Anatomy and Physiology of Stomach

In anatomy, the stomach is a bean-shaped hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication.

The stomach lies between the esophagus and the duodenum (the first part of the small intestine). It is on the left side of the abdominal cavity. The top of the stomach lies against the diaphragm. Lying beneath the stomach is the pancreas, and the greater omentum which hangs from the greater curvature.

In humans, the stomach has a volume of about 50 mL when empty. After a meal, it generally expands to hold about 1 liter of food, but it can actually expand to hold as much as 4 liters. When drinking milk it can expand to just under 6 pints, or 3.4 liter.

Functions

The stomach is a highly acidic environment due to gastric acid production and secretion which produces a luminal pH range usually between 1 and 4 depending on the species, food intake, time of the day, drug use, and other factors. Combined with digestive enzymes, such an environment is able to break down large molecules (such as from food) to smaller ones so that they can eventually be absorbed from the small intestine. The human stomach can produce and secrete about 2 to 3 liters of gastric acid per day with basal secretion levels being typically highest in the evening.Pepsinogen is secreted by chief cells and turns into pepsin under low pH conditions and is a necessity in protein digestion.Absorption of vitamin B12 from the small intestine is dependent on conjugation to a glycoprotein called intrinsic factor which is produced by parietal cells of the stomach.

Other functions include absorbing some ions, water, and some lipid soluble compounds such as alcohol, aspirin, and caffeine.Another function of the stomach is simply a food storage cavity.

Sections

The stomach is divided into four sections, each of which has different cells and functions. The sections are:

Cardia Where the contents of the esophagus empty into the stomach.Fundus Formed by the upper curvature of the organ.Body or corpus

The main, central region.

Pylorus or antrum

The lower section of the organ that facilitates emptying the contents into the small intestine.

Layers

Like the other parts of the gastrointestinal tract, the stomach walls are made of the following layers, from inside to outside:

mucosaThe first main layer. This consists of an epithelium, the lamina propria underneath, and a thin layer of smooth muscle called the muscularis mucosae.

submucosaThis layer lies under the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer. The Meissner's plexus is in this layer.

muscularis externa

Under the submucosa, the muscularis externa in the stomach differs from that of other GI organs in that it has three layers of smooth muscle instead of two.

inner oblique layer: This layer is responsible for creating the motion that churns and physically breaks down the food. It is the only layer of the three which is not seen in other parts of the digestive system. The antrum has thicker skin cells in its walls and performs more forceful contractions than the fundus.

middle circular layer: At this layer, the pylorus is surrounded by a thick circular muscular wall which is normally tonically constricted forming a functional (if not anatomically discrete) pyloric sphincter, which controls the movement of chyme into the duodenum. This layer is concentric to the longitudinal axis of the stomach.

outer longituditinal layer: Auerbach's plexus is found between this layer and the middle circular layer.

serosaThis layer is under the muscularis externa, consisting of layers of connective tissue continuous with the peritoneum.

Anatomy of a Normal Stomach

Picture of a gastritis stomach