Stomach

The stomach is a muscular, hollow organ found in the upper gastrointestinal tract of humans, many other animals, and even some invertebrates. Its ancient Greek name, *gaster*, gives us the medical term "gastric." This dilated organ is a key part of the digestive system, taking over after the cephalic phase—triggered by seeing, smelling, and chewing food—to begin the gastric phase of digestion. Inside, digestive enzymes and gastric acid chemically break down food, and the stomach also helps manage the gut microbiota, which influences both digestion and overall health.

Situated between the esophagus and the small intestine, the stomach sits in the left upper quadrant of the abdominal cavity, with its top against the diaphragm and the pancreas behind it. A large fold of visceral peritoneum called the greater omentum hangs from its greater curvature. Two sphincters keep its contents contained: the lower esophageal sphincter at the junction with the esophagus, and the pyloric sphincter, which controls the release of partially digested food (chyme) into the duodenum, the first part of the small intestine. The stomach is surrounded by parasympathetic and sympathetic nerve networks that regulate both its secretion of digestive juices and the movement of its muscles.

The stomach is distensible, normally expanding to hold about one litre of food, though maximum adult volume ranges from 2 to 4 litres, with extreme cases reaching 15 litres. Its shape changes with distension: empty, it is somewhat J-shaped; partially full, it becomes pear-shaped; in obese individuals, it is more horizontal. A newborn’s stomach holds only about 30 millilitres.

The human stomach has four sections. The cardia, a small region around the esophageal opening, receives food from the esophagus via the cardiac orifice. A cardiac notch marks the start of the greater curvature, and a horizontal line from this notch defines the dome-shaped fundus. The body, or corpus, is the main central region. The pylorus connects to the duodenum at the pyloric sphincter, with the pyloric antrum opening into the body. The cardia begins at the z-line, where the epithelium changes from stratified squamous to columnar, near the lower esophageal sphincter.

The stomach bed—the structures it rests upon—includes the tail of the pancreas, splenic artery, left kidney, left suprarenal gland, transverse colon and its mesocolon, left crus of diaphragm, and left colic flexure. This term was introduced around 1896 but later discredited.

Blood supply comes from the right and left gastric arteries along the lesser curvature, the right and left gastroepiploic arteries along the greater curvature, and 5–7 short gastric arteries from the splenic artery to the fundus and upper greater curvature. Lymphatic drainage occurs via two sets of gastric lymph nodes, which send fluid into the intestinal lymph trunk and then to the cisterna chyli.

The stomach wall, from inner to outer, consists of mucosa, submucosa, muscular layer, subserosa, and serosa. The inner gastric mucosa is a mucous membrane with columnar epithelium, lamina propria, and a thin smooth muscle layer called the muscularis mucosa. Beneath it, the submucosa contains fibrous connective tissue and Meissner's plexus. The muscular layer has three fibre layers—inner oblique, middle circular, and outer longitudinal—arranged at angles to each other. The inner oblique layer is unique to the stomach and creates the churning motion that physically breaks down food. This three-layer structure gives the stomach the thickest muscular wall in the gastrointestinal tract, allowing for maximum peristalsis.