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Medicine LibreTexts

21.4: The Stomach

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Learning Objectives
  • Label on a diagram the four main regions of the stomach, its curvatures, and its sphincter
  • Identify the four main types of secreting cells in gastric glands, and their important products
  • Explain why the stomach does not digest itself
  • Describe the mechanical and chemical digestion of food entering the stomach

Although a minimal amount of carbohydrate digestion occurs in the mouth, chemical digestion really gets underway in the stomach. An expansion of the alimentary canal that lies immediately inferior to the esophagus, the stomach links the esophagus to the first part of the small intestine (the duodenum) and is relatively fixed in place at its esophageal and duodenal ends. In between, however, it can be a highly active structure, contracting and continually changing position and size. These contractions provide mechanical assistance to digestion. The empty stomach is only about the size of your fist, but can stretch to hold as much as 4 liters of food and fluid, or more than 75 times its empty volume, and then return to its resting size when empty. Although you might think that the size of a person’s stomach is related to how much food that individual consumes, body weight does not correlate with stomach size. Rather, when you eat greater quantities of food—such as at holiday dinner—you stretch the stomach more than when you eat less.

Popular culture tends to refer to the stomach as the location where all digestion takes place. Of course, this is not true. An important function of the stomach is to serve as a temporary holding chamber. You can ingest a meal far more quickly than it can be digested and absorbed by the small intestine. Thus, the stomach holds food and parses only small amounts into the small intestine at a time. Foods are not processed in the order they are eaten; rather, they are mixed together with digestive juices in the stomach until they are converted into chyme, which is released into the small intestine.

As you will see in the sections that follow, the stomach plays several important roles in chemical digestion, including the continued digestion of carbohydrates and the initial digestion of proteins and triglycerides. Little if any nutrient absorption occurs in the stomach, with the exception of the negligible amount of nutrients in alcohol.

Structure

There are four main regions in the stomach: the cardia, fundus, body, and pylorus (Figure 21.4.1). The cardia (or cardiac region) is the point where the esophagus connects to the stomach and through which food passes into the stomach. Located inferior to the diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the fundus is the body, the main part of the stomach. The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga.

2414_Stomach.jpg
Figure 21.4.1: Stomach. The stomach has four major regions: the cardia, fundus, body, and pylorus. The addition of an inner oblique smooth muscle layer gives the muscularis the ability to vigorously churn and mix food.

The convex lateral surface of the stomach is called the greater curvature; the concave medial border is the lesser curvature. The stomach is held in place by the lesser omentum, which extends from the liver to the lesser curvature, and the greater omentum, which runs from the greater curvature to the posterior abdominal wall.

Histology

The wall of the stomach is made of the same four layers as most of the rest of the alimentary canal, but with adaptations to the mucosa and muscularis for the unique functions of this organ. In addition to the typical circular and longitudinal smooth muscle layers, the muscularis has an inner oblique smooth muscle layer (Figure 21.4.2). As a result, in addition to moving food through the canal, the stomach can vigorously churn food, mechanically breaking it down into smaller particles.

2415_Histology_of_StomachN.jpg
Figure 21.4.2: Histology of the Stomach. The stomach wall is adapted for the functions of the stomach. In the epithelium, gastric pits lead to gastric glands that secrete gastric juice. The gastric glands (one gland is shown enlarged on the right) contain different types of cells that secrete a variety of enzymes, including hydrochloride acid, which activates the protein-digesting enzyme pepsin.

The stomach mucosa’s epithelial lining consists only of surface mucus cells, which secrete a protective coat of alkaline mucus. A vast number of gastric pits dot the surface of the epithelium, giving it the appearance of a well-used pincushion, and mark the entry to each gastric gland, which secretes a complex digestive fluid referred to as gastric juice.

Although the walls of the gastric pits are made up primarily of mucus cells, the gastric glands are made up of different types of cells. The glands of the cardia and pylorus are composed primarily of mucus-secreting cells. Cells that make up the pyloric antrum secrete mucus and a number of hormones, including the majority of the stimulatory hormone, gastrin. The much larger glands of the fundus and body of the stomach, the site of most chemical digestion, produce most of the gastric secretions. These glands are made up of a variety of secretory cells. These include parietal cells, chief cells, mucous neck cells, and enteroendocrine cells.

Parietal cells—Located primarily in the middle region of the gastric glands are parietal cells, which are among the most highly differentiated of the body’s epithelial cells. These relatively large cells produce both hydrochloric acid (HCl) and intrinsic factor. HCl is responsible for the high acidity (pH 1.5 to 3.5) of the stomach contents and is needed to activate the protein-digesting enzyme, pepsin. The acidity also kills much of the bacteria you ingest with food and helps to denature proteins, making them more available for enzymatic digestion. Intrinsic factor is a glycoprotein necessary for the absorption of vitamin B12 in the small intestine.

Chief cells—Located primarily in the basal regions of gastric glands are chief cells, which secrete pepsinogen, the inactive proenzyme form of pepsin. HCl is necessary for the conversion of pepsinogen to pepsin.

Mucous neck cells—Gastric glands in the upper part of the stomach contain mucous neck cells that secrete thin, acidic mucus that is much different from the mucus secreted by the goblet cells of the surface epithelium. The role of this mucus is not currently known.

Enteroendocrine cells—Finally, enteroendocrine cells found in the gastric glands secrete various hormones into the interstitial fluid of the lamina propria. These include gastrin, which is released mainly by enteroendocrine G cells.

