Overview
This lecture covers how the GI tract secretes fluid, mucus, enzymes, acid and hormones. It classifies secretion as endocrine or exocrine, quantifies the volume and composition of the roughly 8 L secreted daily, sets out the general cellular machinery of epithelial secretion, and then works through specific mechanisms in the stomach, pancreas, salivary glands and small intestine, including the neural and hormonal control of gastric acid secretion.
Functions and Classification of GI Secretion
- The GI tract moves, secretes, digests and absorbs.
- Secretion serves three roles: lubricants (water, mucus), chemicals (enzymes, acid, electrolytes) and controls (hormones).
- Secretions are classified as endocrine or exocrine.
- Endocrine secretions: Gastrin, Secretin, Cholecystokinin (CCK), GIP, Somatostatin. These are peptide hormones released by enterochromaffin cells in the mucosa. They coordinate and control motility and secretion, usually in response to luminal contents. Example: acid chyme entering the duodenum triggers secretin release, which drives HCO3- secretion to neutralise the acid.
- Exocrine secretions: water/electrolytes, mucus, enzymes. Functions: prepare food for absorption (dilution, optimising pH, digestion), lubricate luminal contents, protect the mucosa.
Daily secretion volumes and composition:
| Secretion | Volume (L/day) | Osmolality (mosmol/L) | pH | Na (mmol/L) | K (mmol/L) | Cl (mmol/L) | HCO3 (mmol/L) |
|---|---|---|---|---|---|---|---|
| Saliva | 1.5 | 100 | 7.5 | 40 | 15 | 25 | 30 |
| Gastric | 3.0 | 200 | 1.0 | 50 | 10 | 100 | 0 |
| Pancreatic | 1.5 | 300 | 7.8 | 140 | 10 | 70 | 80 |
| Bile | 0.5 | 300 | 7.5 | 140 | 2 | 80 | 20 |
| Small intestine | 1.5-2.0 | isosmotic NaCl solution | |||||
| Colon | small volume; mucus + alkaline solution rich in HCO3 and K |
Total: roughly 8 litres/day secreted into the GI lumen.
Digestive secretions by organ (recurring roadmap diagram, shown three times):
- Salivary glands: mucus, alpha-amylase
- Stomach: HCl, pepsinogens, mucus (plus water/NaCl)
- Pancreas: HCO3-, hydrolytic enzymes
- Liver and gallbladder: HCO3-, bile salts, bile pigments
- Small intestine: hydrolytic enzymes from desquamated cells
- Large intestine: mucus
Epithelial Secretion: General Principles
- Secretory epithelia sit at an interface between lumen and interstitial fluid.
- Cells are asymmetrical: the apical membrane (facing the lumen) differs from the basolateral membrane (facing interstitial fluid), each with its own set of transport pathways.
- There is a variety of specific transport pathways (channels, cotransporters, exchangers, pumps) distributed asymmetrically between the two membranes.
- Epithelia are classed as “leaky” versus “tight” [slide does not elaborate on the distinction].
- Because the lumen is effectively “the outside world,” secretion represents a loss of fluid and solute from the body unless it is reabsorbed downstream.
General secretory epithelial cell model:
- Basolateral membrane (facing interstitial fluid): a Na+/K+/2Cl- cotransporter brings Na+, K+ and Cl- into the cell; a Na+/K+-ATPase pumps Na+ out and K+ in (using ATP); a K+ channel recycles K+ back out.
- Apical membrane (facing lumen): a Cl- channel lets the accumulated Cl- leave the cell into the lumen.
- Paracellular pathway: the resulting electrochemical gradient drives H2O and Na+ from interstitial fluid into the lumen between cells.
- The apical Cl- channel is the step highlighted separately (on a repeat of this diagram) as the key regulated point setting the rate of net NaCl and water secretion.
Gastric Secretion
- Volume: 2-3 L/day, containing water, electrolytes, HCO3-, HCl, mucus, pepsins, gastric lipase and IF.
- Controlled by both neural and hormonal signals, across cephalic, gastric and intestinal phases [slide does not elaborate on what distinguishes the three phases].
- Gastric gland structure, from the gastric pit down to the base of the gland: surface epithelial cells (secrete HCO3-) -> mucous cells -> oxyntic cells (secrete acid) -> chief cells (secrete enzymes).
HCl secretion by oxyntic cells:
- Inside the cell, carbonic anhydrase catalyses CO2 + H2O <-> H+ + HCO3-.
