Stomach (Encyclopedia of Nursing & Allied Health)
The stomach is a muscular J-shaped organ of the digestive tract. It temporarily stores and mixes food; it also secretes gastric juice into the lumen (the hollow inside the stomach) and a hormone called gastrin into the blood.
The stomach is located in the upper left quadrant of the abdomen, just beneath the diaphragm. It is positioned between the esophagus (the passage between the mouth and stomach) and the small intestine. There is a sphincter (circular muscle) between the esophagus and the stomach, which allows food to pass into the stomach and prevents chyme (the semi-fluid mass into which food is converted by gastric enzymes) from flowing backwards into the esophagus. The pyloric valve is situated between the stomach and the small intestine, which allows chyme to pass into the small intestine and back into the stomach.
The stomach is divided into three general areas. The upper portion of the stomach near the esophagus is called the fundus; the middle section of the stomach is called the body; and the bottom portion of the stomach where the pyloric sphincter is located is called the antrum. When the stomach is completely distended (expanded), it measures about 10 in (26 cm) by about 4 in (10 cm). It can hold about one quart of semiliquid chyme.
The wall of the stomach is made up of four layers: the mucous, submucous, muscular, and peritoneal layers. The mucous and submucous layers are made up of ridges called rugae. Within the ridges are gastric glands made up of mucous cells, parietal cells, chief cells, and G-cells. Each of these cells secretes a chemical that aids in the process of digestion.
The muscular layer is actually composed of three different layers of smooth muscle, each with fibers running in a different direction: horizontal, vertical, and diagonal. The muscles are responsible for mixing the chyme and moving it through the stomach to the small intestine.
The peritoneal layer is the outer layer of stomach tissue. It is part of the peritoneum that lines the inside of the abdomen, covering most of the organs. It does not play a role in digestion.
The stomach has a large supply of blood vessels for the absorption of nutrients from digested food. Branches of the vagus nerve supply both sensory and nervous fibers to the stomach.
The stomach's primary role is to act as a temporary receptacle for food. While the food is in the stomach, it is mixed with gastric juices that are secreted by cells in the mucosal layer.
There are three general phases regulating gastric juice secretion. The first, or cephalic, phase occurs before food is actually eaten. The thought, smell and sight of food cause the brain to send signals to the stomach to increase its gastric secretions. The second phase is the gastric phase, which occurs when food enters the stomach. The food causes the stomach to stretch, which in turn sends nervous impulses to the brain. The brain sends return impulses back to the stomach to begin secreting gastrin. Gastrin then stimulates the release of other gastric juices. The third phase is called the intestinal phase, which occurs when food enters the small intestine. This phase results in a decrease in the movement of chyme into the small intestine, ensuring that the small intestine does not receive too much chyme at one time.
The stomach secretes 2 quarts (liters) of gastric juices every day. There are several types of specialized cells in the stomach that secrete gastric juices. Like the pancreas, the stomach has both exocrine and endocrine glandular functions. Exocrine secretory glands contain three types of secretory cells: chief cells, parietal cells and mucous cells. Endocrine cells called G-cells are scattered throughout the mucosa.
Exocrine glands are located in the fundus and body of the stomach. The chief cells in these exocrine glands secrete pepsinogen, the inactive precursor of pepsin. Pepsin is an enzyme that is responsible for the initial breakdown of protein molecules into smaller polypeptides. If pepsin did not have an inactive form, it would destroy the chief cells as they produced it. Pepsin can be activated in the stomach, because the stomach lining is protected from its action.
The exocrine glands of the fundus and body of the stomach also contain parietal cells. These parietal cells secrete hydrochloric acid (HCl), which makes the stomach strongly acidic, with a pH of about 2 or 3. This is an optimal pH for the action of pepsin. Hydrochloric acid is responsible for transforming the inactive pepsinogen into active pepsin. The hydrochloric acid in the chyme also stimulates the production of pancreatic and biliary secretions that further stimulate digestion. The acidic environment kills most bacteria that enter the digestive tract through the mouth. Parietal cells also release a chemical called the intrinsic factor, which is necessary for the absorption of vitamin B12.
Mucous cells release an alkaline mucous fluid into the gastric wall, protecting it against the damaging action of stomach acid. The fluid neutralizes the hydrochloric acid and also acts as a lubricant, protecting the inner lining of the stomach.
The G-cells are the only endocrine cells located in the stomach and are located mainly in the antrum of the stomach where there are few acid producing cells. G-cells release the hormone gastrin into the bloodstream. Gastrin acts on the parietal cells, stimulating them to release hydrochloric acid.
The stomach must undergo muscular contractions in order to mix food and gastric juices together. These waves of involuntary muscular contractions are called peristalsis. When food is present, peristaltic contractions pass through the stomach muscles about two or three times every minute and continue at a constant rhythm. Pressure will begin to develop in the lower part of the stomach. When there is sufficient pressure, a small amount of the stomach's contents moves through the pyloric sphincter and into the duodenum (the first section of the small intestine). It takes about two to six hours for the entire contents of the stomach to empty, depending on the composition of the person's diet. Low-fat meals leave the stomach more quickly than high-fat meals. Psychological states also affect the rate of stomach emptying; depression and fear may cause the stomach to empty slowly, while anger and aggression may cause the stomach to empty quickly.
