The Stomach

Sto m ach • Stomach can be divided into three parts: fundus, body and pyloric antrum. Ø The function of: Fundus: food storage (very important). Ø Body: digestion and mixing (chemical digestion through gastric secretion(acid and enzymes)& mechanical digestion because of mixing (peristaltic movement) through contraction and relaxation of smooth muscles in the stomach wall). Ø From other stomach functions: initial digestion of proteins ( the digestion of protein start here Ø The initial digestion of carbohydrates starts in the oral cavity by salivary amylase. Ø The pancreatic amylase continues the digestion of carbohydrates in small intestine because the acidic environment in the stomach inhibits salivary amylase and stop its work (salivary amylase works at neutral pH environment such as that in the oral cavity). o The soupy food mixture (particles of food) that mixed to the gastric secretion called chyme. v There are 2 valves (sphincters) which have an important role in regulation the movement of food between the stomach and these parts (esophagus & small intestine). 1) Gastroesophageal sphincter: it is in the upper portion of the stomach (between the stomach and esophagus). 2) Pyloric sphincter: the lower valve. o The smooth muscle layers in the fundus and body are relatively thin, but the lower part of the stomach, the antrum, has heavier musculature v Gastric pits: they are folding in the gastric mucosa (invagination) towards submucosa (inward). • Gastric pit contains different types of cells that collectively forming gland: exocrine gastric gland that move its secretion to the lumen through ducts. § These cells are: 1. Mucous surface cells: secretes mucus (on the surface). 2. Mucous neck cells: secretes mucus (in the folding) ü They differ in the constituents of the mucous The exocrine gastric gland cells: A. Mucous neck cells: secretes mucus that protect the lining of the stomach because of the high acidity of stomach. ü This mucous is 3mm in thickness and alkaline to make neutralization of Hydrochloric acid. ü ü ü ü B. Parietal cells: secretes hydrochloric acid and intrinsic factor. The intrinsic factor is important to vitamin B12 absorption. The problems in most cases of vitamin B12 deficiency are problems in the absorption vitamin B12 because of the problems in the secretion and formation of intrinsic factor. The storage of B12 in the human body is enough even there is reluctance to eat food items contain vitamin B12. Most food items contain B12 then, the vitamin B12 deficiency rarely to be due to the diet, it is mostly due to malabsorption because of the problems in the secretion and formation of intrinsic factor. C. Chief (zymogenic) cells: secretes pepsinogen (the inactive form of pepsin) and gastric lipase. ü Pepsinogen is activated in the lumen to form pepsin which makes partial digestion of proteins. ü The initial digestion of proteins occurs in the stomach by the hydrochloric acid (denaturation) & pepsin (the active form of pepsinogen). ü Gastric lipase: which digest the lipids, its more efficient (very active) in infants not in adult to deal with the lipids in milk. D. Enteroendocrine cells: secrete hormones (unlike first 3 cell types that secrete enzymes, acids or mucous). ü Its located at the end of the foldings and also called G cells that secrete gastrin which have an important role in regulation the stomach activity. ü The other type of cells that secrete hormones; D cells that secrete somatostatin which regulate the G cells work.