Elijah Mobley M.D. FACS

The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as the primary organ of the digestive tract. It is involved in the second phase of digestion, following mastication  (chewing). The stomach is located between the esophagus  and the small intestine. It secretes protein-digesting enzymes and strong acids to aid in food digestion, and also churns food (sent to it via oesophageal peristalsis) through smooth muscular contortions before sending partially-digested food to the small intestines.

The word stomach is derived from the Latin stomachus which is derived from the Greek word stomachos, ultimately from stoma, "mouth". The words gastro- and gastric (meaning related to the stomach) are both derived from the Greek word gaster.

Bolus (masticated food) enters the stomach through the esophagus  via the esophageal sphincter. The stomach releases proteases( protein-digesting enzymes such as pepsin) and hydrochloric acid, which kills or inhibits bacteria  and provides the acidic pH for the proteases to work. Food is churned by the stomach through muscular contractions of the wall - reducing the volume of the fundus, before looping around the fundus and the body of stomach as the boluses are converted into chyme  (partially-digested food). Chyme slowly passes through the pyloric sphincter and into the duodenum, where the extraction of nutrients begins. Depending on the quantity and contents of the meal, the stomach will digest the food into chyme anywhere between 40 minutes and a few hours. 

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

Two smooth muscle valves, or sphincters, keep the contents of the stomach contained. They are the esophageal sphincter (found in the cardiac region) dividing the tract above, and the Pyloric sphincter dividing the stomach from the small intestine.

The stomach is surrounded by parasympathetic (stimulant) and orthosympathetic (inhibitor) plexuses (networks of blood vessels and nerves in the anterior gastric, posterior, superior and inferior, celiac and myenteric), which regulate both the secretions activity and the motor (motion) activity of its muscles.

In humans, the stomach has a relaxed, near empty volume of about 45 ml. It is a distensible organ. It normally expands to hold about 1 litre of food,[4] but will hold as much as 2-3 litres(whereas a newborn baby will only be able to retain 30ml).