CHAPTER XIII 

 THE PHYSIOLOGY OF THE CIRCULATION 



SECTION I 

 GENERAL FEATURES OF THE CIRCULATION 



IN order that the nutrition of the tissues may be properly carried out, 

 and that they may receive a continual supply of nourishment from 

 the alimentary canal, and of oxygen from the lungs, and be able to free 

 themselves of their waste products, the blood which flows through 

 them must be continually renewed. For this purpose every part of 

 the body is supplied with tubes blood-vessels of various sizes and 

 structure. 



In the tissues the blood is passing continuously through a thick 

 rneshwork of capillaries, hair-like vessels with walls consisting of a 

 single layer of delicate endothelial cells which permit of a free inter- 

 change of material by diffusion between the blood within and the 

 tissue fluid outside the vessel. The movement of the blood is main- 

 tained by a hollow muscular organ, the heart, placed in the chest, the 

 blood being brought from the heart to the tissues by thick-walled 

 tubes, the arteries, and being carried back from the tissues to the 

 heart by a system of thin-walled vessels, the veins. 



In all the vertebrates the vascular system is closed, i.e. communi- 

 cates at no point with the tissue spaces or ccelomic cavity. It is found 

 in its simplest form in fishes (Fig. 367, A), where the heart consists of one 

 auricle and one ventricle. The blood is received from the great veins 

 into the auricle. The walls both of auricle and ventricle contract 

 rhythmically. By the contraction of the auricle the blood is forced 

 into the ventricle, and this, when it contracts, sends the blood on 

 into the bulbus arteriosus. From the bulbus the blood passes through 

 the branchial arteries into the gills, where it takes up oxygen from the 

 surrounding water, and then flows on into the aorta, by which it is 

 distributed to the various organs of the body. From the capillaries 

 of these organs the blood is collected by the veins and is carried once 

 more back to the auricle. The fish heart is thus entirely on the 

 venous side of the vascular system. 



In amphibia, such as the frog, the heart consists of two auricles and 



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