[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Cell and membrane](https://durgical.com/en/physiology/areas/cell/) › Transport across epithelial sheets

# Transport across epithelial sheets

- Mechanism
- Exam priority: medium

Epithelia move salt and water in one direction because their two faces carry different transporters; the Na⁺/K⁺ pump at the base powers it all.

Updated Nov 13, 2026

Also called: Apical and basolateral membranes, Paracellular transport, Tight junctions

**See it in 3D:** [Jejunum](https://durgical.com/en/anatomy/jejunum/) · [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- Na⁺/K⁺-ATPase is always basolateral; it creates the low cell Na⁺ that drives apical Na⁺ entry (channels or cotransporters).
- Absorption: Na⁺ in at the apex, out at the base; Cl⁻ follows the voltage, water follows the osmotic gradient.
- Secretion (crypts, airway): NKCC1 loads Cl⁻ at the base, Cl⁻ exits through apical CFTR, Na⁺ and water follow between the cells.
- Leaky epithelia (proximal tubule, small intestine) move large isosmotic volumes partly between cells; tight epithelia (collecting duct, colon, sweat duct) build steep gradients under hormonal control.
- SGLT1 couples glucose to Na⁺ entry and keeps working in cholera: the basis of oral rehydration solution.

## What it is

An epithelium is a sheet of polarized cells joined by tight junctions. The apical (lumen) and basolateral (blood) membranes carry different transporters, so solutes cross the cell in one direction. The basolateral Na⁺/K⁺-ATPase keeps cell Na⁺ low, which pulls Na⁺ in at the apex; Cl⁻ and water follow by transcellular and paracellular routes. This model explains the gut, kidney tubule, sweat glands, airways and the defect in cystic fibrosis.

## How it works, step by step

- The basolateral Na⁺/K⁺-ATPase pumps 3 Na⁺ out and 2 K⁺ in, keeping cell Na⁺ low (≈ 10–15 mmol/L) and the cell interior negative.
- K⁺ leaks back out through basolateral K⁺ channels, which keeps the pump supplied and the membrane potential negative.
- Na⁺ enters across the apical membrane down its gradient, through ENaC channels or with glucose, amino acids or Cl⁻ on cotransporters.
- Na⁺ moved to the interstitium makes the lumen negative relative to blood; Cl⁻ follows through channels or between cells.
- Salt in the lateral spaces makes them slightly hypertonic; water follows through aquaporins and tight junctions.
- Raised interstitial pressure pushes the absorbed fluid into peritubular or villous capillaries.
- For secretion the polarity is reversed for Cl⁻: NKCC1 brings it in at the base, cAMP opens apical CFTR, and Na⁺ and water follow into the lumen.

### The nephron: segments, transporters and diuretics

**Na⁺ reabsorbed** ≈ 65–70% **Water reabsorbed** ≈ 65–70%

#### Proximal tubule

- **What it does:** Reabsorbs ≈ 65–70% of filtered Na⁺ and water (isosmotically), nearly all glucose and amino acids, ≈ 80% of HCO₃⁻ and most K⁺ and phosphate. Secretes organic acids and bases (many drugs) and makes NH₄⁺ from glutamine.
- **Transporters:** NHE3 (Na⁺/H⁺) with carbonic anhydrase; SGLT2 (early segment, ≈ 90% of glucose) and SGLT1; NaPi-2a (phosphate); basolateral Na⁺/K⁺-ATPase.
- **Hormones:** Ang II stimulates NHE3 (more Na⁺ and HCO₃⁻: contraction alkalosis); PTH inhibits NaPi-2a (phosphaturia) and activates 1α-hydroxylase (active vitamin D).
- **Drugs:** Carbonic anhydrase inhibitors (1), SGLT2 inhibitors (2) and osmotic diuretics (3, with the thin descending limb).
- **Disorders:** Fanconi syndrome (loses glucose, amino acids, phosphate and HCO₃⁻); type 2 (proximal) renal tubular acidosis; familial renal glucosuria (SGLT2).

