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 coelom, which may be called 

 blood, becomes enclosed in branching systems of tubes, and its circula- 

 tion 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 



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