- Apical membrane (lumen side): H+/K+-ATPase pumps H+ into the lumen and K+ into the cell (using ATP); a K+ channel recycles K+ back to the lumen; a Cl- channel lets Cl- reach the lumen; H2O follows into the lumen.
- Basolateral membrane (interstitial fluid side): a Cl-/HCO3- exchanger sends HCO3- into the blood in exchange for Cl-; a Na+/H+ exchanger; a Na+/K+-ATPase pump moves Na+ out and K+ in; a K+ channel lets K+ leave. H2O also crosses to the interstitial fluid.
Regulation of gastric acid secretion:
- The vagus nerve releases ACh onto both the antrum and the fundus.
- In the antrum: ACh acts on the D cell and the G cell. The D cell releases somatostatin (SST), which inhibits the G cell; GRP acts on the G cell as a stimulating signal. The G cell releases gastrin.
- Circulating gastrin stimulates both the D cell and the ECL cell.
- In the fundus: ACh acts directly on the oxyntic cell and on the ECL cell; the ECL cell releases histamine, which stimulates the oxyntic cell. The D cell’s somatostatin inhibits both the ECL cell and the oxyntic cell.
- The oxyntic cell integrates three stimulatory inputs (histamine via a Gs-coupled receptor, gastrin, and ACh) acting through Ca2+ and cAMP pathways to drive H+ secretion into the gastric lumen.
- Feedback: luminal acid (H+) inhibits further G cell/gastrin release (negative feedback); luminal amino acids/digested protein stimulate gastrin release (positive feedback).
Pancreatic Duct Secretion
Pancreatic duct cells secrete HCO3- into the lumen, the mirror image of oxyntic cell H+ secretion:
- Inside the cell, carbonic anhydrase catalyses CO2 + H2O <-> HCO3- + H+.
- Apical membrane (lumen side): a Cl-/HCO3- exchanger secretes HCO3- into the lumen in exchange for Cl-; a Cl- channel recycles the Cl-; H2O and Na+ also move into the lumen.
- Basolateral membrane (interstitial fluid side): a Na+/H+ exchanger secretes H+ into the interstitial fluid in exchange for Na+; a Na+/K+-ATPase pump moves Na+ out and K+ in; a K+ channel lets K+ leave.
- Net effect: the oxyntic cell secretes H+ apically and HCO3- basolaterally (acidifying the lumen), while the pancreatic duct cell secretes HCO3- apically and H+ basolaterally (alkalinising the lumen); both generate their H+/HCO3- pair from CO2 + H2O via carbonic anhydrase.
Salivary Gland Secretion
- Three glands: parotid, submandibular, sublingual; combined output ~1.5 L/day.
- Structure: an acinus feeds into an intercalated duct, which joins an intralobular (striated) duct.
- Composition: mucus, water, electrolytes, enzymes.
- Control: neural.
- Functions: lubrication, maintenance, digestion.
Intestinal Secretion
- Intestinal crypts secrete 1.5-2 L/day of an NaCl solution, using the same apical-Cl—channel-driven mechanism described above (highlighted again on a repeat of the general secretory epithelial cell diagram).
- Goblet cells secrete mucus.
- Enzymes come from desquamated epithelial cells.
- Jejunal villus and crypt structure, tip to base: goblet cells sit on the villus; APUD cells sit near the villus/crypt junction; Paneth cells sit at the base of the crypt.
- Disruption of intestinal secretion leads to diarrhoea.
- Colon secretion: small volume of mucus plus an alkaline solution rich in HCO3- and K+.
Self-test
- Distinguish endocrine from exocrine GI secretion, including where each is produced and what it typically achieves.
- List the endocrine hormones named in this lecture and describe, with an example, how their release is typically triggered.
- What are the three broad categories of exocrine GI secretion described, and what functions do exocrine secretions serve overall?
- Approximately how much fluid is secreted into the GI lumen per day, and which single secretion contributes the largest volume?
- List the four general features of epithelial secretion described in this lecture, noting which one is left undefined by the slides.
- Describe the ion transport steps of the general secretory epithelial cell model, from basolateral uptake to luminal secretion.
- Distinguish the oxyntic cell’s H+-secretion mechanism from the pancreatic duct cell’s HCO3—secretion mechanism.
- Describe the steps of HCl secretion by the oxyntic cell, from carbonic anhydrase to the apical membrane.
- List the cell layers of a gastric gland from pit to base, and state what each secretes.