- Glomerulus and arterioles
- Proximal tubule
- Thin descending limb
- Thin ascending limb
- Thick ascending limb
- Distal convoluted tubule
- Collecting tubule and duct
- Juxtaglomerular apparatus (macula densa)
- Cortex
- Medulla
- Interstitial osmolality (≈ 300 → 1200 mOsm/kg)

Proximal tubule: a leaky epithelium that reabsorbs most filtered salt and water. Tap other segments to compare tight ones.

## Leaky and tight epithelia

| Feature | Leaky | Tight |
| --- | --- | --- |
| Examples | Proximal tubule, small intestine, gallbladder | Collecting duct, distal colon, sweat duct, urinary bladder |
| Paracellular route | Large; water and ions cross easily | Small; little backflow |
| Gradient it can build | Small; fluid stays near isosmotic | Large; e.g. very dilute or concentrated urine |
| Volume moved | Large | Small, fine-tuned |
| Hormonal control | Little | Strong (aldosterone, ADH) |

## Clinical notes

- Cholera: toxin keeps cAMP high, CFTR stays open and the crypts secrete large volumes of Cl⁻ and water; SGLT1 still works, so glucose with salt by mouth is absorbed.
- Cystic fibrosis: defective CFTR thickens airway, pancreatic and biliary secretions; in the sweat duct Cl⁻ cannot be reabsorbed, so sweat Cl⁻ is high (sweat test).
- Aldosterone excess adds apical ENaC and basolateral pumps in the collecting duct: Na⁺ retention, K⁺ loss and hypertension.

## Memory aid

Pump at the base, door at the top, water tags along.

## Test yourself

Think of the answer, then tap the question to see it.

## Why must the Na⁺/K⁺-ATPase be on the basolateral and not the apical membrane in an absorbing epithelium?

Pumping Na⁺ out at the base keeps cell Na⁺ low, which pulls Na⁺ in from the lumen; the result is net movement from lumen to blood.

## Why does oral rehydration solution contain glucose as well as salt?

SGLT1 carries Na⁺ and glucose together into enterocytes; this pathway still works in secretory diarrhea, and water follows the absorbed solutes.

## Why is sweat salty in cystic fibrosis?

The sweat duct reabsorbs Cl⁻ through CFTR (and Na⁺ with it); with defective CFTR, NaCl stays in the sweat.

## What distinguishes a tight epithelium from a leaky one, and where is each found?

Tight epithelia have low paracellular permeability and build large gradients under hormones (collecting duct, colon); leaky ones move large isosmotic volumes (proximal tubule, small intestine).

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## Connections

## See it in 3D 2

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

- [Jejunum](https://durgical.com/en/anatomy/jejunum/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related topics 5

- [Primary and secondary active transport](https://durgical.com/en/physiology/primary-and-secondary-active-transport/)
- [Membrane transport: diffusion, channels and carriers](https://durgical.com/en/physiology/membrane-transport-diffusion-channels-and-carriers/)
- [Proximal tubule transport](https://durgical.com/en/physiology/proximal-tubule-transport/)
- [Water and electrolyte absorption and how diarrhea happens](https://durgical.com/en/physiology/water-and-electrolyte-absorption-and-how-diarrhea-happens/)
- [Osmosis, osmolarity and tonicity](https://durgical.com/en/physiology/osmosis-osmolarity-and-tonicity/)

## Related conditions 2

- [Cystic fibrosis](https://durgical.com/en/conditions/cystic-fibrosis/)
- [Cholera](https://durgical.com/en/conditions/cholera/)

## In Biochemistry 2

- [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/)
- [Membrane transport](https://durgical.com/en/biochemistry/topics/membrane-transport/)

## In Pharmacology 2

- [Oral rehydration salts](https://durgical.com/en/pharmacology/oral-rehydration-salts/)
- [Ivacaftor](https://durgical.com/en/pharmacology/ivacaftor/)

[← 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: [Cystic fibrosis: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/cystic-fibrosis/) (MedlinePlus, US National Library of Medicine, 2025); [Cholera](https://www.who.int/news-room/fact-sheets/detail/cholera) (World Health Organization, 2024); [Anatomy, Abdomen and Pelvis: Kidneys](https://www.ncbi.nlm.nih.gov/books/NBK482385/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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