CHAPTER V. 



THE BLOOD 



BLOOD is a red, somewhat viscid fluid, denser than water, and 

 apparently composed of but one substance. This liquid, which is 

 usually spoken of as the nutritive fluid of the body, serves as an 

 internal medium of exchange existing between the foodstuffs found 

 in the outer world and the cells composing the various tissues of the 

 body. It was constantly kept before the student's attention that the 

 main and ultimate end of digestion was the absorption of the food- 

 stuffs into the blood-stream, not as proteoses and peptones, but as 

 native albumins and globulins these latter are the results of the 

 living, vital activity of the epithelial cells of the villi through which 

 pass the proteoses and peptones. Thus, into the blood are poured 

 new products (the work of digestion), which are carried by its circu- 

 lation to all parts of the body, to be given up to the various tissues 

 having need of them. By this means every cell receives the nutri- 

 ment necessary for carrying on its own metabolic processes, either 

 directly or indirectly, for the student will remember that each cell 

 possesses an inherent selective capability. From the pabulum con- 

 tained in the enveloping lymph it is able to take up those factors 

 which it can work up into its own constitution to form an integral 

 part of itself. These constituents, having served their respective 

 purposes, are no longer of any value to the cell they are waste-pro- 

 ducts, and as such must be gotten rid of. Passing out from the cell- 

 substance, they find themselves in the same enveloping lymph, to 

 be eventually carried again into the blood-stream for elimination 

 through the excretory activities of the lungs, kidneys, and skin. 

 Thus, indirectly the blood is a medium of elimination of such dele- 

 terious products as urea, uric acid, water, carbon dioxide, etc. 



However, the afferent function of the blood is not simply single, 

 for it conveys to the tissues in addition that material, all-impor- 

 tant for successful combustion, namely, oxygen, which has been 

 obtained from the respired air of the lungs. Among warm-blooded 

 animals another office served by the blood is to equalize to a certain 

 degree the temperature of the body. 



Color. There are certain characteristics which distinctly mark 

 blood from other fluids. The color of the blood of the vertebrata is 

 generally red. Its shade is, however, not fixed. As the blood-stream 

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