Practice 5 structured short- and long-answer medical questions on Weekly Year 2: GIT Physiology Exam - September 25, 2026 (Sections B & C). Review model answ...
Answer: The cephalic phase of gastric secretion is the anticipatory phase, preparing the stomach for incoming food. It is initiated by the sight, smell, taste, or thought of food. Vagal nerve stimulation leads to acetylcholine release, which stimulates parietal cells to secrete HCl and chief cells to secrete pepsinogen. Three stimuli are: sight of food, smell of food, thought of food, taste of food.
Answer: Secretin is released from the duodenum in response to acidic chyme. It inhibits gastric acid secretion primarily by acting on the parietal cells. Secretin binds to receptors on parietal cells and reduces the intracellular concentration of cyclic AMP (cAMP), thereby dampening the stimulatory effect of histamine and gastrin on acid production. It also indirectly reduces gastric motility.
Answer: Pancreatic bicarbonate plays a crucial role in neutralizing the acidic chyme entering the duodenum from the stomach. This neutralization is vital for several reasons: 1. It protects the duodenal mucosa from acid damage. 2. It creates an optimal pH environment (around 7-8) for the action of pancreatic digestive enzymes, which are most active in a slightly alkaline medium. 3. It facilitates the absorption of certain nutrients by maintaining the appropriate pH.
Answer: Micelles are small, water-soluble aggregates formed in the lumen of the small intestine. They consist of bile salts, phospholipids, cholesterol, and fat-soluble vitamins, with their hydrophobic ends oriented inwards and hydrophilic ends outwards. Free fatty acids and monoglycerides, produced by lipase activity, are incorporated into the hydrophobic core of the micelles. This process increases the surface area of the lipids, bringing them into close contact with the brush border enzymes and the intestinal epithelial cells, thus facilitating their digestion and absorption across the cell membrane.
Answer: Gastric emptying is a precisely regulated process that controls the rate at which stomach contents are delivered into the duodenum. It involves a complex interplay of factors within the stomach, duodenum, and neural pathways. **Gastric Factors:** * **Volume of chyme:** Increased volume in the stomach stretches the stomach wall, activating mechanoreceptors. This leads to increased gastric motility via the vagus nerve, promoting emptying. * **Gastric distension:** Similar to volume, distension activates intrinsic enteric neurons and extrinsic vagal pathways to increase antral contractions. * **Gastrin:** This hormone, released by G cells in the stomach in response to distension and partially digested proteins, enhances gastric motility and relaxes the pyloric sphincter, thus promoting emptying. **Duodenal Factors:** These factors primarily act to inhibit gastric emptying, ensuring that the duodenum is not overwhelmed by acidic or hyperosmotic chyme. * **Acidity:** The presence of acid in the duodenum triggers the release of secretin from S cells. Secretin inhibits gastric emptying by reducing gastric motility and increasing pyloric sphincter tone. This allows time for bicarbonate to neutralize the acid. * **Fatty acids and amino acids:** Their presence in the duodenum stimulates the release of cholecystokinin (CCK) from I cells. CCK is a potent inhibitor of gastric emptying, acting by reducing gastric motility and contracting the pyloric sphincter. It also stimulates gallbladder contraction and pancreatic enzyme secretion, reflecting its role in digestion. * **Osmolarity:** Hyperosmolar chyme entering the duodenum leads to a reflex inhibition of gastric emptying. This prevents excessive water movement from the intestinal mucosa into the lumen. * **Dumping:** Rapid entry of hyperosmolar chyme into the duodenum can cause 'dumping syndrome'. * **Enteric nervous system:** Receptors in the duodenal mucosa sense these factors (acidity, fat, osmolarity) and activate the intestino-gastric reflex, which is mediated by the enteric nervous system and the extrinsic vagal nerves. This reflex inhibits gastric emptying. **Extrinsic Neural Control:** * **Vagus nerve:** The vagus nerve plays a dual role. It promotes gastric motility and relaxation of the pyloric sphincter during the cephalic and gastric phases. However, it also mediates inhibitory reflexes from the duodenum back to the stomach in response to duodenal stimuli (e.g., the intestino-gastric reflex). * **Sympathetic nerves:** Generally, sympathetic stimulation inhibits gastric emptying by reducing motility and increasing pyloric tone. However, their role in normal physiological regulation is less prominent than the vagal influence. **Interaction of Factors:** The rate of gastric emptying is a dynamic balance. When food enters the stomach, gastric factors promote emptying. As chyme enters the duodenum, duodenal factors become dominant, sensing the composition and volume of the chyme. If the chyme is acidic, fatty, or hyperosmolar, duodenal inhibitory mechanisms are activated, slowing gastric emptying to allow for neutralization, digestion, and absorption. Hormonal signals (secretin, CCK) and neural reflexes (vagal, enteric) work in concert. For instance, the presence of fat in the duodenum triggers CCK release, which both slows gastric emptying and stimulates the pancreas and gallbladder to release digestive enzymes and bile, preparing the intestine for further processing of the meal. This coordinated regulation ensures that the small intestine receives chyme at a rate that allows for efficient digestion and absorption, preventing damage to the intestinal mucosa.