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, **. e. communicates at no point 

 with the tissue spaces or ccelomic cavity. It is found in its simplest form in fishes 

 (Fig. 382, 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 cavse and 

 forces it by its contraction into the ventricle. From the ventricle the blood passes 

 into the aorta, whence it is carried partly by the pulmonary artery to the lungs, partly 

 by arteries to the different organs of the body. The blood, which Jias passed through 

 the lungs and been arteriahsed, flows through the pulmonary veins to the left auricle, 

 whence it passes into the ventricle and mixes with the venous blood which is arriving 

 from the right auricle. The pulmonary circulation is thus merely a branch of the 

 general or systemic circulation. The bulbue aortse in the frog is divided into two parts 

 58 913 



