[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Gastrointestinal](https://durgical.com/en/physiology/#s-gi) › Water and electrolyte absorption and how diarrhea happens

[Physiology](https://durgical.com/en/physiology/) · [Gastrointestinal](https://durgical.com/en/physiology/#s-gi)

# 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](https://durgical.com/en/anatomy/jejunum/) · [Ileum](https://durgical.com/en/anatomy/ileum/) · [Ascending colon](https://durgical.com/en/anatomy/ascending-colon/) · [Transverse colon](https://durgical.com/en/anatomy/transverse-colon/) · [Descending colon](https://durgical.com/en/anatomy/descending-colon/)

## Key points

- Water follows solutes: there is no active water transport.
- Na⁺ absorption: with nutrients (SGLT1, jejunum), Na⁺/H⁺ exchange (NHE3), coupled exchange with Cl⁻/HCO₃⁻ (ileum and proximal colon) and ENaC channels in the distal colon (aldosterone).
- Secretion: crypts secrete Cl⁻ through CFTR; cAMP or cGMP opens it (cholera, enterotoxigenic E. coli).
- Osmotic diarrhea: high fecal osmotic gap (> 100–125 mOsm/kg) and it stops with fasting. Secretory: low gap (
- 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

**Fecal osmotic gap**Gap = 290 − 2 × ([Na⁺]stool + [K⁺]stool)

**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 (

| 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.

## Memory aid

'Water is sodium's shadow': where Na⁺ goes, water goes; if Cl⁻ leaves, water leaves too.

## 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 (

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/water-and-electrolyte-absorption-and-how-diarrhea-happens/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Jejunum](https://durgical.com/en/anatomy/jejunum/)
- [Ileum](https://durgical.com/en/anatomy/ileum/)
- [Ascending colon](https://durgical.com/en/anatomy/ascending-colon/)
- [Transverse colon](https://durgical.com/en/anatomy/transverse-colon/)
- [Descending colon](https://durgical.com/en/anatomy/descending-colon/)

## Related topics

- [Carbohydrate digestion and absorption](https://durgical.com/en/physiology/carbohydrate-digestion-and-absorption/)
- [Dehydration and fluid overload](https://durgical.com/en/physiology/dehydration-and-fluid-overload/)
- [Potassium balance](https://durgical.com/en/physiology/potassium-balance/)
- [Metabolic acidosis, the anion gap and compensation](https://durgical.com/en/physiology/metabolic-acidosis-the-anion-gap-and-compensation/)
- [Aldosterone: actions and control](https://durgical.com/en/physiology/aldosterone-actions-and-control/)
- [The enteric nervous system, slow waves and GI control](https://durgical.com/en/physiology/the-enteric-nervous-system-slow-waves-and-gi-control/)
- [Patterns of malabsorption](https://durgical.com/en/physiology/patterns-of-malabsorption/)

## Related conditions

- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/)
- [Normal anion gap (hyperchloraemic) metabolic acidosis](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/)
- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Lactose intolerance](https://durgical.com/en/conditions/lactose-intolerance/)
- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)

## In Biochemistry

- [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/)
- [Membrane transport](https://durgical.com/en/biochemistry/topics/membrane-transport/)
- [G protein-coupled receptors and cAMP](https://durgical.com/en/biochemistry/topics/g-protein-coupled-receptors-and-camp/)
- [Cyclic AMP (cAMP)](https://durgical.com/en/biochemistry/molecules/cyclic-amp/)

## In Pharmacology

- [Opioids and antagonists](https://durgical.com/en/pharmacology/classes/opioids-and-antagonists/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-5-the-small-and-large-intestines) (OpenStax (Rice University), 2022); [23.7 Chemical Digestion and Absorption: A Closer Look](https://openstax.org/books/anatomy-and-physiology-2e/pages/23-7-chemical-digestion-and-absorption-a-closer-look) (OpenStax (Rice University), 2022); [Viral Gastroenteritis](https://www.ncbi.nlm.nih.gov/books/NBK518995/) (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).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/physiology/water-and-electrolyte-absorption-and-how-diarrhea-happens/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/water-and-electrolyte-absorption-and-how-diarrhea-happens/