Gastric and Pancreatic Secretions
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Direct answer
Parietal cells pump hydrogen ions at a concentration of about 150-160 mEq/L — a million-fold gradient, yielding gastric juice at pH 0.8-1 — from a daily output of 2-2.5 litres, and secrete intrinsic factor, the only truly indispensable gastric product. Acid secretion is driven by three potentiating stimuli: acetylcholine from the vagus, gastrin from antral G cells, and histamine from ECL cells acting on H2 receptors — potentiation means blocking one (an H2 blocker) blunts all three. Secretion runs cephalic (about 30 per cent, vagal), gastric (60 per cent, distension and peptides) and intestinal (10 per cent) phases, shut off by somatostatin from D cells when antral pH falls below 2-3. The pancreas delivers 1-1.5 litres daily: secretin from duodenal S cells, released at pH below 4.5, drives the ductal bicarbonate (up to 140-145 mEq/L) that neutralises acid, while cholecystokinin from I cells drives the enzymatic secretion, gallbladder contraction and sphincter of Oddi relaxation. Trypsinogen is activated by duodenal enterokinase, and trypsin then activates every other proenzyme.
What you must remember
- Cell map: parietal cells — HCl plus intrinsic factor (parietal cells are the vitamin B12 story); chief cells — pepsinogen; G cells — gastrin; ECL cells — histamine; D cells — somatostatin brake.
- Acid numbers: H+ concentration 150-160 mEq/L, juice pH 0.8-1, volume 2-2.5 L/day; the proton pump (H-K ATPase) is the omeprazole target.
- Stimulus potentiation: acetylcholine, gastrin and histamine amplify one another — H2 blockade cuts vagal and gastrin-driven acid too; vagotomy removes the cholinergic arm.
- Phase arithmetic: cephalic 30 per cent (sight, taste, vagal), gastric 60 per cent (distension, amino acids and peptides, local reflexes), intestinal 5-10 per cent.
- Feedback brake: antral acid below pH 2-3 releases somatostatin, which suppresses gastrin — the reason pernicious anaemia (achlorhydria) presents with high gastrin.
- Pancreatic numbers: 1-1.5 L/day, juice pH about 8, bicarbonate up to 140-145 mEq/L; secretin (pH under 4.5 trigger) for the watery alkaline load, CCK for enzymes; vagal stimulation adds enzyme-rich secretion.
- Activation cascade: enterokinase on the duodenal brush border converts trypsinogen to trypsin; trypsin activates chymotrypsinogen, proelastase, procarboxypeptidases and prophospholipase; pancreatic secretory trypsin inhibitor prevents autodigestion.
One meal, two organs
Watch a meal arrive. Cephalic first: the sight and smell of food fire the vagus, acetylcholine reaches parietal and ECL cells, and gastrin-releasing peptide nudges G cells — 30 per cent of the acid is out before the first swallow. Food lands, distends the antrum, and amino acids and peptides directly stimulate G cells: the gastric phase adds 60 per cent, gastrin now the strongest driver. Chyme then hits the duodenum at pH 3, and the S cells respond to the acid itself — secretin pours bicarbonate into the duct lumen, neutralising acid to create the pH 6-9 window digestive enzymes require, while CCK, sensing fatty acids and amino acids, orders out enzymes and empties the gallbladder against a relaxed sphincter of Oddi. Enterokinase unlocks trypsin, trypsin unlocks the rest, and within the micelle-rich duodenum the meal is dismantled. The elegance the exam rewards: the duodenum controls its own pH — acid entering triggers the bicarbonate that neutralises it, a classical negative feedback you should be able to quote as "acid in the duodenum inhibits acid secretion and stimulates pancreatic bicarbonate".
Where students slip
The recurring slip is intrinsic factor versus acid: the stomach's only truly essential secretion is intrinsic factor — patients can digest happily after acid blockade or partial gastrectomy, but B12 absorption fails without intrinsic factor, presenting years later with megaloblastic anaemia and neuropathy. Second, secretin versus CCK duties: secretin is the bicarbonate-and-water hormone from S cells triggered by acid; CCK is the enzyme-and-gallbladder hormone from I cells triggered by fat and protein — candidates swap them under time pressure. Third, the gastrin feedback: acid suppresses gastrin, so hypochlorhydria (pernicious anaemia, prolonged proton pump inhibitor use) elevates gastrin and ECL hyperplasia follows. Finally, know why pancreatitis is autodigestion: premature trypsin activation inside the gland overwhelms the trypsin inhibitor — the physiological basis of the screening vignette of acute pancreatitis after a heavy meal with alcohol.
Frequently asked questions
Which gastric secretion is physiologically indispensable?
Intrinsic factor from parietal cells — the stomach can be bypassed for acid, but without intrinsic factor, terminal ileal B12 absorption fails.
What are the three phases of gastric secretion and their contributions?
Cephalic about 30 per cent (vagal), gastric about 60 per cent (distension and peptides via gastrin), and intestinal 5-10 per cent.
How is acidic chyme in the duodenum neutralised?
Secretin from S cells, released when duodenal pH falls below 4.5, drives pancreatic ductal bicarbonate secretion up to 140-145 mEq/L plus biliary and intestinal buffers.
What activates trypsinogen?
Enterokinase (enteropeptidase) on the duodenal brush border; the trypsin generated then activates the other pancreatic proenzymes.
Why do proton pump inhibitors raise serum gastrin?
Blocking acid leaves antral pH high, removing the somatostatin-mediated brake on G cells, so gastrin secretion rises — usually modest and reversible.