The Digestive System 2- Stomach The Digestive System 2- Stomach PHYSIOLOGY-2PHL226 1 Dr/ Abdulaziz Saeedan Pharmacy College Pharmacy College

Stomach  Stomach is a muscular elastic bag.  Volume of the empty stomach is only 50 mL, but after a meal, its volume expands to about 1.5 L, SO stomach acts as a storage container or food reservoir. LES) pyloric  Food enters the stomach through the lower esophageal sphincter (LES) and leaves it through the pyloric sphincter.  The wall of the stomach has a number of folds called gastric rugae which help the stomach to expands during a large meal.  The inner surface of the stomach wall is lined by a mucous membrane known as the gastric mucosa. 2

 The gastric mucosa contains gastric glands which secrete gastric juice.  These glands contain 4 types of secretory cells. mucus 1- Mucus cells : Secrete mucus. hydrochloric acid intrinsic factor 2- Parietal cells : Secrete hydrochloric acid (HCl) and intrinsic factor. pepsinogen 3- Chief cells : Secrete pepsinogen. gastrin 4- G-cells : Secrete gastrin hormone. 3

1- Mucus cells:  Mucus cells secrete mucus.  Mucus is a viscous alkaline fluid that covers the gastric mucosa.  Mucus is made up of a protein (mucin) and glycoproteins.  Functions of mucus:  It forms a coat over the gastric mucosa (about 1mm thick) to protects stomach against the effect of HCl and enzymes SO, stomach wall will not be digested by the acid or enzymes.  Mucus contains some bicarbonates, which helps to neutralize the stomach acid over the gastric mucosa.  Mucus helps to lubricate food in the stomach. 4

2- Parietal cells:  Parietal cells secrete HCl and intrinsic factor. a- HCl a- HCl is responsible for the high acidity of stomach content (pH 1.5 – 2.0) at which gastric enzymes work best.  Functions of HCl:  Denatures proteins SO inactivates salivary amylase and most enzymes in food.  Kills most of the microorganisms ingested with food. pepsinogenpepsin  Activates pepsinogen that secreted by chief cells into pepsin (proteolytic enzyme) which starts protein digestion.  Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+) that can be absorbed and used for Hb synthesis. Q- Carbohydrate digestion stops at the stomach level ….Why?  Salivary amylase which digest carbohydrates in the mouth at pH 6.7; transports with the food bolus into stomach at which pH is (due to HCl).  HCl of the stomach denatures salivary amylase enzyme (because salivary amylase is a protein) so inhibits its activity. 5

 Function of intrinsic factor:  It is essential for the absorption of vitamin B12 from the ileum in the small intestine. NOTE:  Vitamin B12 is necessary for the normal function of nerve cells and for the formation of red blood cells. Q: Which disease results due to a lack of intrinsic factor OR Vitamin B12? Pernicious anemia. A: Pernicious anemia. 6

3- Chief cells: pepsinogen  Chief cells secrete pepsinogen; an inactive proenzyme. proteolytic enzyme  HCl activates pepsinogen into pepsin which is active proteolytic enzyme.  Pepsin functions most effectively at a strongly acidic pH of 1.5–2.0.  Chief cells in the stomach of newborn infants (but not of adults) secrete rennin and gastric lipase enzymes which are important for the digestion of milk. casein Rennin curdles milk proteins into casein to be digested by pepsin, while gastric lipase initiates the digestion of milk fats.  Function of Pepsin:  Pepsin digests dietary proteins (mostly found in meat, fish, eggs and dairy products) to a shorter peptide chain that will be completely digested in the small intestine to amino acids. 7

4- G cells: gastrin  G cells secrete gastrin hormone.  Secretion of gastrin is stimulated by the distension of stomach with food. Caffeine is also a stimulant. increase the stomach motility HCl  Gastrin is carried by the blood stream to increase the stomach motility and to stimulate the parietal cells to secrete HCl.  Gastrin secretion is inhibited by: o Excess stomach acidity. o Cholecystokinin (CCK).  Function of gastrin:  Increase the stomach motility.  Stimulates the production of HCl in stomach. 8

The gastric juice:  The gastric juice is a colorless, watery, acidic, digestive fluid secreted by the stomach glands.  It has a pH ranging from 1.5 to 2.0 which is essential in activating many digestive enzymes and in destroying most pathogens.  The gastric juice is made up of mucus, HCl, pepsinogen, pepsin and intrinsic factor. The gastric juice also contains bicarbonates to neutralize the gastric acidity.  The gastric juice converts the semisolid bolus into a semifluid paste called chyme.  Chyme  Chyme is semifluid paste consists of partially digested food and gastric juice. 9

 Secretion of gastric juice occurs in 3 phases: 1- The cephalic phase:  It is the stage in which the stomach responds to sight, smell or taste of food. BEFORE  About 30% of total gastric secretion occurs BEFORE food enters the stomach.  Sight, smell or taste of food stimulates the dorsal nucleus of the vagus nerve in the medulla oblongata.  Vagus nerve (parasympathetic nerve) stimulate the gastric secretion. 2- The gastric phase:  It is the stage that begins with the arrival of the bolus into the stomach.  50-60% of total gastric secretion occurs DURING this phase.  Distension of stomach with food : a- stimulates the dorsal nucleus of the vagus nerve in the medulla oblongata …… b- stimulates the secretion of gastrin hormone from G-cells which, in turn, stimulates the production of HCl in stomach. 10

3- The intestinal phase:  It is the stage that begins with the arrival of the chyme into the small intestine  Presence of chyme in the small intestine: a- inhibits the dorsal nucleus of the vagus nerve, thus reduces vagal stimulation of the stomach. SO reduces the gastric secretion. b- stimulates duodenal enteroendocrine cells to release secretin (inhibits the chief and parietal cells) and cholecystokinin (inhibits G-cells) thus reduces gastric secretion.NOTES:  The optimum pH for digestion in the Stomach is  Acid secretion is lowest in the morning before awakening and highest at night. may cause  High acidity of the gastric juice may cause damage to the stomach wall, causing gastric (peptic) ulcer.  Infection of the gastric mucosa with Helicobacter pylori are responsible for at least 80% of peptic ulcers. So, treatment of gastric ulcers is commonly involves the administration of antibiotic drugs. 11

o Stomach has 3 regions: o Stomach has 3 regions: 1. Fundus 2. Body 3. Pylorus Food bolus moves slowly from the fundus to the pylorus in 3- 4 h. During this period, food boluses are subjected to: 1- Mechanical digestion: 1- Mechanical digestion: By stomach contractions Contractions and relaxations of stomach help to churn the food bolus and mix it with the gastric juice. boluschyme Churning action of the stomach which occurs every 20 seconds converts the semisolid bolus into a semifluid paste called chyme. pyloric sphincter The pyloric sphincter, keeps the food in the stomach until complete churning and mixing with the gastric juice. 12

2- Chemical digestion: 2- Chemical digestion: By stomach secretions.  Pepsinshorter peptide chain  Pepsin digest dietary proteins to a shorter peptide chain.  Once the gastric juice do its job, the pyloric sphincter opens and the chyme passes gradually into the small intestine. o Functions of the Stomach 1- The bulk storage of ingested food 2- Partial digestion of the food :  Pepsin:  Pepsin: digest proteins to a shorter peptide chain. 3- Production of the intrinsic factor, that is required for the absorption of vitamin B-12 in the small intestine. 13