Chap. 12 



CIRCULATION AND TRANSPORTATION BODY FLUIDS 



215 



cortex 



afferent lymphatics 



capsule 



medulla 



trabeculae 



lymph 

 sinus 



efferent lymphatic 



Fig. 12.14. Diagram of a lymph node sectioned to show its internal structure, 

 the pocket endings of the vessels, and the valves within them that prevent back- 

 ward flow. The lymph spaces are shown with their usual contents of lymphocytes 

 (black). Such nodes are situated at strategic points on the lymph vessels and act 

 as filters removing bacteria otherwise entering the lymph. (Courtesy, Nonidez and 

 Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 

 1953.) 



Main Circuits. Circulating blood goes through two main circuits, the pul- 

 monary route from the heart to the lungs and back to the heart, and the 

 systemic route from the heart over a long course through the body and back 

 to the heart (Figs. 12.15, 12.16). It is calculated that in man the complete 

 double circuit of the blood takes about 23 seconds. 



Two factors are of great importance in the movement of blood. The first is 

 the pumping of the heart and the second, the resistance that the blood en- 

 counters along the sides of the vessels and at the forking of their manifold 

 branches. Blood that is pumped from the right ventricle into the pulmonary 

 artery to the lungs and back to the left auricle meets with little resistance in 

 this short circuit as compared with that in the long route through the aorta 

 and over most of the body. With the contraction of the left auricle, blood 

 flows into the left ventricle, the main pumping chamber of the heart (Fig. 

 12.17). The contraction of this chamber forces the blood past the semilunar 

 valves into the aorta. From there it begins the great systemic circuit through 

 several arteries: to the head through the carotids, to the arms through the 

 subclavians, and to the viscera, trunk, and legs through the dorsal aorta. 



Two special parts of this circuit are of most vital importance. The first is 

 the circulation in the walls of the heart. Although the chambers of the heart 

 are continually receiving blood, the action of the muscle in its walls depends 

 upon the coronary arteries from the aorta for an income of blood, and upon 

 the coronary veins emptying into the right auricle for its outgo. Any reduction 

 of blood to these muscles cripples the heartbeat and a complete block of 



