This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
Before leaving the subject of the circulation, it remains to be pointed out that there are exceptions to the rule that the arteries continually divide till they reach the capillaries, and that the veins emerging from these bring the blood directly back to the heart. Looking at vertebrate animals generally, one finds many instances of arteries breaking up into small branches which reunite before reaching the capillaries, and such an arrangement is called a rete mirabile. There is only one instance of such a thing occurring in the human subject, namely, in the Malpighian corpuscles of the kidney. But there is a notable instance, occurring in man and all vertebrate animals, of a venous trunk branching again into twigs, which open into a second set of capillaries; and that is the portal vein. The portal vein receives all the blood returning from the stomach, intestines, and spleen, and divides into a right and a left branch, which enter the liver, and break up into branches which pour their contents into the capillaries of that organ, and then discharge their blood into the hepatic veins, which open into the vena cava inferior. Thus all the blood which goes to the stomach and intestines has to pass through two sets of capillaries before returning to the heart, and this is the blood on which the liver exercises its purifying power.