The stomach is one of the primary organs of the digestive system. It is located in the middle of the abdominal cavity and extends from the lower end of the esophagus to the duodenum. A curved sac, the stomach has a lesser curvature along the top and a greater curvature along the bottom. The bolus of chewed food paste enters the stomach from the esophagus, propelled by peristaltic waves. The cardiac gastric glands are located near the entrance to the stomach and lubricate the food as it comes in. The stomach secretes hydrochloric acid and the enzymes pepsin, rennin, and lipase which help digest the carbohydrates, proteins and fats in food.
The stomach is lined with a durable mucous lining which protects it from the gastric juices so that the stomach itself is not digested. Occasionally, a portion of this lining wears thin and the digestive juices do aggravate the stomach lining. This condition is known as an ulcer and is a frequent occurrence which may affect the stomach, the lower esophagus, or the duodenum. The lower end of the stomach is guarded by the pylorus (pylorus means gatekeeper), a sphincter muscle which admits digested food into the duodenum after it has been adequately processed by the digestive juices.
The average stomach can hold about a quart (about a liter), but can stretch to hold much more than this. When empty, or nearly empty, the stomach contracts to form folds, or rugae, in its mucous lining. Though it was once thought that the contractions of the stomach in the absence of food was what caused the sensation of hunger, it is now known that the primary impulse of hunger is due to low glucose levels in the bloodstream. However, the contraction of the stomach can often be felt, and this and the "grumbling" heard as food is passed through the lower digestive tract also serve to remind one of the sensation of hunger.