The Human Gastrointestinal (GI) Tract

The strategy

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

  • Digestive enzymes are secreted from cells lining the inner surfaces of various exocrine glands.
  • The enzymes hydrolyze the macromolecules in food into small, soluble molecules that can be
  • absorbed into cells.

The topology

The diagram shows the major topological relationships in the body. The linings of all

  • exocrine glands, including digestive glands,
  • nasal passages, trachea, and lungs,
  • kidney tubules, collecting ducts, and bladder,
  • reproductive structures like the vagina, uterus, and fallopian tubes

are all continuous with the surface of the body. Anything placed within their lumen is, strictly speaking, outside the body. This includes

  • the secretions of all exocrine glands (in contrast to the secretions of endocrine glands, which are deposited in the blood).
  • Any indigestible material placed in the mouth which will appear, in due course, at the other end.


Food placed in the mouth is

  • ground into finer particles by the teeth,
  • moistened and lubricated by saliva (secreted by three pairs of salivary glands)
  • small amounts of starch are digested by the amylase present in saliva
  • the resulting bolus of food is swallowed into the esophagus and
  • carried by peristalsis to the stomach.

The stomach

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

Parietal cells

Parietal cells secrete

  • hydrochloric acid
  • intrinsic factor

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 ā€” as a result of an autoimmune attack against parietal cells ā€” 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.


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