TRANSPORT OF MATERIALS 143 



importance, that in the immediate neighbourhood of the neutral 

 point. The osmotic pressure of the proteins in the plasma is about 

 40 mm. of mercury, as we have seen, so that their molar concen- 

 tration is only 0.0023, and they do not possess many free NH 2 

 groups capable of combining with acids. It appears that it is half 

 the nitrogen of the lysine contained in the protein molecule that is 

 in the form referred to. If so, the proteins of serum would be about 

 equivalent to a 0.0006 normal ammonia solution. 



But there is a salt present in plasma which can combine with 

 acids. This is sodium bicarbonate, and its concentration is 0.03 

 molar. On investigation, solutions of this salt are found to be very 

 effective in preventing a rise in the hydrogen-ion concentration of 

 solutions when acid is added. This is because when an acid 

 stronger than carbonic acid is added to a bicarbonate solution, 

 carbon dioxide gas is given off to the atmosphere^ while that 

 remaining in solution is so little dissociated as to afford only a few 

 hydrogen ions as compared with those of the acid added. We may 

 look at the question from another point of view. Sodium bicar- 

 bonate solutions are alkaline because the salt, as one of a weak acid 

 with a strong base, is hydrolytically dissociated into carbonic acid 

 (H. 2 CO 3 ) and sodium hydroxide. The latter is electrolytically dis- 

 sociated much more than the former, so that there is an excess of 

 OH' ions, conferring alkalinity. Thus, in a solution of sodium bi- 

 carbonate containing dissolved carbon dioxide, we may say that 

 the former confers alkalinity, the latter acidity, hence the hydrogen- 

 ion concentration is given by the ratio between the two. If they 

 vary in proportion, the reaction is unaltered. If the carbon dioxide 

 increases without the bicarbonate changing, the hydrogen ion is 

 raised. If it decreases, the hydrogen ion is lowered. These facts 

 give the clue to the most rapid and effective of the means of 

 regulating the reaction of the blood. Let us suppose that lactic 

 acid has been passed into the blood, as happens in defective supply 

 of oxygen. It immediately combines with a part of the bicar- 

 bonate, and if the content of the plasma in this salt is determined, 

 it is found to be diminished. The state is often called " acidosis," 

 although it does not imply that the blood has become more acid. 

 In fact, it is easy to show, by experiment, that a large amount of 

 acid may be introduced into the blood without raising the hydrogen- 

 ion concentration. Why not ? The answer is found in the activity 

 of the respiratory centre. When bicarbonate combines with acid, 

 carbon dioxide is given off and, for a brief period of time, the 

 hydrogen ion of the blood is raised by its excess. But this excess 

 excites the respiratory centre to increased ventilation of the lungs 

 until the carbon dioxide tension in the alveolar air has become low 

 enough to reduce that in the blood to a level to compensate for 



