FUNCTION OF BLOOD-VESSELS 



145 



result, the blood is pushed for- 

 ward from one section to an- 

 other on its way to the heart. 

 The largest veins of the trunk, 

 however, do not have valves. In 

 them, the forward movement of 

 the blood is largely due to the 

 variations in pressure exerted 

 upon them by the movements of 

 the chest in breathing. 



General function of blood- 

 vessels. The work of the arte- 

 rial system as a whole is thus 

 seen to be to carry the blood in 

 adequate amounts to the capil- 

 laries. The work of the capil- 

 laries is to get the blood into 

 such close contact with the cells 

 of the tissues that oxygen and 

 nourishment may be given to 

 them and waste products re- 

 moved from them. The work of 

 the venous system is to return 

 the used blood back to the oppo- 

 site side of the heart, ready to 

 start on its pulmonary circuit. 



Pulmonary circulation. In 

 the pulmonary system, the struc- 

 ture and relation of the arteries 

 and veins are similar to those in 

 the systemic. The capillaries in 

 the lungs, however, differ from 

 the systemic capillaries in that 

 they are larger and have a dif- 



FIG. 86. Superficial view of 

 right arm showing the fascia- 

 aud veins, and the method of 

 branching of the veins by 

 which many other paths are 

 open to the returning blood 

 in case of obstructing pressure 

 or injury to the direct path. 



