POSTCAVAL 

 VEIN 

 (OR INFERIOR 

 VENA CAVA) 



-RIGHT AURICLE 

 -HEPATIC VEINS 



Fig. 17-16. The hepatic portal circulation of man. 

 Note that blood, after absorbing food substances 

 from the gastrointestinal tract, passes directly to the 

 capillary system of the liver. A portal vein always 

 originates from and terminates in a capillary network. 



the hepatic veins, which carry blood away 

 from the liver. In vertebrate animals, it is 

 generally not difficult to identify the portal 

 vein, because this large vein always origi- 

 nates near the duodenum and runs to the 

 base of the liver, closely accompanied by the 

 bile duct. 



A portal vein not only originates from 

 capillaries but also terminates in capillaries; 

 whereas other veins originate from capil- 

 laries but terminate either by joining some 

 other vein, or by emptying into an auricle. 

 In man and other mammals, the hepatic 

 portal vein is the only large portal vein of 

 the body, but many lower vertebrates also 



The Circulatory System - 333 



possess a renal portal vein, which brings 

 blood to the kidney (Fig. 17-14). 



THE LYMPH AND ITS ORIGIN 



Blood in all capillaries continually loses 

 a small proportion of its fluid volume 

 by seepage through the capillary walls. 

 This fluid is the lymph, which filters from 

 the capillaries into the tissue spaces (Fig. 

 17-13). 



The formation of lymph is not an osmotic 

 process. It is a kind of forced filtration, which 

 is energized by blood pressure. Normal blood 

 is slightly hypertonic to the lymph, but the 

 osmotic tendency for water to return to the 

 blood is counterbalanced by blood pressure. 



At the arterial end of a capillary, blood 

 pressure is highest — being equivalent, on the 

 average, to a pressure of about 30 mm of 

 mercury. This pressure exceeds the lymph 

 pressure by about 22 mm, and since the 

 hypertonicity of the blood is only 15 mm, 

 the effective force that accounts for the 

 formation of new lymph is about 7 mm of 

 mercury. Near the venous end of the capil- 

 laries, where the blood pressure has dis- 

 sipated somewhat, there is a definite tendency 

 for fluid to re-enter the blood from the 

 lymph. However, the re-entering fluid is of a 

 smaller volume than that which filtered out 

 at the arterial end of the network. 



The Composition of Lymph. The lymph is 

 a force filtrate of the blood and consequently 

 lymph has a composition that is closely re- 

 lated to the composition of the plasma. How- 

 ever, the capillary walls tend to hold back 

 the colloidal components of the blood. Ac- 

 cordingly, lymph contains all the crystalloid 

 components (water, salts, glucose, amino 

 acids, urea, etc.) of the plasma, in amounts 

 that very closely approximate those in the 

 plasma. But the plasma proteins, being col- 

 loidal in their molecular dimensions, are 

 present in the lymph in much lesser amounts 

 than in the plasma. In fact the protein con- 

 tent of lymph is only about one third that of 

 plasma. 



