The capillary beds of most tissues drain into veins that lead directly back to the heart, however, blood from the intestines is an exception. The veins draining the intestines lead to a second set of capillary beds in the liver. Here the liver removes many of the materials that were absorbed by the intestine.

The hepatic portal system begins in the capillaries of the digestive organs and ends in the hepatic portal vein. As a result, portal blood contains substances that have been absorbed by the intestines and stomach. Portal blood is passed through the liver, where nutrients and toxins are absorbed or excreted. The liver acts as a filter, by screening blood so that its composition when it leaves will be close to normal for the body.

Restriction of flow through the hepatic portal system can lead to portal hypertension, most commonly associated with cirrhosis. This can lead to serious gastro-intestinal bleeding, among other things.

As organisms perform their various metabolic processes, many proteins and nucleic acids, both of which contain nitrogen, are broken down. While some of this nitrogen is used to produce other molecules, most of it can’t be used and must be disposed of. Many species, including humans, aren’t terribly concerned about water loss, so nitrogen is disposed of by converting it intro urea, which is water soluble, and is excreted in a water-based solution. Many other animals, however, like lizards and birds, are very concerned with water loss, and convert their nitrogen into uric acid, which is not water soluble, which can thus be excreted with little or no water in it (this is the white gooey stuff in bird droppings).

Nitrogenous wastes are excreted through the kidneys. They are supplied with blood with the renal artery. Urine made in the kidneys is stored in the renal pelvis within the kidney, then flows through the ureter and into the bladder. From the bladder, the urine flows outside through the urethra.

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