The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital digestive organ. In the digestive system the stomach is involved in the second phase of digestion, following mastication (chewing).
In humans and many other animals, the stomach is located between the oesophagus and the small intestine. It secretes digestive enzymes and gastric acid to aid in food digestion. The pyloric sphincter controls the passage of partially digested food (chyme) from the stomach into the duodenum where peristalsis takes over to move this through the rest of the intestines.
The stomach is located in the superior aspect of the abdomen. It lies in the epigastric and umbilical regions, mostly protected by the lower portion of the rib cage.
The exact size, shape and position of the stomach can vary from person to person. For example, in thin individuals, it is not uncommon for the stomach to extend into the pelvic region.
The stomach has four main regions; the cardia, fundus, body and pylorus:
- Cardia – surrounds the superior opening of the stomach.
- Fundus – the rounded portion superior to and left of the cardia.
- Body – the large central portion inferior to the fundus.
- Pylorus – connects the stomach to the duodenum.
Greater and Lesser Curvatures
The medial and lateral borders of the stomach are curved, forming the lesser and greater curvatures:
- Greater curvature – forms the long, convex, lateral border of the stomach. Arising at the cardiac orifice, it arches backwards and passes inferiorly to the left. It curves to the right as it continues medially to reach the pyloric antrum. The short gastric arteries and the right and left gastro-omental arteries supply branches to the greater curvature.
- Lesser curvature – forms the shorter, concave, medial surface of the stomach. The most inferior part of the lesser curvature, the angular notch, indicates the junction of the body and pyloric region. The lesser curvature gives attachment to the hepatogastric ligamentand is supplied by the left gastric artery and right gastric branch of the hepatic artery.
Sphincters of the Stomach
There are two sphincters of the stomach, located at each orifice. They control the passage of material entering and exiting the stomach.
Inferior Oesophageal Sphincter
The inferior oesophageal sphincter is located between the oesophagus and the stomach (in contrast to the superior oesophageal sphincter, located in the pharynx).
It is located to the left of the T11 vertebra. Situated immediately superior is the oesophageal hiatus, an opening in the diaphragm through which the oesophagus travels. Histologically, the sphincter is marked by an abrupt change from stratified squamous epithelium to simple columnar.
The inferior oesophageal sphincter is termed a physiological (or functional) sphincter – it does not have any specific muscle (see the oesophagus for more on this).
The pyloric sphincter lies between the pylorus and the duodenum. It controls of the exit of chyme (food and gastric acid mixture) from the stomach.
In contrast to the inferior oesophageal sphincter, this is an anatomical sphincter. It contains smooth muscle, which constricts to limit the discharge of stomach contents through the orifice.
Emptying of the stomach occurs intermittently when intragastric pressure overcomes the resistance of the pylorus. The pylorus is normally contracted so that the orifice is small and food can stay in the stomach for a suitable period. Gastric peristalsis pushes the chyme through the pyloric canal into the duodenum for further digestion.
Greater and Lesser Omenta
Within the abdominal cavity, the organs are covered in a double layered membrane, called the peritoneum. It supports the viscera, and attaches them the abdominal wall.
The greater and lesser omenta are two structures that consist of peritoneum folded over itself (two layers of peritoneum – four membrane layers). Both omenta attach to the stomach, and are useful anatomical landmarks:
- Greater omentum – hangs down from the greater curvature of the stomach. It drapes over the transverse colon and folds back upon itself before reaching the posterior abdominal wall. It features many lymph nodes, which contain macrophages to help combat infections of the GI tract.
- Lesser omentum – continuous with peritoneal layers of the stomach and duodenum. These two layers combine at the lesser curvature, and ascend to attach to the liver. The main function of the lesser omentum is to attach the stomach and duodenum to the liver.
Together, the greater and lesser omenta divide the abdominal cavity into two; the greater and lesser sac. The stomach lies immediately anterior to the lesser sac. The greater and lesser sacs communicate via the epiploic foramen, a hole in the lesser omentum.
The arterial supply to the stomach comes from the coeliac trunk and its branches. Anastomoses form along the lesser curvature by the right and left gastric arteriesand along the greater curvature by the right and left gastro-omental arteries:
- Right gastric – branch of the common hepatic artery, which arises from the coeliac trunk.
- Left gastric – arises directly from the coeliac trunk.
- Right gastro-omental – terminal branch of the gastroduodenal artery, which arises from the common hepatic artery.
- Left gastro-omental – branch of the splenic artery, which arises from the coeliac trunk.
The veins of the stomach run parallel to the arteries. The right and left gastric veins drain into the hepatic portal vein. The short gastric vein, left and right gastro-omental veins ultimately drain into the superior mesenteric vein.
The stomach receives innervation from the autonomic nervous system:
- Parasympathetic nerve supply comes from the posterior vagal trunks, derived from the vagus nerve.
- Sympathetic nerve supply from the T6-T9 spinal cord segments pass to the coeliac plexus. It also carries some pain transmitting fibres.
The gastric lymphatic vessels travel with the arteries along the greater and lesser curvatures of the stomach. Lymph fluid drains into the gastric and gastro-omental lymph nodes found at the curvatures.
Efferent lymphatic vessels from these nodes connect to the coeliac lymph nodes, located on the posterior abdominal wall.
In the mouth, we chew and moisten solid food until it becomes a small mass known as a bolus. When we swallow each bolus, it then passes through the esophagus to the stomach where it is stored along with other boluses and liquids from the same meal.
