SECTION V 



THE BLOOD-VASCULAR SYSTEM 



Revised for the Fifth Edition 

 By H. D. senior, M.B., F.R.C.S. 



PROFESSOR OF ANATOMY, NEW YORK UNrV'ERSITY 



I'^F^HE organs of circulation consist of a system of tubes or vessels which during 

 I life are filled with fluid constantly moving in one direction. The major 

 -L portion of the system is concerned with the continuous distribution of blood 

 throughout the body and is called the haemal or blood-vascular system. A 

 circumscribed part of the haemal circulation is differentiated into a rhythmically 

 contracting propulsory organ called the heart. The minor portion of the system 

 is called the lymphatic system. The lymphatic vessels convey fluid, the lymph, 

 from the tissues to the haemal system. 



The essential functions of the blood-vascular system are performed by the 

 smallest of all the blood-vessels, the capillaries [vasa capillaria], which form a 

 network pervading practically all the tissues of the body. Blood is carried to and 

 from the capillaries by larger vessels called the arteries and veins respectively. 

 The heart receives blood from the veins and propels it, in turn, into the arteries. 



One of the primary functions of the blood is the transmission of oxygen from the atmosphere 

 to the tissues. In order to do this the blood must of necessity pass through the respiratory 

 organ before being delivered to the body at large. In giU-breathing vertebrates, the blood, 

 having received oxygen in its passage through the giUs, passes on directly to the tissues. The 

 entire circuit is here accomplished by a single continuous chain of vessels in which capillaries 

 occur twice, once in the gills and again in the organs and tissues in general. In man, as in other 

 higher vertebrates, lungs assume the function of the gills. Having received oxygen in the lungs 

 the blood is returned again to the heart before being redistributed throughout the body. 

 There are thus in man two separate circuits or systems of blood-vessels, one traversing the lungs 

 and a second ramifying throughout the body. The former is known as the pulmonary circula- 

 tion; the latter as the systemic. Each has its own arteries, capillaries and veins; the heart is 

 common to both. From the time of birth the heart is longitudinally divided into right and left 

 halves, each of which contains its own independent stream of blood. The blood entering the 

 left side of the heart has issued from the pulmonary circulation and is drive« into the systemic; 

 that in the right side, having traversed the systemic circuit, is returned again to the lungs. 



The heart and blood-vessels have a continuous lining of flattened cells called endothelium; 

 the haemal system is, therefore, a closed circuit.* The main thickness of the heart, arteries 

 and veins consists of additional tissue developed around the endothelial lining. It is due to 

 this tissue that the blood is continuously delivered to and withdrawn from the capillaries under 

 suitable pressure and velocity. The heart is mainly composed of rhythmically contracting 

 muscle and its valves are so arranged that the contained blood is driven intermittently in one 

 direction only. The walls of the largest arteries are formed to a great extent of elastic tissue, 

 and, being constantly under tension from within, are instrumental in converting the stream, 

 intermittently received from the heart, into a continuous flow. The walls of the medium sized 

 to smallest arteries are mainly muscular. The smallest arteries are microscopic in size and 

 known as arterioles [arteriolse]. The muscular arteries are capable of general or local alterations 

 of calibre regulated by the nervous system ; they are thus largely concerned in the maintenance 

 of the blood pressure and in the regulation of the volume of blood entering given localities 

 under varj-ing conditions. The veins have much thinner walls than the arteries; the blood in 

 them is under low tension upon which they exercise little or no control. 



When an artery divides, the combined calibre of its branches is greater than that of the 

 vessel itself. Since the arteries divide repeatedly the bed of the blood-stream increases in 

 proportion as the vessels diminish in size. The rate of increase, slow at first, becomes enormous 

 in the arterioles. Conversely, the bed of flow undergoes contraction as the heart is approached 

 from the venous side. The velocity of flow in the capillaries must necessarily be much lower 



* In the spleen and bone marrow the blood-channels intermediate between the arteries and 

 veins are possible exceptions to this statement, but the essential conditions here are still imper- 

 fectly understood. 



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