The Human Gastrointestinal (GI) Tract

The strategy

Humans (and most animals) digest all their food extracellularly; that is, outside of cells.

The topology

The diagram shows the major topological relationships in the body. The linings of all are all continuous with the surface of the body. Anything placed within their lumen is, strictly speaking, outside the body. This includes

Ingestion

Food placed in the mouth is

The stomach

The wall of the stomach is lined with millions of gastric glands, which together secrete 400-800 ml of gastric juice at each meal. Three kinds of cells are found in the gastric glands

Parietal cells

Parietal cells secrete

Hydrochloric acid (HCl)

Parietal cells contain a H+ ATPase This transmembrane protein secretes H+ ions (protons) by active transport, using the energy of ATP. The concentration of H+ in the gastric juice can be as high as 0.15 M, giving gastric juice a pH somewhat less than 1. With a concentration of H+ within these cells of only about 4 x 10-8 M, this example of active transport produces more than a million-fold increase in concentration. No wonder that these cells are stuffed with mitochondria and are extravagant consumers of energy.

Intrinsic factor

Intrinsic factor is a protein that binds ingested vitamin B12 and enables it to be absorbed by the intestine. A deficiency of intrinsic factor causes pernicious anemia.

"Chief" Cells

The "chief" cells synthesize and secrete pepsinogen, the precursor to the proteolytic enzyme pepsin.

Pepsin cleaves peptide bonds, favoring those on the C-terminal side of tyrosine, phenylalanine, and tryptophan residues. Its action breaks long polypeptide chains into shorter lengths.

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:

The liver

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 contains:

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:

So 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

The pancreas

The pancreas consists of clusters if endocrine cells (the islets of Langerhans) and exocrine cells whose secretions drain into the duodenum.

Pancreatic fluid contains: The secretion of pancreatic fluid is controlled by two hormones:

The small intestine

Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi.

The large intestine (colon)

The large intestine receives the liquid residue after digestion and absorption are complete. This residue consists mostly of water as well as materials (e.g. cellulose) that were not digested. It nourishes a large population of bacteria (the contents of the small intestine are normally sterile). Most of these bacteria (of which one common species is E. coli) are harmless. And some are actually helpful, for example, by synthesizing vitamin K. Bacteria flourish to such an extent that as much as 50% of the dry weight of the feces may consist of bacterial cells.

Reabsorption of water is the chief function of the large intestine. The large amounts of water secreted into the stomach and small intestine by the various digestive glands must be reclaimed to avoid dehydration. If the large intestine becomes irritated, it may discharge its contents before water reabsorption is complete causing diarrhea. On the other hand, if the colon retains its contents too long, the fecal matter becomes dried out and compressed into hard masses causing constipation.

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6 June 1999