The stomach is a hollow muscular organ of the digestive system designed to accumulate and digest food. It lies between the esophagus and duodenum, connecting the upper gastrointestinal tract with the lower. Also, it represents the most dilated part of the gastrointestinal tract.

The stomach is positioned in the upper aspect of the abdomen, lying below the left dome of the diaphragm. It provides mechanical and chemical digestion, a small degree of absorption and secretion of various hormones. It has two openings, four parts, two curvatures, two surfaces and two sphincters.

Openings

The stomach has two openings:

Cardiac opening (orifice) - lies between the esophagus and the stomach; situated at the level of the body of the eleventh thoracic vertebra (T11).
Pyloric opening (orifice) - connects the stomach with the duodenum and is located at the level of the first lumbar vertebra (L1).

Parts

The stomach is composed of four parts:

Cardia - surrounds the cardiac openings; the very first portion that receives ingested food;
Fundus - the superior dilation of the stomach; located above and left to the cardiac opening;
Body - also called the corpus; the largest part of the stomach positioned below the fundus;
Pyloric part - also called the pylorus; the most distal part that connects the stomach with the duodenum and can be subdivided into two smaller areas - the pyloric antrum and the pyloric canal; the pyloric antrum is wider, while the pyloric canal is narrower and it is connected to the duodenum via the pyloric orifice; this opening is controlled by the pyloric sphincter - a circular layer of smooth muscles.

Curvatures and surfaces

The stomach has a characteristic J-shape that is created by two curved borders known as its curvatures:

Greater curvature - long and convex curvature found on the left side; it goes from the cardiac notch (formed between the abdominal esophagus and the fundus of the stomach) to the pyloric part;
Lesser curvature - short and concave curvature on the right side of the stomach; its distal ending contains a small incisure called the angular notch that marks the intersection site between the body and the pyloric part of the stomach.

The stomach has two surfaces found between both curvatures. Anterior between the lesser and greater curvatures is the anterior surface, while posterior is the posterior surface.

Sphincters

The stomach has two sphincters that provide chyme movements further to the following parts of the digestive system, and at the same time, they block its backflow. These two sphincters are the lower esophageal sphincter and the pyloric sphincter.

The lower esophageal sphincter is also known as the inferior sphincter of the esophagus. It is located between the stomach and the esophagus at the gastroesophageal junction that corresponds to the eleventh thoracic vertebra (T11). This sphincter is a physiological sphincter made of longitudinal and circular smooth muscle fibers, crura of the diaphragm, and the fibers from the phrenoesophageal ligament.
The pyloric sphincter is located between the pylorus and the duodenum. This sphincter controls the passage of the chyme into the duodenum. The pyloric sphincter is an anatomical sphincter made of circular muscle layer fibers.

Note: Chyme is a thick semi-fluid mass of partly digested food mixed with gastric juice.

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Stomach