[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Cardiovascular](https://durgical.com/en/physiology/#s-cardio) › How edema forms

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

# How edema forms

- Mechanism
- Exam priority: high

Swelling appears when more fluid leaves the blood vessels than the lymph can carry back.

Updated 2026-11-01

Also called: Pitting edema, Third spacing

**See it in 3D:** [Leg veins](https://durgical.com/en/anatomy/anterior-tibial-veins/) · [Great saphenous vein](https://durgical.com/en/anatomy/great-saphenous-vein/) · [Thoracic duct](https://durgical.com/en/anatomy/thoracic-duct/)

## Key points

- Four mechanisms: ↑ capillary hydrostatic pressure, ↓ plasma oncotic pressure, ↑ permeability, ↓ lymph drainage.
- Tissues tolerate some extra filtration (safety factors: lymph flow can rise many-fold, interstitial pressure rises, interstitial proteins wash out) before edema appears.
- Heart failure: high venous pressure + renal Na⁺ and water retention (RAAS, ADH). Left failure → lung edema; right failure → legs, liver, ascites.
- Hypoalbuminemia (nephrotic syndrome, cirrhosis, malnutrition) → generalized soft pitting edema.
- Lymphedema is protein-rich, non-pitting late, and does not improve much overnight.

## What it is

Edema is an excess of interstitial fluid. It appears when filtration out of the capillaries exceeds lymphatic drainage, for one of four reasons: high capillary pressure, low plasma oncotic pressure, leaky capillaries, or blocked lymphatics. The kidney's salt and water retention keeps most chronic edema going.

## How it works, step by step

- A Starling force shifts (e.g. venous pressure rises in heart failure) and filtration increases.
- Interstitial pressure rises and lymph flow increases, absorbing the first extra fluid.
- When filtration outruns maximal lymph flow, fluid accumulates: visible edema needs a few litres of extra extracellular fluid.
- Less effective arterial volume activates RAAS, sympathetic and ADH; the kidney retains Na⁺ and water.
- Retained volume raises venous and capillary pressure further, keeping the edema going.

### Starling forces along a capillary

**Arteriolar NFP** +14 mmHg **Venular NFP** +6 mmHg **Filtration** 10.0 **Reabsorption** 0.0 **Net filtration** 10.0 a.u. (lymph capacity 7.5)

Heart failure: high venous pressure raises Pc along the whole capillary; there is almost no reabsorption and filtration outruns the lymph (peripheral edema in right-sided failure, pulmonary edema in left-sided).

Edema: net filtration (10.0 a.u.) is more than the lymph can return (7.5 a.u.), so fluid collects in the interstitium.

Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.

- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema

Compare the heart failure, low-albumin, inflammation and lymphatic block presets: each changes a different force.

## Edema by mechanism

| Mechanism | Examples | Clue |
| --- | --- | --- |
| [High capillary pressure](https://durgical.com/en/conditions/heart-failure/) | Heart failure, venous insufficiency, DVT, kidney failure (volume overload) | Raised jugular pressure or local venous signs |
| [Low plasma oncotic pressure](https://durgical.com/en/conditions/cirrhosis/) | Cirrhosis, nephrotic syndrome, kwashiorkor | Low albumin, generalized and facial edema |
| [High permeability](https://durgical.com/en/conditions/sepsis-and-septic-shock/) | Inflammation, burns, sepsis, severe dengue, anaphylaxis | Protein-rich fluid, often with shock |
| [Lymphatic obstruction](https://durgical.com/en/conditions/lymphoedema/) | Surgery or radiation, tumours, filariasis | Non-pitting late, skin thickening |

## Clinical notes

- Pitting edema: pressure leaves a dent that persists, because free fluid is pushed aside; graded by depth and recovery time.
- In Colombia, chronic venous insufficiency is a common cause of leg edema in standing workers; it worsens through the day.

## Memory aid

Four faucets of edema: pushing harder, pulling weaker, a leaky pipe, or a blocked drain.

## Test yourself

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

## Why does left-sided heart failure cause pulmonary edema but not leg edema at first?

Raised left atrial pressure is passed back to the pulmonary capillaries, raising their hydrostatic pressure; systemic venous pressure rises only when the right heart also fails.

## Why can interstitial fluid rise considerably before edema is visible?

Safety factors buffer it: lymph flow increases many-fold, interstitial pressure rises from negative values, and interstitial proteins are washed out, lowering their oncotic pull.

## Why does the kidney make the edema of heart failure worse?

Low effective arterial volume activates RAAS, the sympathetic system and ADH, which retain Na⁺ and water and further raise venous and capillary pressure.

## How does lymphedema differ from the edema of hypoalbuminemia?

Lymphedema is protein-rich and localized, and becomes firm and non-pitting with skin thickening; hypoalbuminemic edema is protein-poor, soft, pitting and generalized.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/how-edema-forms/)

## See it in 3D

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

- [Leg veins](https://durgical.com/en/anatomy/anterior-tibial-veins/)
- [Great saphenous vein](https://durgical.com/en/anatomy/great-saphenous-vein/)
- [Thoracic duct](https://durgical.com/en/anatomy/thoracic-duct/)

## Related topics

- [Starling forces at the capillary](https://durgical.com/en/physiology/starling-forces-at-the-capillary/)
- [How the lymphatic system works](https://durgical.com/en/physiology/how-the-lymphatic-system-works/)
- [Heart failure physiology](https://durgical.com/en/physiology/heart-failure-physiology/)
- [The renin–angiotensin–aldosterone system](https://durgical.com/en/physiology/the-renin-angiotensin-aldosterone-system/)
- [Vascular compliance and capacitance](https://durgical.com/en/physiology/vascular-compliance-and-capacitance/)

## Related conditions

- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)
- [Chronic venous insufficiency](https://durgical.com/en/conditions/chronic-venous-insufficiency/)
- [Lymphoedema](https://durgical.com/en/conditions/lymphoedema/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Anaphylaxis](https://durgical.com/en/conditions/anaphylaxis/)

## In Biochemistry

- [Albumin](https://durgical.com/en/biochemistry/enzymes/albumin/)
- [Serum albumin as a lab test](https://durgical.com/en/biochemistry/topics/serum-albumin-as-a-lab-test/)

[← 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: [20.3 Capillary Exchange](https://openstax.org/books/anatomy-and-physiology-2e/pages/20-3-capillary-exchange) (OpenStax (Rice University), 2022); [Venous Insufficiency](https://www.ncbi.nlm.nih.gov/books/NBK430975/) (StatPearls (NCBI Bookshelf), 2025); [Lymphedema](https://www.ncbi.nlm.nih.gov/books/NBK537239/) (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 25). 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.

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Source: https://durgical.com/en/physiology/how-edema-forms/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/how-edema-forms/