can 

 take up from the air as much oxygen as blood containing the same 

 amount of haemoglobin. 1 gram of haemoglobin can combine with 

 approximately 1*34 c.c. oxygen, though this figure varies slightly in 

 different animals, probably on account of slight differences in the 

 character of the protein part of the haemoglobin molecule. 



If 100 c.c. of blood are exposed to an atmosphere of pure oxygen in 

 a closed vessel at atmospheric pressure, they take up about 20 c.c. of 

 oxygen. When the oxygen is slowly withdrawn from the vessel, so 

 that the pressure on the surface of the blood gradually falls, the blood 

 remains almost unaltered until the pressure falls to about 100 mm. Hg. 

 With a further fall of pressure the blood rapidly gives off its oxygen, 

 and, when the pressure falls to zero, all the oxygen has been evolved. 

 Evidently the combination of haemoglobin with oxygen is a reversible 

 one ; haemoglobin gives off its oxygen when the pressure of oxygen is 

 low, and takes it up when the pressure is high. This reversible action 

 is usually indicated thus, HbO^Hb + 0. 7 . It is to this power that 

 haemoglobin owes its value for respiration. In the alveoli of the lungs 

 the partial pressure of oxygen is relatively high, and the blood becomes 

 almost fully saturated with oxygen. The partial pressure of oxygen in 

 the tissues is low, and, when the blood is carried round in the circulation 

 to the tissues, the haemoglobin dissociates, the oxygen set free being- 

 taken up by the tissues. The effect of a varying partial pressure of 

 oxygen upon the combination between haemoglobin and oxygen is shown 

 in fig. 97. The curve shows the percentage of haemoglobin present 

 as oxyhaemoglobin with varying pressures of oxygen ; when all the 

 haemoglobin is in the form of oxyhaemoglobin, it is said to be fully 

 saturated. For pressures above 100 mm. Hg, the curve becomes 

 almost a straight line. 



This curve, which is known as the dissociation curve of haemoglobin, 

 is obtained in the following manner. A small quantity of blood is 

 placed in an aerotonometer containing a mixture of oxygen and nitrogen 

 at atmospheric pressure. A suitable form of tonometer is that shown 

 in fig. 98 ; it consists of a glass cylinder, which can be rotated in 

 water kept at a constant temperature. When the cylinder is rotated, 

 the small amount of blood, previously placed in it, spreads out into 

 a thin film over the inner wall of the cylinder, and after a short time 

 the blood and gas come into equilibrium, and the blood neither takes 

 up nor gives off oxygen. The cylinder is removed from the water 

 bath, and the percentage of oxyhaemoglobin in the blood is estimated 



