Git5.6

• Stomach


  1. shaped, pouch-like organ about 20-30 cm long which hangs under the diaphragm in the upper left portion of the abdominal cavity
• Stomach

  • Capacity of about one liter or more
  • Inner lining marked by thick folds (rugae) of the mucosal and submucosal layers that tend to disappear when its wall is distended
• Stomach function


  1. Receives food from the esophagus

  2. Mixes it with gastric juice
• Regions of the stomach

  • Cardiac region
  • Fundic region
  • Body region
  • Pyloric region (antrum)

  
• Pyloric sphincter (pylorus)


  Circular layer of fibers in the muscular wall that thickens, forming a powerful muscle which prevents regurgitation of food from the intestine back into the stomach and also prevents excessive release of gastric acid into the duodenum
• Types of gastric glands

  • Cardiac glands
  • Gastric (oxyntic) glands
  • Pyloric glands
• Gastric secretion

  About 2500ml (2.5 L) of gastric juice is secreted daily, containing hydrochloric acid, pepsinogen, mucus and intrinsic factor in addition to cations, anions, HCO3- and digestive enzymes
• Cell types in oxyntic (gastric) gland

  • Mucus neck cells
  • Peptic (chief) cells
  • Parietal (oxyntic) cells
• Parietal (oxyntic) cells


  Secrete HCl and intrinsic factor
• HCl in gastric juice

  pH of 0.8, kills many ingested bacteria, aids protein digestion, provides the necessary pH for pepsin to start protein digestion, stimulates the flow of bile and pancreatic juice
• HCl secretion mechanism


  1. Chloride ion (Cl-) is transported actively from cytoplasm of parietal cell into the lumen of canaliculi

  2. Sodium ions are actively transported from lumen of canaliculi into the cytoplasm of parietal cell

  3. Potassium ion (K+) & small number of Na+ diffuse from the parietal cell into the canaliculus

  4. Water becomes dissociated into hydrogen ions (H+) and hydroxyl ion (OH-) in the cell cytoplasm

  5. H+ are actively secreted into the canaliculus in exchange with K+, catalyzed by H+-K+ ATPase

  6. Water passes into the canaliculi by osmosis

  7. Carbon dioxide (CO2) combines with OH- to form bicarbonate ions (HCO3-) under the influence of carbonic anhydrase
• Pepsinogen secretion


  1. Pepsinogen is secreted by peptic and mucus neck cells

  2. When pepsinogen contacts with the HCl from parietal cells, it changes rapidly into pepsin

  3. Pepsin functions as an active proteolytic enzyme in a highly acidic medium (pH: 1.8 – 3.5) but has no proteolytic activity above pH 5
• Intrinsic factor


  Secreted by the parietal cells of oxyntic glands, essential for absorption of vitamin B12 in the ileum
• Mucus secretion


  Secreted by mucous neck cells of oxyntic glands and pyloric glands, coats the stomach mucosa more than 1 millimeter thick providing protection, lubricates food transport, secretes HCO3- to protect the underlying stomach wall from the acidic secretions
• Gastric juice secretion


  1. Produced continuously, controlled neurally and hormonally

  2. Stimulated by histamine via H2 receptors, acetylcholine via muscarinic receptors, and gastrin via gastric receptors on parietal cells
• Enterochromaffin-like (ECL) cells


  Lie in the deep recesses of the oxyntic glands, secrete histamine in direct contact with the parietal cells, have acetylcholine and gastrin receptors, principal pathway by which gastrin and acetylcholine stimulate acid secretion
• Gastrin and acetylcholine


  Have a direct effect on parietal cells and an indirect effect via ECL cells to stimulate histamine secretion, which then promotes acid secretion
• The rate of HCl secretion is directly related to the amount of histamine secreted by the ECL cells
• Mechanisms that stimulate ECL cells to secrete histamine


  • Gastrin hormone from G-cells in the antral portion of the stomach mucosa in response to protein
  • Acetylcholine released from stomach vagal nerve endings
  • Hormonal substances secreted by the enteric nervous system of stomach wall
• Vagal stimulation results in about 500ml of HCl secreted daily, while gastrin release results in about 200ml/day
• Prostaglandins inhibit acid secretion, explaining the increased incidence of ulcers in patients taking anti-inflammatory drugs that inhibit prostaglandin synthesis
• Phases of gastric secretion


  • Cephalic phase
  • Gastric phase
  • Intestinal phase
• Cephalic phase

  Begins before any food reaches the stomach, parasympathetic reflexes operating through the efferent vagus nerves stimulate gastric secretion in response to sight, smell, thought, or taste of food, accounts for about 30% of the gastric secretion associated with eating meals
• Gastric phase


  Starts when food enters the stomach, excites vagovagal reflexes, local enteric reflexes, and gastrin release, accounts for about 60% of total gastric secretion associated with eating a meal and most of the total daily gastric secretion
• Intestinal phase


  Accounts for about 10% of the total secretory response to a meal, the presence of food in the upper portion of the small intestine causes the stomach to secrete a small amount of gastric juice due to the release of gastrin
• Intestinal factors that inhibit gastric secretion


  1. Neural effect (enterogastric inhibitory reflex) triggered by distension, presence of acid, protein breakdown, or irritation of the small intestinal mucosa

  2. Inhibitory hormones: secretin, VIP, GIP
• Emotional state


  Anger and hostility cause hypersecretion and hyperemia, while fear and depression cause hyposecretion, inhibit motility and decrease blood flow
• Other factors that enhance gastric juice release


  • Hypoglycemia
  • Alcohol
  • Caffeine
• Motor functions of the stomach


  • Storage of large quantities of food
  • Mixing of food with gastric secretion to form chyme
  • Slow emptying of chyme for proper digestion and absorption by the small intestine
• Receptive relaxation


  The fundus and upper portion of the body relax before the arrival of food, allowing the stomach's volume to increase to as much as 1-5 liters, mediated by a vagovagal reflex
• Stomach mixing and emptying


  1. Newest food closest to the esophageal opening, oldest food nearest the outer wall

  2. Weak peristaltic constrictor waves (mixing waves) in the mid to upper portions of the stomach wall moving toward the antrum about once every 15-20 seconds

  3. Stronger constrictor rings in the antrum portion force the contents toward the pylorus

  4. Chyme, a murky semifluid or paste, leaves the stomach 1-3 hours after a meal
• Gastric factors that promote emptying


  • Neural effect: increased food volume in the stomach
  • Hormonal: release of gastrin from the antral mucosa in response to digestive products of meat
• Duodenal factors that inhibit stomach emptying


  • Enterogastric reflexes from the duodenum
  • Hormonal feedback: CCK, secretin, GIP
• Type of food ingested


  Carbohydrate-rich food leaves the stomach in a few hours, protein-rich food leaves more slowly, fat-containing food leaves the slowest
• As chyme enters the duodenum, accessory organs like the pancreas, liver and gallbladder add their secretions