146 STUDIES IN PHYSIOLOGY 



roundabout course back to the heart. The veins from these 

 four organs unite to form the large portal vein, which enters 

 the liver, and there divides to supply a system of capillaries. 

 These in. turn send the blood into the he-pat'ic veins (Latin 

 hepaticus, referring to the liver), which empties into the in- 

 ferior vena cava. The peculiarity of the portal system lies 

 in the fact that the blood supplied by the aorta to these organs 

 of the abdomen passes through two sets of capillaries before 

 returning to the heart (see Figs. 55 and 36). In our study of 

 the liver we learned that one of its most important functions 

 is that of storing carbohydrate food in the form of glycogen. 

 The portal system of veins is an adaptation to this function, 

 for the blood from the stomach and the intestines is often 

 well supplied with sugar which may be left for a time in the 

 liver. Blood from the kidneys, on the other hand, has not 

 been supplied with this nutrient, and so passes directly into 

 the inferior vena cava. 



The liver, it will be remembered, contains one fourth of 

 all the blood in the body, and this blood is of two kinds. 

 One kind is furnished direct by a branch of the aorta (hepatic 

 artery); the other kind comes to the liver through the portal 

 vein. The latter is the only vein in the body that supplies 

 blood to capillaries. 



The Circulation but a Single System. To keep the course of 

 the blood clearly in mind, we have carefully distinguished 

 a pulmonary from a systemic circulation. In reality, how- 

 ever, there is but a single circulation in the body. The 

 blood cannot pass across from the right heart to the left 

 heart. In order to get back to the right ventricle, for 

 instance, a drop of blood must first go through a system of 

 capillaries in the lungs, must return to the left heart, and 

 must thence be driven through a second set of capillaries 

 before it reaches the point from which it started. And if, 

 perchance, this drop of blood goes through the tissues of 

 the stomach or intestines, or, in other words, through the 

 portal system, it must pass through three different sys- 



