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 peristalytic 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.
