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stomach
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What is the stomach?
The stomach is a digestive organ between the esophagus and small intestine. It receives ingested food and liquids, retaining them for grinding and mixing with gastric juice. The stomach is located in the upper-left part of the abdomen, below the diaphragm.
The stomach has several parts: the cardia, fundus, body, antrum, and pylorus. The inner surface is lined by the gastric mucosa, which contains cells that secrete mucus, enzymes, and hormones to aid in digestion and protect the stomach lining. The stomach muscles have three layers that help in grinding and mixing food. The stomach can expand to hold more than a quart of food or liquid.
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Where is the stomach located in the human body?
The stomach, a sac-like organ of the digestive system, is in the upper-left section of the abdominal cavity. Positioned just below the diaphragm, the stomach is flanked by the liver, a portion of the diaphragm, and the anterior abdominal wall in the front, while the pancreas, left kidney, adrenal gland, spleen, and colon lie behind it.
The stomach connects to the esophagus and small intestine and has four regions: the fundus, body, antrum, and pylorus. The stomach stores ingested food, initiates carbohydrate and protein digestion, transforms meals into chyme, and then discharges the chyme into the small intestine.
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What are the main functions of the stomach?
The stomach's primary functions involve receiving, storing, and preparing ingested food for digestion in the small intestine. Gastric glands in the stomach lining secrete enzymes and hydrochloric acid to initiate the digestion of proteins and fats. The stomach muscles then contract to churn food into a semifluid mixture called chyme, which is then gradually moved into the small intestine. The stomach also produces a thick mucus that protects it from being digested by its own enzymes. Although the stomach absorbs very little, it can directly absorb iron, alcohol, and some drugs.
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What are the main parts or sections of the stomach called?
The stomach, which receives ingested food from the esophagus, has four main sections:
* Fundus The fundus is the upper part of the stomach that sits above the opening from the esophagus.-
Body The body is the main, central, and largest part of the stomach and acts as a reservoir for food and liquids.
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Antrum The antrum is the lower, funnel-shaped section of the stomach.
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Pylorus The pylorus is a narrowing that connects the stomach to the small intestine. The pylorus has a sphincter muscle that keeps the stomach closed, except when food is passing through.
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How does the stomach help break down food during digestion?
The stomach breaks down food through a combination of mechanical and chemical processes. After food enters, the stomach muscles begin to contract, churning and kneading the food into a semi-fluid mixture called chyme.
Gastric glands in the stomach lining secrete digestive juices, including hydrochloric acid and enzymes like pepsin, which break down proteins and fats. The gastric mucosa, the stomach's inner lining, contains cells that secrete mucus and bicarbonate, protecting the stomach from its own digestive acids. Once the food particles are small enough, they pass into the small intestine for further digestion and absorption.
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What are some important muscles and tissues that make up the stomach's structure?
The stomach's structure comprises several important muscles and tissues. The stomach wall consists of three layers of muscles: the external longitudinal muscle layer, the middle circular muscular layer, and the innermost oblique muscular layer. The pyloric sphincter is a thickening of the circular muscle layer at the pyloric end.
The inner surface of the stomach is lined by the gastric mucosa, a mucous membrane covered by a layer of thick mucus secreted by tall columnar epithelial cells. The gastric mucosa contains different types of cells, including mucoid cells, zymogenic or chief cells, gastrin cells, parietal or oxyntic cells, and enterochromaffin-like cells. Beneath the gastric mucosa is the muscularis mucosae, a thin layer of smooth muscle. The submucosa, composed of loose connective tissue, attaches the gastric mucosa to the stomach walls' muscles.
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stomach, saclike expansion of the digestive system, between the esophagus and the small intestine; it is located in the anterior portion of the abdominal cavity in most vertebrates. The stomach serves as a temporary receptacle for storage and mechanical distribution of food before it is passed into the intestine. In animals whose stomachs contain digestive glands, some of the chemical processes of digestion also occur in the stomach.
Humans
The human stomach is subdivided into four regions: the fundus, an expanded area curving up above the cardiac opening (the opening from the stomach into the esophagus); the body, or intermediate region, the central and largest portion; the antrum, the lowermost, somewhat funnel-shaped portion of the stomach; and the pylorus, a narrowing where the stomach joins the small intestine. Each of the openings, the cardiac and the pyloric, has a sphincter muscle that keeps the neighbouring region closed, except when food is passing through. In this manner, food is enclosed by the stomach until ready for digestion.
The stomach has the ability to expand or contract depending upon the amount of food contained within it. When contracted, the interior walls form numerous folds (rugae), which disappear when the walls are distended. The thick mucous-membrane lining of the walls is densely packed with small gastric glands; these secrete a mixture of enzymes and hydrochloric acid that partly digest proteins and fats.
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The stomach muscles are rarely inactive. Upon entry of food, they relax briefly, then begin to contract. Periodic contractions churn and knead food into a semifluid mixture called chyme; rhythmical pumping (peristaltic) waves move food toward the pylorus and small intestine. Peristaltic contractions persist after the stomach empties and, increasing with time, may become painful. Such hunger pangs may also be related to the amount of sugar in the blood. If the level of sugar decreases significantly, hunger can be experienced without the stomach’s intervention.
The absorption of food, water, and electrolytes by the stomach is practically negligible, but iron and highly fat-soluble substances such as alcohol and some drugs are absorbed directly. Secretions and movements of the stomach are controlled by the vagus nerve and the sympathetic nervous system; emotional stress can alter normal stomach functions. Common stomach disorders include peptic ulcer, cancer, and gastritis.
Other animals
The stomachs of some other animals differ considerably from that of humans; many have multiple-chambered organs or special adaptations. The stomachs of cows and most cud-chewing (ruminant) animals are divided into four separate parts. Food is received first in the rumen, where mucus is added and cellulose is broken down. Next, it goes back to the mouth to be thoroughly rechewed. When swallowed again, it is passed to the second and third chambers, the reticulum and omasum, where water is extracted and absorbed. The food then goes to a final chamber, the abomasum, to receive the digestive enzymes.
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Birds have a three-chambered stomach: the first chamber, the crop, receives the food initially and either stores or begins to moisten and soften (macerate) it; the true stomach area adds digestive juices; and the gizzard, with its stones, or toothlike structures, grinds the food.
Rodents have only one stomach area, and many must eat their food twice before absorption takes place. Food is eaten and passed through the lower digestive tract, where it is coated with metabolites to help break it down. The fecal material is then re-eaten and mixed with additional food. Enzymes and water are removed from the once-passed material by the stomach and used to help digest new nutritional substances. Dry fecal pellets are finally excreted.
The starfish can turn its stomach inside out and extrude it partly from the body to eat the soft contents of shelled animals such as clams. Camels and llamas can regurgitate their stomach contents and spit this material at approaching enemies. Crayfish produce stones of calcium salts in their stomach. These are stored until the animal sheds its external shell, when the stones are reabsorbed by the stomach and used in forming a new shell.