Usually the capillary beds of most tissues drain into veins are lead directly back to the heart. However hepatic portal vein is an exception. In this case, blood draining the intestine is leaded to a second set of capillary beds in the liver. Then the liver removes many of the materials that were absorbed by the intestine:
- Glucose is removed and converted into glycogen.
- Other monosaccharides are removed and converted into glucose.
- Excess amino acids are removed and deaminated.
- The amino group is converted into urea.
- The residue can then enter the pathways of cellular respiration and be oxidized for energy.
- Many nonnutritive molecules, such as ingested drugs, are removed by the liver and, often, detoxified.
The liver serves as a gatekeeper between the intestines and the general circulation. It screens blood reaching it in the hepatic portal system so that its composition when it leaves will be close to normal for the body.
Furthermore, this homeostatic mechanism works both ways. When, for example, the concentration of glucose in the blood drops between meals, the liver releases more to the blood by
- converting its glycogen stores to glucose (glycogenolysis)
- converting certain amino acids into glucose (gluconeogenesis)
ref: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GITract.html