THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 91 



is high near the arterial ends of the capillaries (at X in Fig. 6.4), and here 

 fluid is lost. However, the hydrostatic pressure falls rapidly along the 

 course of each capillary, because of friction and loss of fluid, and the 

 colloidal proteins become more concentrated, causing an increase in 

 osmotic pressure within the capillary. As a consequence, the pressure 

 relations are reversed toward the venous ends of the capillaries (at Y 

 in Fig. 6.4), and here lymph tends to flow back through the capillary 

 wall into the blood stream. 



Diffusion also plays an important part in the blood-lymph relations. 

 Blood entering the capillaries is rich in oxygen and dissolved foods and 

 low in metabolic wastes. Lymph is low in food and oxygen, which are 

 continually removed from it by the cells, and is high in wastes received 

 from the cells. It results that oxygen and food substances continually 

 diffuse through the capillary walls into the lymph, and wastes diffuse in 

 the opposite direction into the blood. Of all the food substances, the 

 colloidal proteins pass through the capillary walls with the greatest 

 difficulty, their concentration being therefore always much greater in the 

 blood than in the lymph. 



During the inactive state of an organ, nearly all the lymph is drained 

 directly into the blood capillaries; but increased activity of the organ 

 brings greater blood supply and increases the formation of lymph. This 

 is partly the result of increased blood pressure but is more largely caused 

 by a great increase in the capillary surface. As was mentioned above, 

 only a fraction of the capillaries of a resting organ contain blood, the 

 remainder being collapsed and empty. In a resting muscle, for example, 

 a cross section of 1 sq. mm. of muscle tissue shows only about 200 open 

 capillaries. When the muscle becomes active, however, a combination of 

 nervous and chemical stimuli causes the small arteries and capillaries to 

 dilate, enormously increasing the blood flow through the organ. In a 

 cross section of 1 sq. mm. of active muscle, there are about 2,500 open 

 capillaries — more than 12 times as many as in the resting muscle. Each 

 capillary is furthermore dilated, offering more surface for filtration of 

 lymph into the tissue spaces. With the increase in leakage, the capillaries 

 can no longer absorb the lymph as rapidly as it forms, and the lymphatic 

 system comes into operation. 



The lymphatic system is an accessory circulatory or drainage system. 

 Like the blood circulatory system, it is made up of tubes, in this case 

 called lymph vessels; but unlike that system, it does not form a continuous 

 closed circuit but is a many-branched one-way drainage system com- 

 parable to a river with its affluents and head waters. It contains no pump- 

 ing organ and has nothing analogous to the arteries, but it may be likened 

 to the capillary and vein portions of the blood circulatory system. In 

 all the tissue spaces there are very delicate tubes, the lymph capillaries 



