Anatomy of the Stomach

Male stomach layers anatomy, illustration
PIXOLOGICSTUDIO/SCIENCE PHOTO LIBRARY / Getty Images

The stomach is an organ of the digestive system. It is an expanded section of the digestive tube between the esophagus and small intestine. Its characteristic shape is well known. The right side of the stomach is called the greater curvature and the left the lesser curvature. The most distal and narrow section of the stomach is termed the pylorus—as food is liquefied in the stomach it passes through the pyloric canal into the small intestine.

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Anatomy of the Stomach

Anatomy of the Stomach
This image shows rugae (folds) on the surface an equine stomach. Richard Bowen

The wall of the stomach is structurally similar to other parts of the digestive tube, with the exception that the stomach has an extra oblique layer of smooth muscle inside the circular layer, which aids in performance of complex grinding motions. In the empty state, the stomach is contracted and its mucosa and submucosa are thrown up into distinct folds called rugae; when distended with food, the rugae are "ironed out" and flat. The image above shows rugae on the surface of a dog's stomach.

If the lining of the stomach is examined with a hand lens, one can see that it is covered with numerous small holes. These are the openings of gastric pits which extend into the mucosa as straight and branched tubules, forming gastric glands.

Source:
Republished with permission by Richard Bowen - Hypertexts for Biomedical Sciences

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Types of Secretory Epithelial Cells

Stomach Wall Tissue
Gastric mucosa showing gastric pits, pockets in the epithelium. Corbis via Getty Images / Getty Images

Four major types of secretory epithelial cells cover the surface of the stomach and extend down into gastric pits and glands:

  • Mucous cells: secrete an alkaline mucus that protects the epithelium against shear stress and acid.
  • Parietal cells: secrete hydrochloric acid!
  • Chief cells: secrete pepsin, a proteolytic enzyme.
  • G cells: secrete the hormone gastrin.

There are differences in the distribution of these cell types among regions of the stomach—for example, parietal cells are abundant in the glands of the body, but virtually absent in pyloric glands. The micrograph above shows a gastric pit invaginating into the mucosa (fundic region of a raccoon stomach). Notice that all the surface cells and the cells in the neck of the pit are foamy in appearance—these are the mucous cells. The other cell types are farther down in the pit.

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Gastric Motility: Filling and Emptying

Anatomy of the human stomach.
Anatomy of the human stomach. Stocktrek Images / Getty Images

Contractions of gastric smooth muscle serves two basic functions. First, it allows the stomach to grind, crush and mix ingested food, liquefying it to form what is called "chyme." Second, it forces the chyme through the pyloric canal, into the small intestine, a process called gastric emptying. The stomach can be divided into two regions on the basis of motility pattern: an accordion-like reservoir that applies constant pressure on the lumen and a highly contractile grinder.

The proximal stomach, composed of the fundus and upper body, shows low frequency, sustained contractions that are responsible for generating a basal pressure within the stomach. Importantly, these tonic contractions also generate a pressure gradient from the stomach to small intestine and are thus responsible for gastric emptying. Interestingly, swallowing of food and consequent gastric distention inhibits contraction of this region of the stomach, allowing it to balloon out and form a large reservoir without a significant increase in pressure—this phenomenon is called "adaptive relaxation."

The distal stomach, composed of the lower body and antrum, develops strong peristaltic waves of contraction that increase in amplitude as they propagate toward the pylorus. These powerful contractions constitute a very effective gastric grinder; they occur about 3 times per minute in people and 5 to 6 times per minute in dogs. There is a pacemaker in the smooth muscle of the greater curvature that generates rhythmic slow waves from which action potentials and hence peristaltic contractions propagate. As you might expect and at times hope, gastric distention strongly stimulates this type of contraction, accelerating liquefaction and hence, gastric emptying. The pylorus is functionally part of this region of the stomach—when the peristaltic contraction reaches the pylorus, its lumen is effectively obliterated—chyme is thus delivered to the small intestine in spurts.

Motility in both the proximal and distal regions of the stomach is controlled by a very complex set of neural and hormonal signals. Nervous control originates from the enteric nervous system as well as parasympathetic (predominantly vagus nerve) and sympathetic systems. A large battery of hormones have been shown to influence gastric motility—for example, both gastrin and cholecystokinin act to relax the proximal stomach and enhance contractions in the distal stomach. The bottom line is that the patterns of gastric motility likely are a result from smooth muscle cells integrating a large number of inhibitory and stimulatory signals.

Liquids readily pass through the pylorus in spurts, but solids must be reduced to a diameter of less than 1-2 mm before passing the pyloric gatekeeper. Larger solids are propelled by peristalsis toward the pylorus, but then refluxed backwards when they fail to pass through the pylorus - this continues until they are reduced in size sufficiently to flow thought the pylorus.

At this point, you may be asking "What happens to solids that are indigestible - for example, a rock or a penny? Will it remain forever in the stomach?" If the indigestible solids are large enough, they indeed cannot pass into the small intestine, and will either remain in the stomach for long periods, induce a gastric obstruction or, as every cat owner knows, be evacuated by vomition. However, many of the indigestible solids that fail to pass through the pylorus shortly after a meal do pass into the small intestine during periods between meals. This is due to a different pattern of motor activity called the migrating motor complex, a pattern of smooth muscle contractions that originates in the stomach, propagates through the intestines and serves a housekeeping function to periodically sweep out the gastrointestinal tract.

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Bailey, Regina. "Anatomy of the Stomach." ThoughtCo, Nov. 6, 2017, thoughtco.com/anatomy-of-the-stomach-373482. Bailey, Regina. (2017, November 6). Anatomy of the Stomach. Retrieved from https://www.thoughtco.com/anatomy-of-the-stomach-373482 Bailey, Regina. "Anatomy of the Stomach." ThoughtCo. https://www.thoughtco.com/anatomy-of-the-stomach-373482 (accessed December 14, 2017).