Secretion by the gastric glands is stimulated by the hormone gastrin. Gastrin is released by endocrine cells in the stomach in response to the arrival of food.
Absorption in the stomach
Very little occurs. However, some water, certain ions, and such drugs as aspirin and ethanol are absorbed from the stomach into the blood (accounting for the quick relief of a headache after swallowing aspirin and the rapid appearance of ethanol in the blood after drinking alcohol).
As the contents of the stomach become thoroughly liquefied, they pass into the duodenum, 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.
The liver secretes bile. Between meals it accumulates in the gall bladder. When food, especially when it contains fat, enters the duodenum, the release of the hormone cholecystokinin (CCK) stimulates the gall bladder to contract and discharge its bile into the duodenum.
- bile acids. These amphiphilic steroids emulsify ingested fat. The hydrophobic portion of the steroid dissolves in the fat while the negatively-charged side chain interacts with water molecules. The mutual repulsion of these negatively-charged droplets keeps them from coalescing. Thus 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 pigments. These are the products of the breakdown of hemoglobin removed by the liver from old red blood cells. The brownish color of the bile pigments imparts the characteristic brown color of the feces.
The hepatic portal system
The capillary beds of most tissues drain into veins that lead directly back to the heart. But blood draining the intestines is an exception. The veins draining the intestine 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:
- Glucose is removed and converted into glycogen.
- Other monosaccharides are removed and converted into glucose.
- Excess amino acids are removed and deaminated.
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