Questions: Gastric Secretion and Digestion Physiology

Question 1

A patient regularly takes ibuprofen (an NSAID that inhibits prostaglandin synthesis) for chronic pain and develops a gastric ulcer. The most likely mechanism is:

• A. Decreased gastric acid production, because prostaglandins are required for parietal cell activation
• B. Impaired mucosal barrier — prostaglandins normally stimulate mucus and bicarbonate secretion and maintain mucosal blood flow
• C. Delayed gastric emptying, because prostaglandins control pyloric sphincter relaxation
• D. Excessive pepsin activity, because prostaglandins normally inhibit chief cell pepsinogen secretion

Answer: (B)

Prostaglandins protect the gastric mucosa by stimulating surface epithelial cells to secrete mucus and bicarbonate, and by maintaining the mucosal microcirculation that clears any acid penetrating the barrier. NSAIDs inhibit COX enzymes, blocking prostaglandin synthesis and stripping away this protection. The result is that even the normally manageable level of gastric acid erodes the now-unprotected epithelium. Acid production is actually increased or unchanged — the problem is lost protection, not reduced acid.

Question 2

Chief cells secrete pepsinogen rather than active pepsin. Why is this arrangement physiologically necessary?

• A. Pepsin is too large a molecule to be secreted by exocytosis and must be cleaved to a smaller form first
• B. Active pepsin secretion would digest the chief cells themselves and the gastric epithelium before reaching the luminal contents
• C. Pepsinogen must first bind to intrinsic factor in the lumen before it can acquire protease activity
• D. Pepsin is only effective at neutral pH, so it must be stored as pepsinogen and converted to its active form outside the acidic stomach

Answer: (B)

This is a safety mechanism against autodigestion. Pepsin is a potent protease that works optimally at pH 1.5–2.5. If chief cells secreted it in active form, pepsin would digest the cellular machinery producing it and erode the surrounding mucosa. By secreting the inactive zymogen pepsinogen instead, the cell is protected. Pepsinogen is only converted to pepsin in the gastric lumen when it encounters the low pH created by parietal cells — a tight functional coupling that ensures enzyme activation occurs only where it can safely work.

Question 3

The cephalic phase of gastric acid secretion begins primarily after food physically enters the stomach and distends the gastric wall.

• T. True
• F. False

Answer: False

The cephalic phase is a feedforward response triggered by the sight, smell, taste, or even the thought of food — before food reaches the stomach at all. Vagal activation directly stimulates parietal cells (via acetylcholine) and triggers gastrin release from antral G cells, accounting for roughly 30% of total acid output. It is the gastric phase that begins when food physically arrives and distends the stomach. The cephalic phase is the stomach anticipating and preparing for the meal.

Question 4

The intestinal phase of gastric secretion serves primarily as an inhibitory feedback mechanism — duodenal hormones like secretin and CCK slow gastric acid secretion and gastric emptying when chyme enters the duodenum.

• T. True
• F. False

Answer: True

The intestinal phase is the stomach's feedback shutoff. When acidic, fat- and protein-rich chyme enters the duodenum, S cells release secretin (in response to acid) and I cells release CCK (in response to fats and proteins). Secretin inhibits gastric acid secretion; CCK slows gastric emptying by reducing antral contractions and increasing pyloric tone. Together these hormones prevent the small intestine from being overwhelmed by more chyme than it can neutralize and absorb — matching delivery rate to processing capacity.

Question 5

Explain the functional logic of pepsinogen secretion — why is secreting an inactive protease precursor a better design than secreting the active enzyme directly?

Model answer: Secreting pepsinogen rather than active pepsin prevents autodigestion. If chief cells secreted active pepsin, it would digest the protein machinery of the secreting cells and the surrounding gastric epithelium before reaching the luminal food. Pepsinogen is an inactive zymogen that only converts to active pepsin in the acidic gastric lumen (pH < 2), where low pH cleaves the inhibitory peptide. Active pepsin then autocatalytically activates more pepsinogen, rapidly amplifying enzyme activity in the lumen — the one place where proteolysis is useful and safe.

This zymogen strategy is used throughout the digestive system (trypsinogen, chymotrypsinogen, proelastase) and illustrates a general principle: dangerous enzymes are stored and transported in inactive forms, activated only at the site of action. The tight functional coupling here — parietal cells create the acidic environment that activates the chief-cell enzyme — means pepsin activity is automatically localized to the gastric lumen when acid is present. Proton pump inhibitors (PPIs) exploit this coupling: by raising intragastric pH, they indirectly reduce pepsin activity even though they don't target pepsinogen or pepsin directly.