492 



ORGAN SYSTEMS OF MAN 



qill capillaries 



body capillaries 

 lunq capillaries 



Fig. 19-2. A schematic portrayal of the evolution of the 

 vertebrate circulatory system. While the simple two- 

 chambered heart v/as satisfactory for an aquatic form, 

 it could not handle the problem of transporting oxygen 

 in sufficient quantities for the active air-breathing land 

 dvtrellers. The second heart (pulmonary heart) had its 

 beginnings in the amphibians and reached its full- 

 fledged condition in the higher reptiles, birds, and 

 mammals. 



ments. Because this is so slow, the animal 

 itself responds feebly to its environment; 

 hence these are all slow-moving creatures. 



Although a rudimentary circulatory sys- 

 tem is found for the first time among the 

 nemertines ( Fig. 10-8 ) , it was not until the 

 annelids appeared that a well-differentiated 

 system of closed tubes was established as 

 an essential organ system. The circulatory 

 system of the earthworm, for example, is a 

 well-defined, efficient system. Like all trans- 

 port systems, it is composed of a circulating 

 fluid, blood, confined to closed vessels 

 which at some point break up into very 

 tiny tubules. These tubules have walls only 

 one cell layer thick and they are in close 

 association with all of the cells of the body. 

 A pump is provided to keep the fluid flow- 

 ing continuously. Once such a system was 

 established, the bulk of the animal could 

 increase very greatly with safety and this 

 is exactly what happened, as demonstrated 

 by the great dinosaurs of the past and the 

 huge whales of today. 



Evolution in the vertebrate circulatory 

 system is one of the many interesting 

 changes that accompanied the transition 

 from aquatic to terrestrial existence as de- 

 scribed in an earlier chapter. This can be 

 briefly summed up with the aid of a sketch 

 (Fig. 19-2). In fish, the circulatory flow of 

 blood was between the respiratory organ 

 (gills) and the body tissues. The two- 

 chambered heart forced the blood tiirough 

 arteries into the gill capillaries where exter- 

 nal respiration occurred. These capillaries 

 coalesced to form arteries which carried 

 blood to tissue capillaries in all parts of 

 the body where internal respiration took 

 place. The capillaries then united to form 

 veins which conveyed the blood back to 

 the heart — a very simple system. 



When vertebrates made the transition to 

 land life, a circuit to care for the newly 

 acquired lungs was established. This meant 

 the construction of a new heart superim- 

 posed on the old one. The beginning of this 

 development is seen in amphibians where 



