The Circulatory System 161 



(H2O and CO=). The kidneys also help this function by eliminating ex- 

 cess acids or alkaline salts. Thus the acid-base balance of the blood 

 and body is stable at all times. 



THE ANATOMY OF THE VERTEBRATE CIRCULATORY 

 SYSTEM 



For the carrying out of its many functions, the vertebrate circula- 

 tory system consists of several essential parts. These are the central 

 pumping mechanism, the heart; the blood vessels which carry blood 

 away from the heart, the arteries; and those which carry the blood 

 toward the heart, the veins; and finally the small vessels which con- 

 nect the veins and arteries, the capillaries. 



The Heart. — The heart is basically a series of muscular enlarge- 

 ments of a main blood vessel, which have become variously modified. 

 (Fig. 47) Among the fish there are four main chambers: the sinus 

 venosus, a thin-walled sac for receiving venous blood ; the atrium or 

 auricle, also a thin-walled sac ; the muscular ventricle which is the 

 contractile portion; and the conus arteriosus which carries the blood 

 forward to the gills for oxygenation. From the gills the blood circu- 

 lates to the body. At this stage, there is but one auricle and one ven- 

 tricle, and all the blood is unoxygenated as it passes through the heart. 



The heart of the amphibians is modified from this basic structure, 

 a change which is correlated with terrestrial life and the development 

 of lungs of many of these forms. It is advantageous to keep the oxy- 

 genated and unoxygenated blood as separate as possible, and a second 

 shorter route to the lungs is established. The blood is returned to the 

 heart after being oxygenated in the lungs. This makes two separate cir- 

 cuits : the pulmonary, to and from the lungs, and the systemic, to and 

 from the body. The auricle in the amphibians is divided into a right 

 and left chamber by a median septum. The sinus venosus, which re- 

 ceives blood from the systemic circulation, opens into the right auricle. 

 The left auricle receives the blood from the lungs. Both of the 

 auricles, however, open into the single ventricle. The conus or truncus 

 arteriosus has separate branches going to the lungs, skin, and body. 

 Separation of the oxygenated from the unoxygenated blood is made pos- 

 sible by the spongy nature of the ventricle, by a special valve in the 

 conus arteriosus, and by the timing of the entrance of the blood into 

 the ventricle from the two auricles. 



