Day: May 2, 2010

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

As the contents of the stomach become thoroughly liquefied, they pass into the duodenum(Small intestine). Duodenum is the first segment (about 10 inches long) of the small intestine.

Two ducts enter the duodenum:

• one draining the gall bladder and hence the liver
• the other draining the exocrine portion of the pancreas.

Bile

• pH 7.6
• bile acid: large globules of fat (liquid at body temperature) are emulsified into tiny droplets (about 1 µm in diameter) that can be more easily digested and absorbed.
• Bile contains the products of the breakdown of hemoglobin removed by the liver from old red blood cells. The brownish/greenish color of the bile pigments imparts the characteristic brown color of the faeces.

Pancreatic juice

• The pancreas consists of clusters if endocrine cells and exocrine cells whose secretions drain into the duodenum.
• Contains:

1. sodium bicarbonate (NaHCO₃): This neutralizes the acidity of the fluid arriving from the stomach raising its pH to about 8.
2. pancreatic amylase: This enzyme hydrolyzes starch into a mixture of maltose and glucose.
3. pancreatic lipase: The enzyme hydrolyzes ingested fats into a mixture of fatty acids and monoglycerols. Its action is enhanced by the detergent effect of bile.
4. trypsin