906 PHYSIOLOGY 



constant reaction, even though fixed acids are added to it, is of immense 

 importance in the economy of the body. All cellular functions are acutely 

 sensitive to changes in reaction, and, as we shall see later, the activity of 

 the respiratory centre is primarily dependent on the hydrogen ion con- 

 centration of the blood with which it is bathed. This hydrogen ion con- 

 centration depends in the normal animal on the partial pressure of the C0 2 

 in the medium with which the blood is in contact, so that the slightest rise 

 in the C0 2 tension in the alveolar air of the lungs causes at once a corre- 

 sponding increase in the H ion concentration of the blood, to which the 

 centre responds by increased activity. On the other hand, considerable 

 quantities of lactic acid, for instance, can be produced by the muscles and 

 poured into the blood without affecting more than a transitory alteration 

 in the activity of the respiratory centre. 



The alkaline reserve of the blood is significant, since any diminution 

 indicates in all probability the production of fixed acids in the tissues, and 

 a progressive reduction will precede the point at which the ' buffer ' action 

 of the sodium bicarbonate is lost, and the blood then responds to any addition 

 of acid by an appreciable change in reaction. It is only when the alkaline 

 reserve has been reduced to a minimum that a true condition of c acidosis,' 

 with its rapidly fatal effects, can come into being. 



THE OSMOTIC PRESSURE OF THE BLOOD 



Since the blood serves as a circulating medium, by means of which 

 the composition of the tissues juices forming the immediate environment 

 of all the cells of the body is maintained constant, its osmotic pressure 

 must be of considerable importance in regulating the normal exchanges 

 of the cells with their surrounding fluid. The osmotic pressure of the 

 blood depends on its molecular concentration and can be determined by 

 any of the methods mentioned earlier (p. 125). Of these the most con- 

 venient is the determination of the freezing-point. The depression of 

 freezing-point, A, of -mammalian blood is about O56 and varies between 

 0-54 and 0-60. The depression of the freezing-point observed in blood 

 is equal to that of a 0-9 per cent, sodium chloride solution, which is there- 

 fore taken as isotonic with the blood. Since the corpuscles are in osmotic 

 equilibrium with the plasma, their osmotic pressure must be equal to that 

 of the plasma, and laking the blood does not alter its freezing-point or its 

 osmotic pressure. The blood of the frog has a lower osmotic pressure, 

 the normal saline fluid for the frog's tissues being equivalent to 0-65 per 

 cent, sodium chloride solution. 



THE ELECTRICAL CONDUCTIVITY OF THE BLOOD 



In a solution it is only the dissociated ions which have the power of 

 carrying electric discharges. The conductivity of a solution of pure urea 

 or pure glucose would not differ appreciably from that of distilled water, 



