530 



COLLEGE ZOOLOGY 



Right ptrium- 



Superior 

 vena cava 



Aorta 



Pulmonary 



artery 



Pulmonary 

 veins 

 Atrium- 



vena cava 

 Tricuspid valve 



atrium 



Bicuspid 



valve 



Semilunar 



valves 



Valve from atrium 



to ventricle (closed) 



Left ventricle 



Right ventricle 



Ventricle 



Semilunar valve 



(closed) 



Valve from 



atrium to 



ventricle (open) 



Semilunar valve 



(open) 



Figure 384. Diagram illustrating chambers of the heart and the action of the valves in man. 

 A, internal structure of heart and direction of blood flow through it. B, blood is flowing fiom 

 atrium to ventricle, and the semilunar valve is closed. C, the valve between the atrium and 

 ventricle is closed, and blood is forced past the semilunar valve into the artery. Note that be- 

 cause of the upright position of the body of man, the anterior vena cava is called the superior 

 vena cava, and the posterior vena cava is termed the inferior vena cava. (A after Walter and 

 B after Peabody and Hunt.) 



is forced from the right ventricle through 

 the pulmonary artery into the lungs, and 

 from the left ventricle through the aorta 

 to the rest of the body. Valves prevent the 

 blood from flowing back from the ventri- 

 cles into the atria and from returning to 

 the ventricles from the arteries (Fig. 384). 

 The human heart contracts about 72 times 

 per minute. What is responsible for the 

 heart beat is not known; it appears to be 

 due to factors within the heart itself and 

 not to nervous control from outside, since 

 some hearts continue to beat after they 

 have been removed from the body. 



Elastic tissue and muscle tissue are espe- 

 cially abundant in the walls of the arteries 

 (Fig. 385); they regulate the flow of blood 

 forward to the capillaries in the tissues. The 

 pressure exerted by the blood against the 

 walls of the arteries is known as the blood 

 pressure. When an artery is cut, blood spurts 

 from it, indicating that it is under pressure. 

 Blood from a cut vein flows out continu- 



ously, indicating low pressure. Blood pres- 

 sure depends on the force of the contracting 

 ventricles of the heart, on the elasticity of 

 the walls of the blood vessels, and on the re- 

 sistance to the flow through the vessels. High 

 blood pressure may be brought about by 

 reduction in the elasticity of the small 

 arteries, as in arteriosclerosis. If a finger is 

 placed on the radial artery on the thumb 

 side of the wrist, or on other arteries, a 

 distention will be felt corresponding to the 

 beat of the heart. This is due to the alter- 

 nate dilatation and elastic recoil of the 

 artery, due to the forcing of blood through 

 it by the contractions of the heart, and con- 

 stitutes the pulse. The character of the 

 heart's action can be determined by feeling 

 the pulse. 



The capillaries (Fig. 383) are minute 

 tubes with a thin wall, consisting of a single 

 layer of cells which allow passage of mate- 

 rial through them to and from the tissues. 

 They form networks connecting the smallest 