After the chyme has entered the duodenum and the pyloric sphincter has closed, some of the food returns to the stomach through retropulsion. Retropulsion is a process in which the stomach contents are squirted back into the stomach at a rate of about three times per minute. Retropulsion mixes the food with gastric juices and breaks larger clumps of food into smaller pieces.
Role in human health
The stomach prepares food for digestion in the small intestine. If the stomach is not functioning properly, there are many problems that can arise with regard to digestion. Further, the contents of the stomach are so acidic and caustic to other organs of the body that they can cause problems if they leak out of the stomach, as may happen with perforating ulcers of the duodenum or penetrating wounds of the abdomen.
Common diseases and disorders
Gastritis is a common health problem. It is an inflammation of the gastric mucosal layer caused by a range of factors, including bacterial infections, medications (particularly NSAIDs), acute stress, and spicy foods or alcohol. Gastritis can result in a lowered functioning of chief cells and parietal cells. Less pepsin is provided to the stomach, resulting in incomplete breakdown of proteins. There is also less stomach acid secretion, allowing overgrowth of microbial populations and a decrease in the absorption of vitamin B12. Gastritis can occur at any age, but chronic gastritis is frequently seen in the elderly.
Gastroenteritis is another common disorder of the digestive tract, characterized by inflammation of the stomach and the intestines. It is the most common cause of mortality in underdeveloped nations, and in the United States it ranks second to the common cold as a cause of lost work time. Gastroenteritis is caused by specific bacteria (Staphylococcus aureus, Escherichia coli, etc.), amebae or other parasites. The symptoms of gastroenteritis include diarrhea, nausea, vomiting, and abdominal cramping. Patients can become dehydrated and malnourished if this disorder continues over an extended period of time.
Peptic ulcer disease
Peptic ulcers can occur in the stomach, although they are more likely to develop in the small intestine. Small lesions develop in the mucosal membrane, causing bleeding. Other symptoms include heartburn and indigestion. Researchers think that a bacterium (Helicobacter pylori) or heavy consumption of aspirin can cause this type of ulcer. Although peptic ulcer disease can occur in children, it usually affects people 200 years of age.
Gastroesophageal reflux (GER) or gastroesophageal reflux disease (GERD) is caused by a malfunctioning sphincter between the esophagus and the stomach, resulting in a release of chyme back into the esophagus. The esophagus cannot tolerate the acidic nature of the chyme. Consequently, the acid causes a burning sensation called "heartburn." Generally, taking antacids after meals or medications to reduce acid secretion can relieve GERD. Severe cases may require surgery.
Cancer of the stomach
Cancer of the stomach affects about 24,000 people in the United States each year. It occurs most often in adults over 55; it is more common in men than in women, and more common in African Americans than in Caucasians. Stomach cancer may develop in any part of the stomach and metastasize (spread) to other parts of the digestive tract or to such distant organs as the ovaries or lungs. The early symptoms of stomach cancer are often vague and nonspecific, which means that they can be caused by a range of other health problems. A definite diagnosis of stomach cancer requires a series of laboratory tests and a biopsy of a tissue sample obtained by an instrument called a gastroscope.
The most common treatment for stomach cancer is surgical removal of part or all of the stomach. This procedure is called a gastrectomy. Patients with stomach cancer may also be treated with chemotherapy, radiation therapy, or immunotherapy.
Antrumhe lower portion of the stomach near the pyloric sphincter.
Chymehe mass of semiliquid, partially digested food found in the stomach.
Fundushe upper portion of the stomach near the esophagus.
Gastrin hormone that stimulates the secretion of gastric juice.
Gastritisnflammation of the stomach.
Gastroenteritisnflammation of the stomach and the intestines.
Lumenhe hollow inside a tubular organ such as the digestive tract.
Pepsinn enzyme produced in the stomach that breaks down proteins in the presence of hydrochloric acid.
Peristalsisuscular contractions that move food through the digestive tract.
Retropulsion process in which muscular contractions push food that has entered the duodenum backward into the stomach. Retropulsion helps to mix the chyme with gastric juices, and to break large lumps of food into smaller pieces.
Rugaeidges or folds in the mucosal and submucosal layers of tissue in the wall of the stomach.
Sphincter circular band of muscle that encircles an orifice of the body or one of its hollow organs, such as the digestive tract.
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American College of Gastroenterology. 4900 B South 31st Street, Arlington, VA 22206-1656. (703) 820-7400. Fax:(703) 931-4520. <<a href="http://www.acg.gi.org">http://www.acg.gi.org>.
American Gastroenterological Association. 7910 Woodmont Avenue, 7th Floor, Bethesda, MD 20814. (301) 654-2055. Fax: (301) 652-3890. <<a href="http://www.gastro.org">http://www.gastro.org>.
National Cancer Institute, Office of Cancer Communications. 31 Center Drive, MSC 2580, Bethesda, MD 20892-2580. (800) 4-CANCER (1-800-422-6237). TTY: (800) 332-8615. <<a href="http://www.nci.nih.gov">http://www.nci.nih.gov>.
National Digestive Diseases Information Clearinghouse. 2 Information Way, Bethesda, MD 20892. (301) 654-3810 or (800) 891-5389.
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Sally C. McFarlane-Parrott