224 HUMAN BIOLOGY 



In their course the lymphatics are interrupted by nodes or 

 "glands," which act as sieves and hold back small particles, 

 e.g. bacteria and cancer cells, and keep them from being 

 widely spread through the rest of the body. When protecting 

 the body in this way they become enlarged and can then be 

 felt. Thus, when the tonsils are inflamed the lymph glands 

 of the neck may be swollen and tender. 



The multitudes of fmely branched arterioles which the 

 blood must pass through on its way to the capillaries offer a 

 considerable frictional resistance. When the heart beats 

 and discharges its contents, it must develop a pressure 

 which will drive the blood not only past this resistance, 

 but also through the capillary net and the veins. With each 

 fresh delivery of blood from the heart the elastic arteries 

 stretch to accommodate the extra contents, and while 

 the heart is resting and filling behind the outlet valves the 

 elastic recoil of the arterial walls presses the blood contin- 

 uously onward. Measurements show that the blood in the 

 arteries is under a high head of pressure, equal (in young 

 adults) to a column of about 120 millimeters of mercury 

 (about 5 inches) at the peak of the cardiac discharge into 

 them, and to about 80 millimeters (about 3 inches) just 

 before the next discharge. In the capillaries the pressure 

 has fallen to about 25 millimeters (about i inch), and it 

 falls progressively in the veins until its lowest point is found 

 as the blood enters the right chamber of the heart. 



Clearly the same amount of blood must pass through 

 the heart, the lungs, arteries, capillaries and veins at the 

 same time, or otherwise the circulation could not continue. 

 Since the total bed of the capillaries is much greater than 

 the cross-area of the aorta or the large veins, the blood 

 moves much more slowly in the capillaries than in either 

 the arterial or venous trunks. This slow flow in the capillaries 

 provides time for the important exchanges which occur in 

 this region. 



As we shall soon see, the circulation must vary greatly 

 in its service to the needy cells, according to their degree 

 of activity. The adjustments are brought about through 

 nervous control of the heart and blood vessels. The heart 

 can be made to beat rapidly or slowly by action of two 



