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CHAPTER XII 

 THE BLOOD 



IN the unicellular animals and in the lowest metazoa, the cells are bathed 

 by the medium in which the organisms live, and are therefore exposed to 

 all the changes in the composition of this fluid which may be brought about 

 by cosmic events. With the evolution of a body cavity filled with fluid, 

 the tissue cells are set free from the necessity of adapting their metabolism 

 to wide ranges of chemical composition, being bathed by an internal medium 

 which is maintained practically constant in its characters for any given 

 type. With increasing differentiation the fluid of the ccelom, which may 

 be called blood, becomes enclosed in branching systems of tubes, and its 

 circulation is provided for by the development of contractile chambers at 

 me point or points of the tubes. In all the higher animals, the blood, 

 the common medium and means of exchange for all parts of the body, 

 circulates through a closed system of tubes, a constant flow being kept up 

 by the action of the heart. It is separated from the tissue elements them- 

 selves by the walls of the blood vessels. The free interchange of material 

 between blood and tissues is facilitated by the tenuity of the vascular wall. 

 The interstices of the tissues contain a fluid, the ' tissue fluid/ any excess 

 of which is drained off by special channels known as lymphatics and carried 

 ack to the blood. Interchange between the blood and the tissue cells 

 can be effected partly by diffusion, partly by a direct exudation or filtration 

 of the fluid parts of the blood with certain of its constituents through the 

 capillary walls. Since the function of the blood is to act as the common 

 nutritive medium of all parts of the body, it has to convey food materials 

 from the digestive organs and oxygen from the lungs to the tissues. From 

 these it receives in exchange their waste products, namely, carbon dioxide 

 d the results of nitrogenous metabolism, and carries them away to the 

 excretory organs, such as the lungs and kidneys, by which they are elimin- 

 ated. It is evident that the composition of the blood must vary from time 

 time and place to place according to the condition of activity and the 

 ction of the organ which it is traversing. The organs of the body are 

 justed to respond to very minute changes in the composition of the cir- 

 ating fluid, and add to or subtract from its constituents according as 

 ese are present in deficiency or excess. The changes are therefore kept 

 ithin infinitesimal limits; in most cases they are within the limits of 



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