METABOLISM IN VERTEBRATES 



Fig. 3.7. Heart of the fros;, 

 with the ventral walls of the 

 \,entricle and atria rerriOved. 

 The arrows indicate the di- 

 rection of circulation. 



Carotid artery 

 Systemic artery 



Pulmocutaneous 



artery 



Right atrium 

 Semilunar valve 



Truncus 

 arteriosus 



Opening of 

 pulmonary vein 



Opening of 

 sinus venosus 

 Left atrium 

 Atrio ventricular 

 valve 



Ventricle 



by the endothelium, and continued branching increases the capacity of the 

 bed over that of the small arteries feeding it. All this is significant in con- 

 nection with blood pressure and its rate of flow. When the heart contracts, 

 a quantity of blood is suddenly forced into an artery which has much less 

 capacity than the ventricle. The blood pressure in the aorta of man in a 

 normal resting state is 90-100 mm. of mercury. As the cross-sectional area of 

 all the branches increases, that is, as the capacity of the vessels increases, 

 the blood pressure drops until it is only about 25 30 mm. of mercury as it 

 enters the capillary bed. Here, the pressure falls still more, down to about 

 15 mm. of mercury, and the blood moves relatively slowly; this is important in 

 connection with the exchanges that occur between blood and cells in capillary 

 beds. Veins have walls containing some non-striated muscle in addition to 

 the endothelial lining but have little ability to resist expansion; hence, the 



Fig. 3.8. Heart of man, with the 

 ventral walls of the ventricles and 

 atria removed. The arrows indi- 

 cate the direction of circulation. 



Innominate artery 

 Aorta 

 Precaval vein 



Right atrium 



Semilunar 

 valve 



Postcaval vein 

 Right ventricle 



Left carotid artery 



Left subclavian 

 artery 



Atrio- 

 ventricular 

 valve 



Left ventricle 



53 



