Small Intestinal Secretions
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Direct answer
The small intestine secretes 1-2 litres of succus entericus daily — a pH 7-8 watery fluid from the crypts of Lieberkuhn — while Brunner's glands in the duodenum add alkaline (pH 8.5-9.0) mucus. Most digestive chemistry, however, is membrane-bound: enterokinase on the brush border activates trypsinogen (the master switch of pancreatic proteases), and disaccharidases (lactase, sucrase-isomaltase, maltase) plus aminopeptidases complete carbohydrate and protein digestion at the villus surface. Paneth cells at the crypt base secrete defensins and lysozyme as innate immunity; secretion is driven by vagal and hormonal stimulation, and sympathetic activity inhibits it — the classical link between stress and duodenal ulceration.
What you must remember
- Volumes and pH: about 1.5-2 L of succus entericus daily at pH 7-8, joining saliva, gastric and pancreaticobiliary secretions in the 6-7 L daily turnover of gastrointestinal fluid, of which all but 100-200 mL is reabsorbed.
- Brunner's glands: duodenum only; alkaline mucus plus epidermal growth factor shielding the duodenal mucosa from gastric acid; sympathetic stimulation inhibits them — the textbook rationale for stress-related duodenal ulcers.
- Crypt-versus-villus division of labour: crypts secrete (chloride-driven), villi absorb; stem cells at the crypt base renew the epithelium every 3-5 days.
- Paneth cells: crypt-base granulated cells releasing alpha-defensins 5 and 6, lysozyme and phospholipase A2 — the intestine's own antimicrobial armoury (also the site of mutation in Crohn's disease pathways).
- Enterokinase (enteropeptidase): the brush-border enzyme that converts trypsinogen to trypsin; trypsin then activates everything else including more trypsinogen — its deficiency mimics pancreatic exocrine failure.
- Disaccharidases: lactase, sucrase-isomaltase and maltase are brush-border (not secreted); adult-type hypolactasia is common in South Asian populations — lactose intolerance here is the norm, not the exception.
- Chloride secretion via CFTR at the crypt apex drives water out; cholera toxin locks it open — secretory diarrhoea up to 1 L per hour.
- Regulation: vagal parasympathetics and secretin/CCK stimulate; sympathetic activity inhibits secretion while favouring sphincter tone.
A worked case: cholera and the logic of ORS
A patient with rice-water stool loses a litre of fluid an hour. The toxin binds GM1 receptors, ADP-ribosylates the Gs alpha subunit, and adenylate cyclase runs permanently — cyclic AMP opens crypt CFTR chloride channels, chloride pours into the lumen, sodium and water follow, and the villi continue absorbing normally even as crypts flood the gut. That single asymmetry is the therapeutic opening: glucose-coupled sodium absorption through SGLT1 on villous enterocytes remains intact, so the World Health Organization's low-osmolarity oral rehydration solution (about 75 mmol/L sodium with glucose) drags sodium, and water with it, across even a secreting mucosa. ORS does not stop the secretion; it out-absorbs it — which is why a disease that can kill an adult in hours is survived with a sachet of salts and sugar, and why the national diarrhoeal disease control programme has ORS at its centre. Antibiotics shorten the illness (azithromycin or doxycycline); fluids carry the patient across it.
How the exam frames it
Two differentiating questions recur. First, "which intestinal enzyme is actually secreted into the lumen rather than membrane-bound?" — essentially only enterokinase anchors the list; the rest of terminal digestion is a brush-border event, which is why pancreatic enzymes alone cannot rescue disaccharidase deficiency. Second, lactase: brush-border, latest to mature and earliest to decline, uninvolved in osmotic diarrhoea of other causes — its deficiency gives acidic, fermentative stools with a positive hydrogen breath test, treatable simply by removing lactose (or using lactase-free milk or curd, whose bacteria pre-digest much of the lactose — an Indian dietary staple advantage).
Frequently asked questions
How much succus entericus is secreted daily, and at what pH?
About 1.5-2 litres per day at a slightly alkaline pH of 7-8, produced chiefly by the crypts of Lieberkuhn.
What is the role of enterokinase?
The brush-border enzyme that activates trypsinogen to trypsin, which then activates the remaining pancreatic zymogens — the master switch of protein digestion.
What do Paneth cells secrete, and why?
Alpha-defensins, lysozyme and phospholipase A2 into the crypt lumen, maintaining the sterile-ish crypt niche as innate immunity against luminal microbes.
Why does cholera cause massive secretory diarrhoea?
Toxin-driven permanent activation of adenylate cyclase keeps crypt CFTR chloride channels open, and the resulting chloride-rich secretion drags sodium and water into the lumen at rates up to 1 L per hour.
Why does oral rehydration solution work despite ongoing secretion?
Glucose-coupled sodium transport (SGLT1) on villous cells remains intact, so glucose-sodium-water absorption continues and can match or exceed the crypt secretion.