Gastro Enterology => Stomach
Stomach, organ of the digestive system. Most animals, like humans, have a single stomach, but birds and ruminants (see Ruminant) have digestive organs composed of two or more chambers. The outer surface of the stomach is smooth; the inner surface is folded into numerous complex ridges, which assist in the mixing of food with digestive juices and channel this material through the stomach into the intestines. Only water, alcohol, and certain drugs seem to be absorbed from the stomach; most food absorption takes place in the small intestine (see Intestine).
LOCATION OF STOMACH
In humans the stomach is situated in the upper part of the abdominal cavity (see Abdomen), mostly to the left of the midline. The large, domed end of the stomach, the fundus, lies in the left vault of the diaphragm; the esophagus enters the upper side, or lesser curvature, a short distance below the fundus. The region immediately below the fundus is called the body. The upper part of the stomach, spoken of as the cardiac portion, includes the fundus and body. The lower, or pyloric, portion curves downward, forward, and to the right and includes the antrum and pyloric canal. The latter is continuous with the upper part of the small intestine, the duodenum.
The tissues of the stomach include an outer fibrous coat derived from the peritoneum and, beneath this, a coat of smooth muscle fibers (see Muscle) arranged in diagonal, longitudinal, and circular layers. At the junction of the esophagus and stomach the circular muscles are much enlarged, forming the cardiac sphincter. Contraction of this muscle prevents the passage of esophageal contents into the stomach and the regurgitation of gastric contents into the esophagus. A similar structure, the pyloric sphincter, is found at the junction of the pylorus and the duodenum. Another layer of the stomach, the submucosa, is made up of loose connective tissue in which are found numerous blood and lymph vessels (see Circulatory System) and nerves of the autonomic nervous system. The innermost layer, the mucosa, contains secretory cells. One type secretes hydrochloric acid, which not only neutralizes the alkaline reaction of the saliva (see Salivary Glands), but renders the gastric contents distinctly acid and activates the gastric digestive juices. These juices are secreted by a different type of cell. The enzymes found in gastric juice are pepsin, which in the presence of acid splits proteins to peptones; rennin, which curdles milk; and perhaps lipase, which splits fats to fatty acids and glycerol (see Enzyme). A third type of cell secretes mucus for the protection of the stomach from its own products.
The tissues of the stomach are digestible by the gastric juices, as is mucus. Under normal conditions, however, the mucous coating is renewed more rapidly than it is removed. When a pathological or psychosomatic condition prevents the proper secretion of mucus, the gastric mucosa becomes eroded and an ulcer forms. If neglected, this ulcer may perforate the gastric wall and allow the stomach contents to escape into the abdominal cavity, causing peritonitis.
The introduction into the stomach of meat extracts, cooked grains, and partly digested products of proteins stimulates the flow of gastric juice (see Protein). These agents, which are called secretagogues, cause the formation of a hormone, gastrin, in the pyloric end of the stomach. Gastrin, when absorbed, stimulates the secretory glands. Gastric secretion also may be stimulated by the mere sight or smell of food. This is called cephalic or reflex stimulation.
The walls of the empty stomach are in contact with each other. As food enters the organ, the walls yield and the cavity enlarges without change in intragastric pressure. The cardiac portion of the stomach stores the ingested food. Waves of contraction of the circular muscle, preceded by waves of relaxation (peristalsis), start about midway in the body of the stomach and travel downward, ending just before reaching the pyloric canal. Such waves of contraction, which may occur at a rate of three per minute, macerate and thoroughly mix the food with gastric juice.
The periodic discharge of food from the stomach into the duodenum is caused by the contraction of the muscles in the stomach wall. These muscles are innervated by the cranial vagus nerves, which stimulate contraction of the gastric musculature and allow the sphincter between the stomach and the duodenum to open. Because severing these nerves leads to paralysis lasting only a few days, the stomach, like the heart, must be regarded as an automatic organ. Whether the automaticity is determined in the musculature or in an intrinsic nerve mechanism is unknown. Sympathetic nerve fibers in the splanchnic nerves have opposite effects to the vagal nerves, preventing gastric emptying.
See Nervous System.
Contractions of the empty stomach are known to be associated with hunger pangs in humans. The mechanism of hunger is more complicated than this, however, and is only incidentally related to the stomach.
Many of the symptoms popularly ascribed to diseases of the stomach may be caused by psychosomatic conditions (see Behavioral Medicine), general systemic disease, or diseases of neighboring organs, such as the heart, liver, or kidneys. Abnormal stomach conditions include, in addition to ulcer and cancer, dyspepsia, or gastric indigestion, catarrh, and strictures such as those caused by the scars of healed ulcers (see Indigestion). Ordinary stomach disorders are best treated by allowing the organ to rest, that is, by a one- or two-day fast, followed by a bland or mild diet, with gradual additions until the normal diet is restored. See Gastritis.