- Describe how the vagus nerve, gastrin, histamine and somatostatin interact to regulate gastric acid secretion, including the two feedback loops from luminal contents.
- Describe the composition and duct structure of salivary gland secretion, and state what controls it.
- Describe the structure of the jejunal villus and crypt, naming the specialised cell types and their locations.
- Integrative: explain what “secretion = loss unless reabsorbed” means physiologically, and use the pancreatic duct/oxyntic cell mechanisms and diarrhoea to illustrate why disrupting GI secretion matters.
Answers
Reveal answers
- Endocrine secretions are peptide hormones (e.g. gastrin, secretin) released by enterochromaffin cells in the mucosa into the blood to coordinate motility and secretion elsewhere in the tract; exocrine secretions (water/electrolytes, mucus, enzymes) are released directly into the GI lumen to prepare food for absorption, lubricate contents and protect the mucosa.
- Gastrin, secretin, cholecystokinin (CCK), GIP and somatostatin; release is usually triggered by luminal contents, e.g. acid chyme entering the duodenum triggers secretin release, which drives HCO3- secretion to neutralise the acid.
- Lubricants (water, mucus), chemicals (enzymes, acid, electrolytes) and controls (hormones); overall exocrine secretions prepare food for absorption (dilution, optimising pH, digestion), lubricate luminal contents and protect the mucosa.
- About 8 L/day; gastric secretion contributes the largest single volume at 3.0 L/day.
- Epithelia act as an interface; cells are asymmetrical (apical vs basolateral); they have a variety of specific transport pathways; and they are “leaky” vs “tight” (this last distinction is stated but not explained on the slide).
- At the basolateral membrane, a Na+/K+/2Cl- cotransporter brings Na+, K+ and Cl- into the cell, while a Na+/K+-ATPase pumps Na+ out and K+ in and a K+ channel recycles K+ out; at the apical membrane a Cl- channel releases the accumulated Cl- into the lumen; the resulting gradient drives H2O and Na+ from interstitial fluid to lumen via the paracellular pathway.
- The oxyntic cell secretes H+ across its apical membrane (via H+/K+-ATPase) and HCO3- across its basolateral membrane, acidifying the lumen; the pancreatic duct cell secretes HCO3- across its apical membrane (via a Cl-/HCO3- exchanger) and H+ across its basolateral membrane, alkalinising the lumen. Both generate H+ and HCO3- internally from CO2 + H2O via carbonic anhydrase.
- Carbonic anhydrase converts CO2 + H2O to H+ and HCO3- inside the cell; at the apical membrane, H+/K+-ATPase pumps H+ into the lumen (K+ in), with a K+ channel recycling K+ and a Cl- channel releasing Cl- into the lumen, and H2O following; at the basolateral membrane a Cl-/HCO3- exchanger sends HCO3- into the blood in exchange for Cl-, alongside a Na+/H+ exchanger, a Na+/K+-ATPase pump and a K+ channel.
- Surface epithelial cells (secrete HCO3-), mucous cells (secrete mucus), oxyntic cells (secrete acid) and chief cells (secrete enzymes), running from the gastric pit down to the base of the gland.
- Vagal ACh stimulates the G cell (alongside GRP) to release gastrin and also drives the D cell’s somatostatin output in the antrum, while acting directly on the oxyntic cell and on the ECL cell (which releases histamine) in the fundus; circulating gastrin further stimulates the D cell and ECL cell. The oxyntic cell integrates histamine, gastrin and ACh signals via Ca2+ and cAMP to secrete H+. Luminal acid feeds back negatively on gastrin release, while luminal amino acids/digested protein feed back positively on gastrin release.
- Secretion comes from the parotid, submandibular and sublingual glands (~1.5 L/day total), flowing from an acinus into an intercalated duct and then an intralobular (striated) duct; it contains mucus, water, electrolytes and enzymes, and is under neural control.
- From tip to base: goblet cells (mucus) sit on the villus, APUD cells sit near the villus/crypt junction, and Paneth cells sit at the base of the crypt.
- Because the gut lumen is effectively “outside” the body, any secretion into it is a net loss of fluid and solute unless reabsorbed; the pancreatic duct and oxyntic cell mechanisms show secretion is metabolically active and tightly coupled (H+ and HCO3- are generated and exchanged across two membranes to move ~8 L/day through the tract), so disrupting the normal secretion/reabsorption balance, as in diarrhoea, causes significant net loss of fluid and electrolytes.