The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The GI tract differs substantially from animal to animal. For instance, some animals have multi-chambered stomachs. The major topological relationship in the body is:
- 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
Food placed in the mouth is first grounded into finer particles by the teeth, moistened and lubricated by saliva (secreted by three pairs of salivary glands). Then small amounts of starch are digested by the amylase present in saliva and the resulting bolus of food is swallowed into the esophagus and carried by peristalsis to 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, chief cells and the mucus-secreting cells. (Canagatnam, 2004)
Parietal cells secrete hydrochloric acid and intrinsic factor. 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 is a protein that binds ingested vitamin B12 and enables it to be absorbed by the intestine.................
