BLOOD 481 



sodium, to which the osmotic pressure of the plasma is partly 

 due, and which is present in the proper proportion of 

 sodium with the other cations, potassium, calcium, and 

 magnesium, to maintain the activity of the tissues (p. 218). 

 Perhaps the most important salt is sodium bicarbonate, which 

 maintains the reaction, the Cjj (see Appendix III.) of 

 the blood, and of the body fluids, at the level at which 

 their chemical activity can best be carried on. Sodium phos- 

 phate is also present in very small quantities. Calcium, 

 potassium, and magnesium occur in very small amounts. 



Sodium bicarbonate is an ideal salt for maintaining the 

 balance of H and OH ions in the blood. When stronger 

 acids, such as sarcolactic acid, are liberated from the tissues, 

 they combine with some of the sodium, and the weak, 

 slightly dissociated CO.2 is set free, and is at once got rid of 

 by the lungs (p. 527). When, on the other hand, alkalies are 

 absorbed and added to the blood, there is available in the 

 tissues an abundant supply of CO.,, with which they will 

 combine as bicarbonates. The maintenance of the propor- 

 tion of NaHCOs in the blood is of the utmost importance. 

 It may be termed its alkaline reserve. Only when this 

 alkaline reserve is drawn upon can anything like a real 

 condition of acidosis, a real increase in the Cg, occur. This 

 has been termed an uncompensated acidosis, to distinguish 

 it from the condition in which an increased production of 

 acids has been met by the alkaline reserve {compensated 

 acidosis). 



In venous blood there are something like 60 parts of CO.2 

 per 100 parts of blood. Of this, at the temperature of the 

 body and the pressure of COg in the lungs to which the 

 blood is subjected (p. 539), some 3 parts are dissolved; 

 the remainder is in combination with sodium as NaHCOg, 

 so that the balance is 



H.,C03 _^^ l_ 

 NaHCOs ~ 60 ~ 20' 



If the amount of CO., is increased, then it must either be 

 got rid of from the lungs, or the proportion of Na in the 

 denominator must be increased, while if the Na is combined 

 31 



