The Circulatory System - 327 



SYSTEMCS 

 COWS 



RIGHT AURICLE 



VENTRICLE 



AORTA 

 PULMONARY 



ARTERY 



RIGHT AURICLE 

 LEFT AURICLE 



RIGHT VENTRICLE 

 LEFT VENTRICLE 



Fig. 17-8. Stages in the development of the mammalian heart (ventral view). 

 Note the resemblance of stages B and C to the fish and amphibian hearts, 

 respectively. Also see page 284, embryonic development of the heart. 



direction, due to the presence of numerous 

 valves in the blood vessels. 



The Blood Vessels. The blood vessels of 

 an adult vertebrate are defined in relation to 

 the heart: arteries lead away from the heart; 

 veins lead toward the heart; and capillary 

 networks connect the arteries with the veins. 



In man there are two arterial systems: 

 (1) the pulmonary system, which carries 

 blood from the right ventricle to the lungs; 

 and (2) the systemic system, which leads from 

 the left ventricle to all other organs in the 

 body. The main trunk of the pulmonary sys- 

 tem is the pulmonary artery (Fig. 17-10), 

 which sends a branch directly to each lung; 



and the main trunk of the systemic circuit is 

 a very large artery, the aorta, which sends 

 smaller branches to every part of the body, 

 except the lungs (Fig. 17-10). The artery to 

 each organ ends in a capillary network that 

 permeates all the tissues of that organ. The 

 venous system begins as many small tribu- 

 taries from the capillaries in the various 

 organs, and terminates in the large veins, 

 which enter the auricles (Fig. 17-10). In gen- 

 eral, the artery to a particular organ runs 

 side by side with the vein from the same 

 organ, so that the artery and vein are given 

 corresponding names (for example, renal 

 arteries and renal veins; see Fig. 17-10). How- 



AORTIC LOOPS 



WALL OF DIGESTIVE TRACT 



.DORSAL VESSEL 



BODY WALL 



COELOM 



VENTRAL VESSEL 



Fig. 17-9. The main blood vessels of the earthworm. The dorsal vessel runs above the 

 digestive tract and conducts the blood forward (left); the ventral vessel is below the 

 digestive tract and conducts the blood backward (right); and the five pairs of aortic loops 

 encircle the esophagus. 



