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 meta- 

 bolism 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 some 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 

 themselves 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 back 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 and the results of nitrogenous metabolism, 

 and carries them away to the excretory organs, such as the lungs and 

 kidneys, by which they are eliminated. It is evident that the composition 

 of the blood must vary from time to time and place to place according 

 to the condition of activity and the function of the organ which it is 

 traversing. The organs of the body are adjusted to respond to very 

 minute changes in the composition of the circulating fluid, and add 

 to or subtract from its constituents according as these are present in 

 deficiency or excess. The changes are therefore kept within infinitesimal 

 limits ; in most cases they are within the limits of errors of analysis, 



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