Water and electrolyte absorption and how diarrhea happens !
- Mechanism
- Exam priority: high
The gut handles about 9 L of fluid a day and returns nearly all of it to the blood by following sodium; diarrhea is what happens when that balance fails.
Updated 2026-11-01
Also called: Secretory diarrhea, Osmotic diarrhea
See it in 3D: Jejunum · Ileum · Ascending colon · Transverse colon · Descending colon
What it is
Every day ≈2 L of dietary water and ≈7 L of secretions (saliva, gastric juice, bile, pancreatic and intestinal juice) enter the gut. The small intestine reabsorbs most of it and the colon nearly all the rest, so only ≈100–200 mL is left in stool. Water is not absorbed on its own: it follows Na⁺, Cl⁻ and nutrients. Diarrhea comes from excess unabsorbed solute (osmotic), active chloride secretion (secretory), inflammation, or transit that is too fast.
How it works, step by step
- The basolateral Na⁺/K⁺-ATPase keeps enterocyte Na⁺ low and creates the gradient that drives all absorption.
- In the jejunum Na⁺ enters with glucose and amino acids; nutrient absorption drags in a lot of water.
- In the ileum and proximal colon Na⁺ and Cl⁻ enter electroneutrally (Na⁺/H⁺ and Cl⁻/HCO₃⁻ in parallel).
- In the distal colon Na⁺ enters through ENaC, regulated by aldosterone, which also increases K⁺ secretion.
- In the crypts, Cl⁻ enters via basolateral NKCC1 and leaves via apical CFTR; Na⁺ and water follow paracellularly.
- Cholera toxin locks Gs on → cAMP rises → CFTR stays open and litres of isotonic fluid are secreted.
- Rotavirus and norovirus damage villi: less absorption and fewer disaccharidases, with an osmotic component.
- Loss of fluid rich in Na⁺, K⁺ and HCO₃⁻ causes hypovolemia, hypokalemia and hyperchloremic metabolic acidosis.
Equations
- 290
- assumed stool osmolality, equal to plasma (mOsm/kg)
- [Na⁺], [K⁺]
- concentrations in stool water (mEq/L)
High (> 100–125): unmeasured solute, osmotic diarrhea. Low (< 50): secretory diarrhea. Cut-offs vary between sources.
Approximate daily balance (adult)
| Parameter | Value | Note |
|---|---|---|
| Fluid entering the gut | ≈8–9 L/day | ≈2 L diet + ≈7 L secretions |
| Absorbed in the small intestine | ≈7–8 L/day | |
| Absorbed in the colon | ≈1–1.5 L/day | maximum capacity ≈5–8 L/day |
| Water in stool | ≈100–200 mL/day |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Acute diarrheal disease is still a major cause of dehydration in children under 5 in Colombia; oral rehydration salts use the intact SGLT1.
- Warning signs of dehydration: intense thirst, dry mucosa, slow capillary refill, lethargy and little urine.
- Diarrhea with acidic stool pH and a high gap: unabsorbed sugars (lactose, sorbitol, osmotic laxatives).
- Opioids reduce secretion and motility, which is why they cause constipation.
Test yourself
Think of the answer, then tap the question to see it.
How does the fecal osmotic gap separate osmotic from secretory diarrhea?
In osmotic diarrhea an unmeasured solute (such as lactose) takes up the osmolality, so the gap is high (> 100–125). In secretory diarrhea secreted electrolytes make up the osmolality, so the gap is low (< 50).
Why does profuse diarrhea cause a normal anion gap metabolic acidosis?
Intestinal secretions are rich in bicarbonate; when it is lost the kidney keeps Cl⁻ to maintain electroneutrality, giving a hyperchloremic acidosis with no added anions.
What mechanism explains why cholera diarrhea continues even when the patient is fasting?
The toxin keeps cAMP permanently high and holds CFTR open; Cl⁻ and water secretion does not depend on what is eaten, so it persists during fasting.
Why does chronic diarrhea tend to cause hypokalemia?
The colon secretes K⁺ and rapid flow increases that loss; hypovolemia also activates aldosterone, which raises K⁺ excretion in the colon and kidney.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Carbohydrate digestion and absorption
- Dehydration and fluid overload
- Potassium balance
- Metabolic acidosis, the anion gap and compensation
- Aldosterone: actions and control
- The enteric nervous system, slow waves and GI control
- Patterns of malabsorption
Related conditions
- Acute (viral) gastroenteritis
- Hypokalaemia
- Normal anion gap (hyperchloraemic) metabolic acidosis
- Hypovolaemic shock
- Lactose intolerance
- Hyponatraemia
In Biochemistry
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 23.5 The Small and Large Intestines (OpenStax (Rice University), 2022); 23.7 Chemical Digestion and Absorption: A Closer Look (OpenStax (Rice University), 2022); Viral Gastroenteritis (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 66). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.