Stomach Anatomy and Functions

Stomach Activities

• Two main activities in the stomach are gastric secretion and gastric motility.

Stomach Anatomy

• The stomach is sac-like and divided into three regions: fundus, body, and antrum.
• The stomach wall has distinct layers.
• The mucosa has folds (rugae) and pits opening into gastric glands.
• Different cell types in the mucosa secrete various gastric juice components, including hydrochloric acid (HCl), intrinsic factor, pepsinogen, gastrin, mucus, and bicarbonate.

Stomach Regions

• The esophagus connects to the stomach at the lower esophageal sphincter.
• The fundus is the top portion of the stomach.
• The body is the main part of the stomach.
• The antrum is the lower portion of the stomach, which connects to the duodenum via the pyloric sphincter.
• The duodenum is the first part of the small intestine.

Stomach Functions

• Storage: The stomach stores food and controls its emptying into the duodenum (most important functions)
• Digestion: The stomach digests food using enzymes, HCl, and motility, three main food types are digested
• Secretion: The stomach secretes HCl, pepsinogen, intrinsic factor, gastric lipase, mucus, and bicarbonate.
• Absorption: The stomach absorbs water, alcohol, and some drugs.
• Protection: The stomach protects its lining using bicarbonate mucus barrier, and secretion of inactive pepsinogen.

Gastric Secretion

• Composition: Gastric juice contains hydrochloric acid, pepsinogen & gastric lipase, mucus, bicarbonate, and intrinsic factor.
• Rate: The rate of secretion is 2-3 liters per day.
• pH: Gastric juice is highly acidic (pH 1-4).
• Protection: Mucosa needs protection from its own secretions.

Glands and Cell Types in Gastric Mucosa

• Body: Parietal glands contain oxyntic cells, chief cells, and mucus neck cells.
• Antrum: Pyloric glands contain mucus neck cells and G cells.

Gastric Hydrochloric Acid Secretion

• Origin: Secreted by parietal cells in oxyntic glands.
• Functions: Essential for activating pepsinogen to pepsin, for acidifying stomach contents, and to sterilize foods, for absorbing iron & calcium
• pH: Stomach acidity is maintained between 1-4

Mechanisms for HCl Secretion

• The process involves complex cellular mechanisms and transporters to move hydrogen ions (H+) into the lumen of the stomach.

Cellular Mechanisms of HCl Secretion

• Secretion stimulated by acetylcholine, gastrin, histamine.

Regulation of Gastric Acid Secretion

• Cephalic Phase: Responsible for about 30% of gastric acid secretion that occurs through reflex, involving several stimuli like smell, taste, and thinking of food.
• Gastric Phase: Responsible for about 60% of gastric acid secretion, stimulated by the presence of food in the stomach.
• Intestinal Phase: Responsible for about 10% of gastric acid secretion, triggered by the presence of food in the small intestine.

Peptic Ulcer

• Cause: A lesion in the stomach or duodenum caused by the erosive action of HCl and pepsin.
• Types: Gastric ulcer (stomach) and duodenal ulcer (duodenum).
• Causes of ulcers: Loss of protective mucus barrier, damage factors (HCl, pepsin, H pylori, NSAIDs, smoking, stress, and alcohol).
• Treatment: antacids, omeprazole, cimetidine, ranitidine, and surgery.

Secretion of Pepsinogens, Gastric Lipase, Intrinsic Factor, Mucus & Bicarbonate, Gastrin

• Pepsinogens: Inactive precursors of pepsin, activated by HCl and pepsin.
• Gastric Lipase: Enzyme to digest fat.
• Intrinsic Factor: Glycoprotein to absorb vitamin B12.
• Mucus and Bicarbonate: Provide a protective barrier for the stomach lining.
• Gastrin: Hormone stimulating gastric acid secretion.

Gastric Lipase

• Hydrolyzes triacylglycerols (particularly those with short-chain fatty acids).
• Acts at a pH of 4-7.
• Less important than pancreatic lipase.

Intrinsic Factor

• Secreted by parietal cells.
• Combines with vitamin B12 to protect it during passage in the small intestine.
• Absorbs vitamin B12 in the distal ileum.

Mucus & Bicarbonate

• Secreted by mucus neck cells and surface epithelial cells.
• Creates a protective barrier against mechanical and chemical damage.

Gastrin

• Secreted by G cells in the antrum and upper small intestine.
• Stimulates gastric acid secretion and other functions. (physiological & pharmacological functions)
• Regulated by many factors including emotions and high stomach acidity.

Gastric Motility

• Orad Region: Fundus and proximal body, thin walled, and responsible for receptive relaxation.
• Caudal Region: distal body and antrum, thick walled, responsible for peristaltic contractions, responsible for mixing and grinding food.
• Slow Wave Activities: The electrical basis of gastric motility, originating in the greater curvature at a rate of 3/minute.

Components of Gastric Motility During Fed State

• Receptive relaxation, peristaltic contractions, and gastric emptying.

Receptive Relaxation

• Relaxation of the proximal part of the stomach, including the fundus and proximal body to accommodate food.
• Continuation of esophageal motility.
• Mediated through the vagovagal reflex.

Peristaltic Contractions

• Main motility function in the distal part of the stomach.
• Propulsion and retropulsion of food.
• Grinding and mixing of food.
• Initiated by distension and mediated by vagovagal reflexes.

Gastric Emptying

• The process releases partially digested food into the duodenum.
• It is regulated by coordinated motility activities in the antrum, pylorus, and duodenal bulb (gastroduodenal pump).
• Emptying time varies from 1.5-6 hours for healthy stomachs.

Regulation of Gastric Emptying

• The rate of emptying is dependent on conditions like the state of the stomach, pyloric sphincter, and duodenum.
• Factors that control gastric emptying: characteristics of food in stomach (chemical & physical), factors that influence rate such as: gastrin & presence of H+ & fats in duodenum.,

1. Factors enhance gastric emptying: gastrin & large volumns of food.
2. Factors slow gastric emptying: presence of H+ & fats in the duodenum

Migrating Myoelectric Complexes (MMCs)

• Peristaltic contractions that replace the fed state gastric motility functions.
• Occur in fasting state.
• Clean the stomach.

Vomiting (Emesis)

• Mechanism: Forceful expulsion of stomach and intestinal contents through the mouth involving a number of steps: (salivation, retching, increased pressure in the stomach)
• Regulation: Centrally regulated by a vomiting center in the medulla, and also receives inputs from different areas of the brain, and the gastrointestinal tract.
• Stimulation: Stimulated by irritant substances, over distension, and over stimuli from internal and external environments.
• Components: Autonomic changes including increased heart rate, increased breathing rate, sweating, and dilated pupils, accompanied by peripheral or central stimulation of various receptors (irritation of mucosa, etc.)

Control of Vomiting

• Medulla centers: Two centers in the medulla are involved in the regulation.
  • Vomiting center
  • Chemosensitive trigger zone
• Sensory Input: Sensory signals from the gastrointestinal tract (distension or irritation), vestibular system, and other areas reach the vomiting center.
• Drug effects: Drugs like morphine affect the chemosensitive trigger zone which influences the vomiting center

Explore the anatomy and functions of the stomach through this informative quiz. Learn about gastric secretion, motility, and the specific regions of the stomach. Test your knowledge on the roles and structures that contribute to digestion and food storage.