by Dr. Gary Farr on 20 June 2003
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What is it?
The stomach is a hollow and muscular organ whose interior surface consists of a series of expandable folds, called rugae. The rugae allow the stomach to expand and contract in relation to the volume of food and fluid. Although rarely necessary, the stomach can expand to a capacity of seven liters! The stomach functions to mix, store, and begin digestion of food. In smaller amounts, food is allowed to pass through the pyloric sphincter, at the bottom of the stomach, into the small intestine. Click on the image to the right for a larger view.
How does it work?
The stomach stores, dissolves, and partially digests the contents of a meal, then delivers this partially digested food to the small intestine in amounts optimal for maximal digestion and absorption. Parietal cells within gastric glands in the folds of the stomach lumen secrete hydrochloric acid (HCl), which makes gastric juice acidic, with a pH less than 2. During a meal, the rate of HCl production increases markedly—seeing, smelling, tasting, and chewing food sends information through the vagus nerves to the parietal cells, causing them to increase acid production. Stomach distention, hydrogen ion concentration, and peptides send messages through long and short neural reflexes to increase gastrin release, which increases HCl production. On average, the stomach produces 2 liters of HCl daily.
icroscopic View: Gastric Mucosa
The lining of the stomach contains deep collections of cells organized into gastric glands. These gastric glands secrete various substances into the stomach. The openings of the gastric glands into the surface of the stomach are called gastric pits. Mucous cells in the gastric pits secrete mucus. In the deeper part of the gland, parietal cells secrete hydrochloric acid. G cells, which are present predominantly only in the last portion of the stomach, secrete gastrin. ECL cells secrete histamine, and chief cells secrete pepsinogen (an inactive form of the pepsin-digesting enzyme pepsin). Intrinsic factor, needed for the absorption of vitamin B12, is also secreted by the gastric mucosa (most likely the parietal cells).
Parietal cells: Acid Production
There are three pathways leading to acid production by the parietal cell: the acetylcholine, gastrin, and histamine receptor pathways. These three pathways interact and overlap with each other significantly. Acetylcholine is secreted at the sight, smell, and taste of food; gastrin and histamine are released as a result of swallowed food in the stomach. When acetylcholine, gastrin, or histamine binds to its receptor on the parietal cell, a process is initiated that results in acid production.
The sight, smell, and taste of food cause salivation and the stimulation of the vagus nerve to release acetylcholine. View a Shockwave™ Animation of an Acetylcholine Receptor Pathway. (128kB, 53 seconds at 28.8kbps).
Digested food in the stomach chemically stimulates the release of gastrin from G cells located in the last portion of the stomach. Distention (an expansion) of the stomach causes release of acetylcholine from the vagus nerve, and this further stimulates the G cells to produce gastrin. Gastrin travels through the bloodstream and binds to the gastrin receptor on the parietal cells, located in the body of the stomach. View a Shockwave™ Animation of an Gastrin Receptor Pathway. (122kB, 51 seconds at 28.8kbps).
When histamine binds to its receptor, the parietal cell allows calcim to move into the cell. Through a series of chemical reactions, hydrogen (H+) eventually combines with chloride ions (Cl-) to form hydrochloric acid (HCl). View a Shockwave™ Animation of a Histamine Receptor Pathway. (177kB, 1 minute, 14 seconds at 28.8kbps).
Gastroesophageal reflux occurs when the stomach's contents back up into the esophagus. View a Shockwave™ Animation of gastroesophageal reflex here.
Conditions of the Stomach
To view the various conditions involving the stomach go to the {conditions} Condition Center. {fre_test_digest}{find_doctor_chiro}
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