Practice 5 structured short- and long-answer medical questions on Weekly Year 2: GIT Physiology Exam - October 9, 2026 (Sections B & C). Review model answers...
Answer: Salivary amylase (ptyalin) is an enzyme present in saliva that begins the hydrolysis of complex carbohydrates (starches) into smaller polysaccharides (dextrins) and disaccharides (maltose). It acts by breaking alpha-1,4 glycosidic bonds. Its action is limited in the stomach because the acidic environment (pH 1.5-3.5) inactivates salivary amylase. The enzyme functions optimally at a neutral to slightly alkaline pH found in the mouth (pH 6.7-7.0). Upon entering the stomach, the bolus of food is surrounded by gastric acid, quickly lowering the pH of the luminal contents and denaturing the amylase.
Answer: The mucus barrier in the gastric mucosa serves several critical functions: 1) Physical barrier: A thick layer of mucus traps bicarbonate ions, creating a pH gradient where the surface epithelium is near neutral while the lumen is highly acidic, protecting cells from HCl. 2) Lubrication: Facilitates the passage of food. 3) Antimicrobial: Contains antimicrobial substances like lysozyme and defensins. 4) Trapping growth factors: Mucus can trap epidermal growth factor (EGF) which promotes epithelial cell repair. Regulatory mechanisms include: 1) Prostaglandins (PGE2, PGI2): Stimulate mucus and bicarbonate secretion and increase mucosal blood flow, which is crucial for maintaining the barrier. 2) Hormones: Gastrin can stimulate mucus secretion. 3) Neural control: Vagal stimulation can also increase mucus production. 4) Autoregulation: Mechanical stimulation by food or irritants can also trigger mucus secretion.
Answer: Villi are finger-like projections that increase the surface area of the small intestine by approximately 10-fold. Microvilli are even smaller projections on the apical surface of the enterocytes lining the villi, further increasing the surface area by another 20-fold. Collectively, villi and microvilli increase the absorptive surface area of the small intestine by about 600 times compared to a smooth tube. This vast surface area is essential for efficient absorption of digested nutrients. Three specific nutrients absorbed primarily in the duodenum include: iron, calcium, and water-soluble vitamins (like folate and vitamin C).
Answer: The cephalic phase of gastric secretion is the initial phase, triggered by the sight, smell, taste, or thought of food. It prepares the stomach for the arrival of food by stimulating the secretion of gastric acid and pepsinogen. The primary stimuli that activate this phase include: 1) Sensory input: Seeing, smelling, or tasting food. 2) Cognitive input: Thinking about food, anticipating a meal. 3) Psychological factors: Stress or anxiety can inhibit this phase. The neural pathway involves the vagus nerve, which stimulates the parietal cells to secrete HCl and the chief cells to secrete pepsinogen, and also stimulates ECL cells to release histamine, which further potentiates acid secretion.
Answer: The pancreas has both exocrine and endocrine functions. The exocrine pancreas secretes digestive enzymes and bicarbonate into the duodenum, while the endocrine pancreas (islets of Langerhans) secretes hormones like insulin and glucagon into the bloodstream. **Exocrine Pancreatic Secretion Regulation:** Following a meal, pancreatic exocrine secretion is regulated by a combination of neural and hormonal factors: 1. **Cephalic Phase:** Similar to gastric secretion, the sight, smell, and thought of food can trigger a mild vagal stimulation, leading to a small release of pancreatic enzymes. 2. **Gastric Phase:** As the stomach fills and acidifies, gastric distension and the presence of partially digested proteins can stimulate vagal reflexes and gastrin release, contributing to pancreatic secretion. 3. **Intestinal Phase:** This is the most significant phase of regulation. * **Secretin:** Released from S cells in the duodenal mucosa in response to low pH (acid) entering the duodenum from the stomach. Secretin primarily stimulates the pancreas to secrete a bicarbonate-rich fluid, which neutralizes gastric acid, creating an optimal pH for enzyme activity. * **Cholecystokinin (CCK):** Released from I cells in the duodenal and jejunal mucosa in response to fatty acids and amino acids (products of fat and protein digestion) in the lumen. CCK is the primary stimulator of pancreatic enzyme secretion. It also stimulates gallbladder contraction and relaxes the sphincter of Oddi, facilitating bile and pancreatic juice flow into the duodenum. * **Vasoactive Intestinal Peptide (VIP) and Gastric Inhibitory Peptide (GIP):** Also contribute to pancreatic secretion, often in conjunction with secretin and CCK. **Role of Pancreatic Enzymes in Macronutrient Digestion:** * **Carbohydrate Digestion:** Pancreatic amylase continues the digestion of starches that began in the mouth, breaking them down into disaccharides (maltose, sucrose, lactose) and smaller oligosaccharides. * **Protein Digestion:** The pancreas secretes inactive zymogens that are activated in the duodenum: * **Trypsinogen:** Activated by enterokinase (a brush border enzyme) to trypsin. Trypsin then activates other trypsinogen molecules and other pancreatic zymogens (chymotrypsinogen, procarboxypeptidase, proelastase). * **Chymotrypsinogen:** Activated to chymotrypsin. * **Procarboxypeptidase:** Activated to carboxypeptidase. * **Proelastase:** Activated to elastase. These enzymes break down proteins into smaller peptides and finally into amino acids and di- or tri-peptides, which are then further digested by brush border enzymes. * **Fat Digestion:** The primary enzyme is pancreatic lipase. It hydrolyzes triglycerides into fatty acids and monoglycerides. Bile salts, secreted by the liver, emulsify fats, increasing the surface area for lipase action. Colipase, also secreted by the pancreas, binds to lipase and prevents its inhibition by bile salts, ensuring efficient fat digestion. Other lipases include phospholipase A2 and cholesterol esterase, which digest phospholipids and cholesterol esters, respectively.