162 



Mechanical and Electrical Character of the Heartbeat /9 : 3 



Sinus j 



Venosus f 



Superior Aorta 

 Vena Cava 



Pulmonary 

 Artery 



Right 

 Auricle 



Inferior 

 Vena Cava 



Pulmonary 

 Vein 



Left 



Auricle 



Incomplete 

 Septum 



(a) 



(b) 



Figure 3. Diagrams offish and reptile hearts, (a) Fish heart. 

 The muscular walls develop successively higher pressures in the 

 sinus venosus, auricle, and finally ventricle, (b) Reptile heart. 

 Note the incomplete septum allowing mixing of blood from 

 both auricles within the ventricle. 



Superior 



Vena Cava Aorta 

 from Head to Bod y 

 and Neck 



Pulmonary Artery 

 to Lungs 



Semilunar 

 Valve 



S-A Node 



A- V Node 



Tricuspid 

 Valve 



Pulmonary 

 Valve 



Inferior 

 Vena Cava 

 from Trunk 

 and Limbs 



Pulmonary Vein 

 from Lungs 



Mitral Valve 



A-V Bundle 



Complete 

 Septum 



Figure 4. Diagram of the human heart. Arrows show direc- 

 tion of blood flow. 



