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 ali- 

 mentary 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 structures. 



In the tissues the blood is passing continuously through a thick mesh- 

 work of capillaries, hair-like vessels with walls consisting of a single layer 

 of delicate endothelial cells which permit of a free, interchange of material 

 by diffusion between the blood within and the tissue fluid outside the 

 vessel. The movement of the blood is maintained 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. communicates 

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

 simplest form in fishes (Fig. 371, 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 

 one ventricle. The right auricle receives venous blood from the body by 

 means of the venae cavae and forces it by its contraction into the ventricle. 



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