[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Renal, fluids and acid–base](https://durgical.com/en/physiology/areas/renal/) › Physiology of dialysis

# Physiology of dialysis

- Mechanism
- Exam priority: medium

Dialysis cleans blood across a thin membrane: wastes diffuse out into a cleansing fluid, and pressure or osmosis removes extra water.

Updated Nov 13, 2026

Also called: Hemodialysis, Peritoneal dialysis, Ultrafiltration

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

## Key points

- Diffusion clears small solutes (urea, K⁺, creatinine) best; convection (solvent drag with ultrafiltration, as in hemofiltration) clears middle molecules better.
- Countercurrent flow of blood and dialysate keeps a gradient along the whole membrane and maximizes clearance.
- Dialysate has no urea or creatinine, low K⁺, physiological Na⁺, and bicarbonate (or acetate/lactate) to correct acidosis.
- Peritoneal dialysis: glucose in the dialysate draws water osmotically; the peritoneal capillaries are the membrane.
- Disequilibrium syndrome: urea leaves blood faster than brain; the brain becomes relatively hyperosmolar, water enters and cerebral edema follows.
- Dialysis does not replace EPO, calcitriol or gluconeogenesis; anemia and bone disease need separate treatment.

## What it is

Dialysis replaces the kidney's excretory work, not its hormones. Blood and a prepared fluid (dialysate) run on either side of a semipermeable membrane: an artificial one in hemodialysis, the peritoneum in peritoneal dialysis. Solutes move down their concentration gradients by diffusion, and water is removed by ultrafiltration, driven by hydrostatic pressure in hemodialysis and by osmosis (glucose) in peritoneal dialysis.

## How it works, step by step

- Blood flows through hollow fibres; dialysate flows outside them in the opposite direction.
- Urea, creatinine and K⁺ are high in blood and absent or low in dialysate, so they diffuse out.
- HCO₃⁻ (and Ca²⁺ as set) diffuses from dialysate into blood, correcting acidosis.
- A pressure difference across the membrane pushes plasma water out (ultrafiltration), removing fluid gained between sessions.
- Large proteins and cells cannot cross; middle molecules (β₂-microglobulin) cross poorly unless high-flux membranes are used.
- In peritoneal dialysis, fluid dwells in the abdomen; solutes equilibrate across peritoneal capillaries and glucose draws water by osmosis.
- Rapid removal of urea from plasma lowers osmolality before the brain equilibrates; water shifts into neurons (disequilibrium).

## Hemodialysis vs peritoneal dialysis

| Feature | Hemodialysis | Peritoneal dialysis |
| --- | --- | --- |
| Membrane | Synthetic hollow fibres | Peritoneum (capillary wall + mesothelium) |
| Water removal | Hydrostatic pressure (ultrafiltration) | Osmosis (glucose in dialysate) |
| Schedule | Intermittent, a few sessions per week | Continuous, daily exchanges |
| Access | AV fistula, graft or catheter | Peritoneal catheter |
| Typical problems | Hypotension, disequilibrium, access infection | Peritonitis, glucose absorption, protein loss |

## Clinical notes

- Urgent dialysis is considered for refractory hyperkalemia, severe acidosis, fluid overload, uremic complications (pericarditis, encephalopathy) and dialysable toxins.
- Intradialytic hypotension: fluid is removed from plasma faster than it refills from the interstitium.
- Peritonitis in peritoneal dialysis presents as cloudy effluent and abdominal pain.
- Calciphylaxis and CKD-mineral bone disease persist despite dialysis because phosphate is poorly cleared and calcitriol is missing.

## Memory aid

Diffusion moves the dirt; ultrafiltration moves the water; neither makes hormones.

## Test yourself

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

## Why do blood and dialysate flow in opposite directions in a hemodialyser?

Countercurrent flow keeps a concentration gradient along the whole length of the membrane, so diffusion and clearance are maximal.

## How is water removed in peritoneal dialysis, which has no pump to create pressure?

Glucose in the dialysate makes it hyperosmolar, so water moves from peritoneal capillaries into the abdominal fluid by osmosis.

## A patient with very high urea becomes confused with headache during a first, aggressive hemodialysis. What is the mechanism?

Dialysis disequilibrium: plasma urea falls faster than brain urea, so the brain is relatively hyperosmolar and water moves in, causing cerebral edema.

## Why does a patient on long-term dialysis still need treatment for anemia?

Dialysis replaces only excretion; the kidney's erythropoietin production is not restored.

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## Connections

## See it in 3D 2

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

- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Peritoneum](https://durgical.com/en/anatomy/peritoneum/)

## Related topics 4

- [Physiology of acute and chronic kidney failure](https://durgical.com/en/physiology/physiology-of-acute-and-chronic-kidney-failure/)
- [Diffusion and Fick's law](https://durgical.com/en/physiology/diffusion-and-fick-s-law/)
- [Osmosis, osmolarity and tonicity](https://durgical.com/en/physiology/osmosis-osmolarity-and-tonicity/)
- [Starling forces at the capillary](https://durgical.com/en/physiology/starling-forces-at-the-capillary/)

## Related conditions 4

- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)
- [Calciphylaxis (calcific uremic arteriolopathy)](https://durgical.com/en/conditions/calciphylaxis/)

## In Biochemistry 3

- [Urea](https://durgical.com/en/biochemistry/molecules/urea/)
- [Urea and blood urea nitrogen (BUN) as lab tests](https://durgical.com/en/biochemistry/topics/urea-and-blood-urea-nitrogen-as-lab-tests/)
- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)

[← 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: [Dialysis | Hemodialysis | Peritoneal dialysis | MedlinePlus](https://medlineplus.gov/dialysis.html) (MedlinePlus, US National Library of Medicine, 2025); [Dialysis - peritoneal: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/007434.htm) (MedlinePlus, US National Library of Medicine, 2025); [Chronic Kidney Disease](https://www.ncbi.nlm.nih.gov/books/NBK535404/) (StatPearls (NCBI Bookshelf), 2025); [Anatomy, Abdomen and Pelvis, Peritoneum](https://www.ncbi.nlm.nih.gov/books/NBK534788/) (StatPearls (NCBI Bookshelf), 2025).

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