Practice 5 structured short- and long-answer medical questions on Weekly Year 2: GIT Physiology Exam - May 15, 2026 (Sections B & C). Review model answers fo...
Answer: Intrinsic innervation (enteric nervous system): Composed of myenteric and submucosal plexuses. Myenteric plexus controls muscle contractions (motility) via peristalsis. Submucosal plexus regulates secretion from glands and blood flow. It can operate independently but is modulated by extrinsic input. Extrinsic innervation (autonomic nervous system): Parasympathetic (vagus nerve): Generally stimulates gastric motility and secretion (acetylcholine is the primary neurotransmitter). It enhances gastric emptying and acid production. Sympathetic (celiac ganglia): Generally inhibits gastric motility and secretion (norepinephrine). It can slow gastric emptying and reduce secretion, playing a role in stress responses.
Answer: Gastric acid is secreted by parietal cells in the gastric glands. Stimulation: 1. Acetylcholine (from parasympathetic nerves, vagal stimulation): Binds to muscarinic receptors on parietal cells, increasing intracellular calcium and activating the H+/K+-ATPase pump. 2. Gastrin (from G cells in the antrum): Binds to CCK2 receptors on parietal cells, also increasing intracellular calcium and activating the pump. 3. Histamine (from ECL cells): Binds to H2 receptors on parietal cells, increasing intracellular cAMP, which synergistically enhances the action of acetylcholine and gastrin on the pump. Inhibition: 1. Somatostatin (from D cells in the antrum): Released in response to low pH. Inhibits gastrin release and directly inhibits parietal cell activity. 2. Secretin (from S cells in the duodenum): Released in response to acidic chyme. Inhibits parietal cell secretion and stimulates bicarbonate secretion from the pancreas and duodenum.
Answer: Carbohydrates are absorbed as monosaccharides (glucose, galactose, fructose). 1. Digestion: Disaccharides (sucrose, lactose, maltose) are broken down into monosaccharides by brush border enzymes (sucrase, lactase, maltase) located on enterocytes. 2. Glucose and Galactose Absorption: Absorbed via secondary active transport using the sodium-glucose cotransporter 1 (SGLT1) located on the apical membrane of enterocytes. This transport is driven by the sodium gradient maintained by the basolateral Na+/K+-ATPase. 3. Fructose Absorption: Absorbed via facilitated diffusion through glucose transporter 5 (GLUT5) on the apical membrane. 4. Monosaccharide Exit: All absorbed monosaccharides exit the enterocyte into the bloodstream via facilitated diffusion through GLUT2 transporters on the basolateral membrane.
Answer: Bile salts are produced by the liver and secreted into the duodenum. Their primary role in fat digestion and absorption is emulsification and micelle formation. Emulsification: Bile salts are amphipathic molecules. They break down large fat globules into smaller droplets, increasing the surface area available for pancreatic lipase to act upon. This is achieved by the hydrophobic portion of the bile salt interacting with the fat and the hydrophilic portion interacting with the surrounding aqueous environment. Micelle Formation: Bile salts surround the products of fat digestion (fatty acids, monoglycerides) along with fat-soluble vitamins and cholesterol, forming small aggregates called micelles. The hydrophobic tails of bile salts face inwards, enclosing the lipid components, while the hydrophilic heads face outwards, allowing the micelle to remain soluble in the aqueous intestinal lumen. This facilitates the transport of these digested lipids to the brush border of enterocytes for absorption.
Answer: The small intestine is the primary site for nutrient absorption, with specialized functions across its segments (duodenum, jejunum, ileum). Duodenum: - Primary site for the action of pancreatic enzymes and bile. - Absorption of iron, calcium, folate, and vitamin D begins here. The acidic environment is favorable for iron absorption (Fe2+). Calcium absorption is vitamin D-dependent. - Minimal absorption of carbohydrates, proteins, and fats occurs here as digestion is still ongoing. Jejunum: - The principal site for the absorption of most nutrients. - Carbohydrate Absorption: Monosaccharides (glucose, galactose, fructose) are absorbed. Glucose and galactose via SGLT1 (secondary active transport with Na+) on apical membrane, then facilitated diffusion via GLUT2 on basolateral. Fructose via GLUT5 (facilitated diffusion) on apical, then GLUT2 on basolateral. - Protein Absorption: Amino acids, dipeptides, and tripeptides are absorbed. Amino acids and small peptides by Na+-dependent cotransporters (secondary active transport). Di/tripeptides by PepT1 (H+-dependent cotransporter). - Fat Absorption: Products of fat digestion (monoglycerides, free fatty acids, cholesterol, fat-soluble vitamins) form micelles with bile salts. Micelles transport lipids to the brush border. Lipids diffuse into enterocytes, re-esterified to triglycerides, packaged into chylomicrons, and enter lymphatics (lacteals). - Water-soluble vitamins (B vitamins, C) are absorbed here via various transporters (active and passive). Ileum: - Absorption of vitamin B12 and bile salts is crucial here. - Vitamin B12: Absorbed bound to intrinsic factor (produced by parietal cells in the stomach) via specific receptors in the terminal ileum. - Bile Salts: Undergo enterohepatic circulation. Reabsorbed via Na+-dependent bile salt cotransporter (ASBT) on the apical membrane and exit via facilitated diffusion on the basolateral membrane. Crucial for further fat absorption downstream. - Significant absorption of remaining water and electrolytes occurs. General Mechanisms: - Brush Border Enzymes: Disaccharidases (sucrase, lactase, maltase) and peptidases at the brush border complete digestion. - Transporters: Numerous specific transporters (active and facilitated) for monosaccharides, amino acids, peptides, ions, and vitamins. - Micelles: Essential for efficient absorption of fats and fat-soluble vitamins. - Chylomicrons: Formed within enterocytes to transport absorbed fats into the lymphatic system. - Water Absorption: Occurs passively via osmosis, driven by solute gradients created by nutrient absorption. Primarily occurs throughout the small intestine, with a significant amount absorbed in the ileum. - Electrolyte Absorption: Sodium is absorbed via various mechanisms (cotransport with nutrients, Na+/H+ exchange, epithelial sodium channels). Chloride, potassium, and bicarbonate are also actively and passively absorbed. Calcium and iron absorption are regulated processes.