MULTICELLULAR ANIMALS 



,83 



vessels the confined blood exerts an appreciable pressure. Particu- 

 larly noticeable is the pressure in the arteries, as the heart is con- 

 stantly adding to their contents. The elastic walls of the arteries 

 yield to each pulse wave and thus tend to absorb the increase of 

 pressure. Arterial blood pressure is dependent on four conditions: 

 (i) The quantity of blood in the whole circulatory system; (2) The 

 resistance offered the flow of blood in the capillaries, small arteries 



ENDOTHELIUM , 





ARTERY 



VEIN 



Fig. 125. — Diagram of a cross section through an artery and a vein. Note the very 



great difference in the muscle layers. 



and small veins in the tissues; (3) The force and rate of heart beat; 

 (4) The elasticity of the walls of the arteries. If any one of these is 

 disturbed, the result is a disturbance of the blood pressure. On 

 occasion a blood vessel may rupture as a result of excessively high 

 pressure. 



Disposal of Wastes of Metabolism. Among other func- 

 tions performed by the liver is the conversion of nitrogenous wastes 



.NH2 

 in the blood to urea (C0\ ), which is the final chemical 



NH2 



