332 - Multicellular Animals, Especially Man 



tract, kidneys, and abdominal body wall. 

 Finally, the aorta terminates in the lower 

 extremity of the abdominal region, by fork- 

 ing into the iliac arteries, which carry blood 

 to the legs. 



DEVELOPMENT OF THE CIRCULATORY 

 SYSTEM 



At an early stage of development, the em- 

 bryonic circulation of man and other mam- 

 mals is distinctly similar to the circulation of 

 the dogfish and other primitive vertebrates. 

 In the embryo, there are five pairs of gill 

 clefts, and the six aortic arches branch off 

 from the ventral aorta (Fig. 17-15). However, 

 this arrangement does not persist as develop- 

 ment proceeds. Arches I, II and V are des- 

 tined to disappear. Arch IV, on the left side, 

 becomes the adult aortic arch; and on the 

 right arch IV becomes the right subclavian 

 artery. Of the other arteries associated with 

 the human heart, the carotids (Fig. 17-10) are 

 derived partly from arch III; and the pul- 

 monary arteries come from arch VI. 



THE HEPATIC PORTAL SYSTEM 



The vertebrate circulation displays still 

 another distinctive feature. Blood from the 



gastrointestinal tract does not return directly 

 to the heart, but first passes through the 

 capillaries of the liver (Fig. 17-16). This part 

 of the circulation is called the hepatic portal 

 system, and it has an important bearing 

 upon the metabolism of vertebrate animals 

 generally. Blood in the gastrointestinal capil- 

 laries becomes laden with a wide variety of 

 absorbed products. However, many of these 

 compounds never reach the general circula- 

 tion — because the liver intervenes between 

 the intestinal tract and all other parts of the 

 circulation (p. 333). 



The hepatic portal system of man is shown 

 in Figure 17-16. The hepatic portal vein is 

 formed by the confluence of three tributaries: 

 (1) the gastrosplenic vein, from the stomach 

 and spleen; (2) the superior mesenteric vein, 

 from the upper parts of the small intestine, 

 and (3) the inferior mesenteric vein, from 

 the lower small intestine and colon. The 

 portal vein passes directly to the liver and, 

 upon entering it, gives rise to a set of capil- 

 laries that extend throughout the liver tis- 

 sue. Eventually blood drains from the liver 

 by way of the hepatic veins, which join the 

 postcava. Accordingly it is important to dis- 

 tinguish clearly between the hepatic portal 

 vein, which brings blood to the liver, and 



Pulmonary artery 



Mouth 



entricle 



Fig. 17-15. Head of a hu- 

 man embryo, side view, show- 

 ing the development of the 

 arterial system. I, II, III, IV, V, 

 VI, aortic arches. I, II, and V 

 do not persist, while III forms 

 the internal carotid arteries, 

 IV forms a portion of the 

 aorta, and VI forms the pul- 

 monary arteries of the adult. 

 (From General B/o/ogy, by 

 James W. Mavor. The Macmil- 

 lan Co.) 



