330 - Multicellular Animals, Especially Man 



The exchanges between capillary blood 

 and the surrounding tissues of a working 

 muscle are typical of such exchanges gen- 

 erally. Blood entering the muscle is well sup- 

 plied with oxygen absorbed from the lungs, 

 and with food compounds absorbed from the 

 gastrointestinal tract. The muscle fibers, how- 

 ever, contain very little of these substances, 

 which are continually consumed in metab- 

 olism. Consequently oxygen, glucose, and the 

 other nutrients keep passing from the blood 

 across the lymph and into the muscle fibers. 

 Conversely carbon dioxide and the other 

 metabolic wastes tend to reach a relatively 

 high concentration in the fibers, where they 

 are produced, and so the metabolic wastes 

 keep passing from the fibers, through the 

 lymph, and into the capillaries. 



SINGLE vs. DOUBLE CIRCULATIONS 



The circulatory system of man and other 

 higher vertebrates is a highly modified and 

 improved version of the circulation of the 

 lower vertebrates. The general pattern of 

 the circulation in primitive vertebrates is 

 well illustrated by the dogfish (Fig. 17-14). 

 This typical form possesses a relatively 

 simple two-chambered heart that pumps 

 the blood through just a single cir- 



cuit; and in passing through this one circuit 

 the blood performs all necessary func- 

 tions. 



As may be seen in Figure 17-14, the un- 

 aerated (unshaded) blood leaves the ventri- 

 cle via a large arterial trunk, the ventral 

 aorta; and there are five pairs of branches, 

 the aortic arches, which surround the gill 

 clefts and carry the blood to the dorsal aorta. 

 Before reaching the dorsal aorta, however, 

 the blood must flow through the capillaries 

 of the gills, where the blood becomes aerated 

 (shaded). 



The branches of the dorsal aorta extend to 

 all other parts of the body, carrying aerated 

 blood: to the kidneys, digestive tract, and all 

 structures in head, trunk, and tail (Fig. 17- 

 14). In all these regions the blood must flow 

 through a second network of capillaries, and 

 as it flows through the various organs, the 

 blood returns to an unaerated condition (un- 

 shaded in Fig. 17-14). The many smaller 

 veins, which collect unaerated blood from 

 all parts of the body, empty mainly into four 

 large veins: the two precardinal veins, which 

 drain the two sides of the head, and the two 

 postcardinal veins, which drain the kidneys, 

 gonads, and the body wall musculature in 

 the tail and trunk. These four major veins 

 then empty, via the sinus venosus, into the 



POSTCARDINAL VEIN 

 SINUS VENOSUS 

 DORSAL AORTA 

 PHARYNX 



PRECARDINAL VEIN 

 AORTIC ARCHES 



MOUTH 



Fig. 17-14. Circulation of the dogfish. Note that the aerated parts of the system are 

 shaded, whereas unaerated parts are unshaded, 