Table 21.4.1 describes the digestive functions of important hormones secreted by the stomach.

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Watch this animation that depicts the structure of the stomach and how this structure functions in the initiation of protein digestion. This view of the stomach shows the characteristic rugae. What is the function of these rugae?

Table 21.4.1: Hormones Secreted by the Stomach
Hormone Production site Production stimulus Target organ Action
Gastrin Stomach mucosa, mainly G cells of the pyloric antrum Presence of peptides and amino acids in stomach Stomach Increases secretion by gastric glands; promotes gastric emptying
Gastrin Stomach mucosa, mainly G cells of the pyloric antrum Presence of peptides and amino acids in stomach Small intestine Promotes intestinal muscle contraction
Gastrin Stomach mucosa, mainly G cells of the pyloric antrum Presence of peptides and amino acids in stomach Ileocecal valve Relaxes valve
Gastrin Stomach mucosa, mainly G cells of the pyloric antrum Presence of peptides and amino acids in stomach Large intestine Triggers mass movements
Ghrelin Stomach mucosa, mainly fundus Fasting state (levels increase just prior to meals) Hypothalamus Regulates food intake, primarily by stimulating hunger and satiety
Histamine Stomach mucosa Presence of food in the stomach Stomach Stimulates parietal cells to release HCl
Serotonin Stomach mucosa Presence of food in the stomach Stomach Contracts stomach muscle
Somatostatin Mucosa of stomach, especially pyloric antrum; also duodenum Presence of food in the stomach; sympathetic axon stimulation Stomach Restricts all gastric secretions, gastric motility, and emptying
Somatostatin Mucosa of stomach, especially pyloric antrum; also duodenum Presence of food in the stomach; sympathetic axon stimulation Pancreas Restricts pancreatic secretions
Somatostatin Mucosa of stomach, especially pyloric antrum; also duodenum Presence of food in the stomach; sympathetic axon stimulation Small intestine Reduces intestinal absorption by reducing blood flow

Chapter Review

The stomach participates in all digestive activities except ingestion and defecation. It vigorously churns food. It secretes gastric juices that break down food and absorbs certain drugs, including aspirin and some alcohol. The stomach begins the digestion of protein and continues the digestion of carbohydrates and fats. It stores food as an acidic liquid called chyme, and releases it gradually into the small intestine through the pyloric sphincter.

Interactive Link Questions

Watch this animation that depicts the structure of the stomach and how this structure functions in the initiation of protein digestion. This view of the stomach shows the characteristic rugae. What is the function of these rugae?

Answer: Answers may vary.

Review Questions

Q. Which of these cells secrete hormones?

A. parietal cells

B. mucous neck cells

C. enteroendocrine cells

D. chief cells

Answer: C

Q. Parietal cells secrete ________.

A. gastrin

B. hydrochloric acid

C. pepsin

D. pepsinogen

Answer: B

Critical Thinking Questions

Q. Describe unique anatomical features that enable the stomach to perform digestive functions.

A. The stomach has an additional inner oblique smooth muscle layer that helps the muscularis churn and mix food. The epithelium includes gastric glands that secrete gastric fluid. The gastric fluid consists mainly of mucous, HCl, and the enzyme pepsin released as pepsinogen.

Glossary

body
mid-portion of the stomach
cardia
(also, cardiac region) part of the stomach surrounding the cardiac orifice (esophageal hiatus)
cephalic phase
(also, reflex phase) initial phase of gastric secretion that occurs before food enters the stomach
chief cell
gastric gland cell that secretes pepsinogen
enteroendocrine cell
gastric gland cell that releases hormones
fundus
dome-shaped region of the stomach above and to the left of the cardia
G cell
gastrin-secreting enteroendocrine cell
gastric emptying
process by which mixing waves gradually cause the release of chyme into the duodenum
gastric gland
gland in the stomach mucosal epithelium that produces gastric juice
gastric phase
phase of gastric secretion that begins when food enters the stomach
gastric pit
narrow channel formed by the epithelial lining of the stomach mucosa
gastrin
peptide hormone that stimulates secretion of hydrochloric acid and gut motility
hydrochloric acid (HCl)
digestive acid secreted by parietal cells in the stomach
intrinsic factor
glycoprotein required for vitamin B12 absorption in the small intestine
intestinal phase
phase of gastric secretion that begins when chyme enters the intestine
mixing wave
unique type of peristalsis that occurs in the stomach
mucosal barrier
protective barrier that prevents gastric juice from destroying the stomach itself
mucous neck cell
gastric gland cell that secretes a uniquely acidic mucus
parietal cell
gastric gland cell that secretes hydrochloric acid and intrinsic factor
pepsinogen
inactive form of pepsin
pyloric antrum
wider, more superior part of the pylorus
pyloric canal
narrow, more inferior part of the pylorus
pyloric sphincter
sphincter that controls stomach emptying
pylorus
lower, funnel-shaped part of the stomach that is continuous with the duodenum
ruga
fold of alimentary canal mucosa and submucosa in the empty stomach and other organs
stomach
alimentary canal organ that contributes to chemical and mechanical digestion of food from the esophagus before releasing it, as chyme, to the small intestine

Contributors and Attributions


This page titled 21.4: The Stomach is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by OpenStax via source content that was edited to the style and standards of the LibreTexts platform.

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