CHAPTER XIII 

 THE CIRCULATION OF THE BLOOD 



Each organ and tissue of the body is the seat of a more or less active 

 metabolism, the maintenance of which is essential to its physiologic activity. 

 This metabolism is characterized by the assimilation of food materials and 

 the production of waste products; that it may be maintained it is imperative 

 that there shall be a continuous supply of the former and a continuous 

 removal of the latter. Both conditions are subserved by the blood. 



The Circulatory Apparatus. The apparatus by which the foregoing 

 results are attained consists of (i) a central double organ, the heart, 

 which by its pumping action imparts movement to the blood. (2) A 

 series of branching diverging tubes, the arteries, which originating in 

 the'left side of the heart distribute the blood to all regions of the body. 



(3) A network of minute vessels, the capillaries, which, by reason of the 

 thinness of their walls, permit on the one hand the passage of the nutritive 

 materials of the blood into the surrounding tissue spaces and on the other 

 hand, the passage of waste materials from the tissue spaces into the blood. 



(4) A series of uniting and converging tubes, the veins, which, originating 

 in the capillaries, collect and return the blood to the right side of the heart. 



From the right side of the heart a second series of arteries, capillaries, and 

 veins, respectively distribute the blood to the lungs, permit of the passage 

 of carbon dioxid into the interior of the lungs and of 'oxygen from the 

 interior of the lungs into the blood, and return the blood to the left side of 

 the heart. These structures are so related one to the other as to form a 

 closed system within which the blood is kept in continuous movement, 

 is made to flow into and out of the tissues in volumes varying with their 

 activities, with a certain velocity and under a given pressure. On the 

 maintenance of these conditions only can the blood fulfill its functions 

 and the irritability and the physiologic activity of the tissues be maintained. 



In this system a particle of blood which passes any given point will 

 eventually return to the same point, no matter how intricate or tortuous the 

 route may be through which it in the meanwhile travels; for this reason 

 the blood is said to move in a circle, and the movement itself is termed 

 the circulation. 



In order to understand the reasons for the movement of the blood in one 

 direction only, as well as for many other phenomena connected with the 

 circulation, a knowledge of the structure of the heart and its internal mechan- 

 ism is of primary importance. 



THE PHYSIOLOGIC ANATOMY OF THE HEART 



The heart is a conic or pyramid-shaped hollow muscle situated 

 in the thorax just behind the sternum. The base is directed upward and to 



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