THE TRANSPORTATION SYSTEM 



497 



Systemic 

 aorta 



pre cava 



pulmonary 

 aorta 



pulmonory v«ins 



IcFt auricia 

 right auricia 



bicuspid valve 

 tricuspid valve- 



post cava 



left ventricle 



right ventricle 



Fig. 19-7. The human heart is shown here cut so that the 

 are indicated in solid black, whereas the parts that are 

 way the parts operate during the cardiac cycle. 



capitulated in the developing mammalian 

 embryo (Fig. 19-6). 



The adult human heart is a muscular 

 organ composed of four principal cham- 

 bers, two auricles and two ventricles, and 

 a system of valves beautifully designed to 

 keep the blood going in one direction ( Fig. 

 19-7). The auricles are thin-walled, sac-like 

 chambers whose principal function is to col- 

 lect sufficient blood from the great veins to 

 fill the ventricles quickly the moment they 

 are empty. The ventricles, on the other 

 hand, are thick-walled chambers whose sole 

 function is to keep the blood forever on the 

 move throughout the vast network of ves- 

 sels. The left ventricle is thicker and larger 

 than the right because its task is a bigger 

 one, namely, forcing the blood throughout 

 the body, whereas the right heart merely 

 keeps the blood moving through the lungs. 

 The two hearts, however, are intimately as- 

 sociated and beat simultaneously even 

 though their circuits are distinct and sepa- 

 rate. The left auricle and ventricle are 

 separated by a pair of flaps, called the bi- 

 cuspid (mitral) valve; similarly, the right 

 auricle and ventricle are separated by three 



chambers and valves can be seen. The contracting portions 

 relaxed are left white. The two figures demonstrate the 



flaps, the tricuspid valve. These flaps of 

 tough tissue are held rigidly in place when 

 under pressure by tiny cords that extend to 

 the inner muscular walls of the ventricles. 

 The ventricles open into large arteries, 

 the left into the systemic aorta and the 

 right into the pulmonary artery. Very near 

 the openings are half-moon-shaped valves, 

 the semilunar valves, which prevent the 

 blood from returning to the ventricles once 

 it is forced out. Each valve is composed of 

 three thin-walled cups that fit very tightly 

 together when under pressure. These, as 

 well as the other valves, are arranged so 

 that the blood can pass only in one direc- 

 tion. This, then, is tlie pump that keeps the 

 blood circulating continuously throughout 

 life. 



The blood path through the heart 



Blood passes from the pre- and post- 

 cavas into the right auricle, the walls of 

 which then contract (systole), forcing the 

 blood through the mitral valve into the 

 right ventricle (Fig. 19-7). Actually, when 

 the ventricle empties and begins to relax 

 (diastole), the blood requires very little 