The size of the stomach varies from person to person, but on average it can comfortably contain 1-2 liters of food and liquid during a meal. When stretched to its maximum capacity by a large meal or overeating, the stomach may hold up to 3-4 liters. Distention of the stomach to its maximum size makes digestion difficult, as the stomach cannot easily contract to mix food properly and leads to feelings of discomfort.
After the stomach has been filled with food from a meal, it stores the food for about 1-2 hours. During this time, the stomach continues the digestive process that began in the mouth and allows the intestines, pancreas, gallbladder, and liver to prepare to complete the digestive process.
At the inferior end of the stomach, the pyloric sphincter controls the movement of food into the intestines. The pyloric sphincter is normally closed to keep food and stomach secretions within the stomach. Once chyme is ready to leave the stomach, the pyloric sphincter opens to allow a small amount of chyme to pass into the duodenum. This process, known as gastric emptying, slowly repeats over the 1-2 hours that food is stored in the stomach. The slow rate of gastric emptying helps to spread out the volume of chyme being released from the stomach and maximizes the digestion and absorption of nutrients in the intestines.
The stomach produces and secretes several important substances to control the digestion of food. Each of these substances is produced by exocrine or endocrine cells found in the mucosa.
- The main exocrine product of the stomach is gastric juice – a mixture of mucus, hydrochloric acid, and digestive enzymes. Gastric juice is mixed with food in the stomach to promote digestion.
- Specialized exocrine cells of the mucosa known as mucous cells secrete mucusinto the lumen of the stomach and into the gastric pits. This mucus spreads across the surface of the mucosa to coat the lining of the stomach with a thick, acid- and enzyme-resistant barrier. Stomach mucus is also rich in bicarbonate ions, which neutralize the pH of stomach acid.
- Parietal cells found in the gastric pits of the stomach produce 2 important secretions: intrinsic factor and hydrochloric acid. Intrinsic factor is a glycoprotein that binds to the vitamin B12 in the stomach and allows the vitamin to be absorbed in the small intestine. Vitamin B12 is an essential nutrient for the formation of red blood cells. Hydrochloric acid protects the body by killing pathogenic bacteria naturally found in food. Hydrochloric acid also helps to digest proteins by denaturing them into an unfolded shape that is easier for enzymes to digest. The protein digesting enzyme pepsin is activated by exposure to hydrochloric acid inside the stomach.
- Chief cells, also found within the gastric pits of the stomach, produce two digestive enzymes: pepsinogen and gastric lipase. Pepsinogen is the precursor molecule of the very potent protein-digesting enzyme pepsin. Because pepsin would destroy the chief cells that produce it, it is secreted in its inactive pepsinogen form. When pepsinogen reaches the acidic pH found in the stomach thanks to hydrochloric acid, it changes shape and becomes the active enzyme pepsin. Pepsin then breaks dietary proteins into their amino acid building blocks. Gastric lipase is an enzyme that digests fats by removing a fatty acid from a triglyceride molecule.
- G cells are endocrine cells found at the bottom of the gastric pits. G cells release the hormone gastrin into the bloodstream in response to many stimuli, such as signals from the vagus nerve; the presence of amino acids in the stomach from digested proteins; and the stretching of the stomach wall during a meal. Gastrin travels through the blood to various receptor cells throughout the stomach where it stimulates the glands and muscles of the stomach. Glandular stimulation by gastrin leads to increased secretion of gastric juice to increase digestion. Stimulation of smooth muscles by gastrin leads to stronger contractions of the stomach and the opening of the pyloric sphincter to move food into the duodenum. Gastrin also binds to receptor cells in the pancreas and gallbladder where it increases the secretion of pancreatic juice and bile.
Digestion in the stomach can be divided into 2 classes: mechanical digestion and chemical digestion. Mechanical digestion is the physical division of a mass of food into smaller masses while chemical digestion is the chemical conversion of larger molecules into smaller molecules.
- The mixing action of the stomach walls allows mechanical digestion to occur in the stomach. The smooth muscles of the stomach produce contractions known as mixing waves that mix the boluses of food with gastric juice. This mixing leads to the production of the thick liquid known as chyme.
- While food is being physically mixed with gastric juice to produce chyme, the enzymes present in the gastric juice chemically digest large molecules into their smaller subunits. Gastric lipase splits triglyceride fats into fatty acids and diglycerides. Pepsin breaks proteins into smaller amino acids. The chemical digestion begun in the stomach will not be completed until chyme reaches the intestines, but the stomach prepares hard-to-digest proteins and fats for further digestion.
The activity of the stomach is under the control of several hormones that regulate the production of stomach acid and the release of food into the duodenum.
- Gastrin, produced by the G cells of the stomach itself, increases the activity of the stomach by stimulating increased gastric juice production, muscle contraction, and gastric emptying through the pyloric sphincter.
- Cholecystokinin (CCK), produced by the mucosa of the duodenum, is a hormone that acts to slow gastric emptying by contracting the pyloric sphincter. CCK is released in response to food rich in proteins and fats, which are difficult for the body to digest. By inhibiting gastric emptying, CCK allows food to be stored in the stomach longer to promote improved digestion by the stomach and to give the pancreas and gallbladder time to release enzymes and bile to increase digestion in the duodenum.
- Secretin, another hormone produced by the duodenum’s mucosa, responds to the acidity of chyme entering the duodenum from the stomach. Secretin travels through the bloodstream to the stomach where it slows the production of gastric juice by the exocrine glands of the mucosa. Secretin also promotes the production of pancreatic juice and bile that contain acid-neutralizing bicarbonate ions. The net effect of secretin is to protect the intestines from the damaging effects of acidic chyme.