PHYSIOLOGY FOR BEGINNERS 



CHAP. 



walls fall together. When an artery is cut across the vessel 

 does not collapse, because the walls are thicker and contain 

 much elastic tissue, which keeps the vessel open. 



Blood Pressure 



The aorta and its branches form, then, a system of elastic 

 tubes, which lead through an enormous number of very fine tubes 

 the capillaries into wider tubes again, the veins. The blood has 

 to be forced through an enormous number of capillaries, some 

 of which are, as we have seen, so small that the red corpuscles 



A V 



FIG. 47. An artery and corresponding vein cut across. 



A, artery ; V, vein ; e.c, endothelial cells ; rn, muscular coat ; c, connective tissue 

 coat ; n, nuclei of endothelial cells. 



are squeezed out of shape as they are being forced through 

 them. This gives rise to a large amount of friction. The friction 

 of the blood in passing through the capillaries and small arteries 

 is the resistance which the left ventricle has to overcome. The 

 aorta and the other arteries, the capillaries and the veins, are 

 always full of blood, for there is never any space in them un- 

 filled with blood. If you take a piece of soft rubber-tubing, 

 open at both ends, and put it in a basin of water, the tubing 

 will become full of water. If, now, you tie one end of the tubing 

 you can, by means of a syringe at the other end, pump still 

 more water into the tubing, distending it in so doing. You could 

 do the same with a piece of artery. Now during life the aorta 

 and the arteries are naturally more than full, they are always 



