216 VETERINARY PHYSIOLOGY 



is exposed to an atmosphere poor in C0 2 , the P 2 5 again seizes 

 on the Na, turning out the C0 2 . 



But the proteins of the blood also act in the same way as 

 weak acids, being turned out of their combination with bases 

 by the mass action of C0 2 , and thus acting as carriers of C0 2 . 



In the corpuscles the haemoglobin acts as an acid. When 

 the amount of C0 2 is great, haemoglobin is turned out of its 

 combination with bases. But when the pressure of C0 2 is low 

 the haemoglobin turns it out. For this reason it is possible to 

 remove all the C0 2 from whole blood in an air pump, but not 

 possible to remove it from blood plasma. In fact, the carriage 

 of carbon dioxide and its excretion are mainly the result of a 

 struggle between that gas on the one hand and the proteins and 

 phosphoric acid on the other, for the bases of the plasma. 



Nitrogen. The amount of nitrogen in the blood is not in 

 excess of what can be held in solution, and we may therefore 

 infer that it is simply dissolved in the blood plasma. 



V. Source of the Blood Constituents 



A. Of the Plasma. The water of the blood is derived almost 

 entirely from the water ingested. 



The source of the proteins has not been fully investigated. 

 Undoubtedly they are partly derived, somewhat indirectly as 

 we shall afterwards see, from the proteins of the food. Very 

 probably, too, they are in part derived from the tissues. But 

 the significance of the two proteins, albumin and globulin, 

 and of their variations has not yet been elucidated. 



The glucose is derived from the carbohydrates and possibly 

 from the proteins of the food, and during starvation it is con- 

 stantly produced in the liver and poured into the blood (p. 395). 



The fats are derived from the fats and carbohydrates and 

 possibly from the proteins of the food. 



The urea and other waste constituents are derived from 

 the various tissues. 



B. Of the Cells. I. Leucocytes. In the embryo these are 

 first developed from the mesoblast cells generally. In extra- 

 uterine life they are formed in the lymph tissue and in the 

 red marrow of bone. 



1. Lymph Tissue (see p. 30) is very widely distributed in 



