BLOOD AS THE COMMON CARRIER 105 



explained on the basis of diffusion from regions of higher to those 

 of lower concentration. Toxic products of intestinal putrefaction 

 are carried in the blood plasma to the hver, lungs, and skin, 

 from which they are discharged. Secretions of the endocrine 

 glands are distributed throughout the body to produce chemical 

 coordinations between metabolic processes in different regions. 

 Finally, in passing through parts where oxidations are occurring 

 rapidly with production of heat, the blood becomes heated, an-d 

 thus acts, in warm-blooded animals, as the agency for heat trans- 

 mission and equalization. 



In summary, the blood as a common carrier performs four 

 functions: first, it carries the nutrient substances, including simple 

 sugars, amino acids, fats, lipins, vitamins, water, inorganic salts, 

 and oxygon, to all the cells of the body; second, it removes the 

 waste products of metabolism, carbon dioxide, water, and nitrog- 

 enous compounds, from all cells; third, it distributes the endo- 

 crine secretions, the chemical coordinators, to all body cells; and 

 fourth, it transfers heat from regions of rapid oxidation to those 

 of a lower rate. 



An understanding of the functions that have been discussed 

 in the present chapter must be based on an understanding of the 

 necessities of cells as units of function. Complicated systems 

 of organs exist for the maintenance of metabolism in cells through- 

 out the body. If the physiology of metabolism in vertebrate 

 animals is considered and mastered from the point of view of 

 the chemical reactions occurring in its indi\'idual cells, the study of 

 function in other types of animals will be merely a reaffirmation of 

 the facts brought out in this chapter. The protoplasmic require- 

 ments and reactions are assumed to be in general the same through- 

 out the animal kingdom. Another important fact to keep in mind 

 is that, while the cell is the unit of function, the reactions of the 

 animal as a whole depend upon the coordinated activities of all 

 cells — upon what we shall call the physiological balance of the 

 organism. 



