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The Stomach Location: abdominal cavity Physical description: a "J" shaped organ with muscular walls lined with long folds it is the widest part of the digestive tract has 2 muscular valves called "sphincters" (at the top & bottom of the stomach) has a mucous lining easily holds 1.5 litres (a maximum holding capacity of about 4 litres) The Stomach bbbbbbbbbbbbbbbbbb 1 The Stomach The mass of chewed and moistened (by saliva) food that moves from the mouth into the esophagus to the stomach is called the bolus. When the bolus reaches the stomach its bathed in a liquid called gastric juice. Gastric Juice is a mixture of hydrochloric acid, salts, enzymes, water and mucus that is produced by glands in the stomach to help digest food The mucus protects the walls of the stomach, protecting it from attach by the strongly acidic gastric juice. The rest of the gastric juice continues chemical digestion of the food. Approximately every 20 seconds mixing waves combine the food with gastric juices to form chyme. Liquids empty from the stomach faster than solids, taking about 20 minutes. A mixed meal takes about 2 to 6 hours to be emptied. The Stomach Mechanical Digestion: Mechanical digestion in the stomach involves ... Stomach Muscles (smooth muscle) churning food (mixing food with enzymes and acids) moving food (from stomach to the intestines) Sphincter muscles (valves) keep food from flowing in the wrong direction (a) Esophageal (Cardiac) Sphincter prevents stomach acids from flowing back up the esophagus (b) Pyloric Sphincter keeps food (chyme) in the stomach while acids and enzymes chemically digest it Mucous (lining) prevents the stomach from digesting itself 2 The Stomach Chemical Digestion in the Stomach: Chemical digestion in the stomach involves gastric glands that produce ... HCl (Hydrochloric Acid) breaks down protein into polypeptides which will be broken down into amino acids Pepsin a protease that breaks down protein into polypeptides which will be broken down into amino acids Digestive enzymes speed up the breaking down process by holding the large particle (substrate) in place and breaking bonds. The stomach does not usually break down the protein that make up its own cells because the pepsin is inactive until the hydrochloric acid is present The stomach lining produces chemicals used in digestion. (a) Lumen of the stomach which stores the food from a meal (b) Gastric pits from which mucus , enzymes and acid are secreted (c) Mucus secreting cells. Mucus protects the surface of the stomach from autodigestion (d) Parietal cells that produce HCl which kills microorganism that enter the digestive system (food & tracheal mucus). This also converts inactive pepsinogen to active pepsin (e) Chief cells: produces pepsinogen a protease enzyme 3 The Stomach Interesting Facts about the Stomach: Powerful hydrochloric acid in the stomach helps break down the bolus into a liquid called chyme. HCl also kills most contaminating microorganisms. A thick mucus layer that lines the stomach walls prevents the stomach from digesting itself. When the mucus is limited, an ulcer (erosion of tissue) may form. 4
