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 · Great saphenous vein · Thoracic duct
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 mmHgVenular NFP +6 mmHgFiltration 10.0Reabsorption 0.0Net 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
Edema by mechanism
| Mechanism | Examples | Clue |
|---|---|---|
| High capillary pressure | Heart failure, venous insufficiency, DVT, kidney failure (volume overload) | Raised jugular pressure or local venous signs |
| Low plasma oncotic pressure | Cirrhosis, nephrotic syndrome, kwashiorkor | Low albumin, generalized and facial edema |
| High permeability | Inflammation, burns, sepsis, severe dengue, anaphylaxis | Protein-rich fluid, often with shock |
| Lymphatic obstruction | 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.
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.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Starling forces at the capillary
- How the lymphatic system works
- Heart failure physiology
- The renin–angiotensin–aldosterone system
- Vascular compliance and capacitance
Related conditions
In Biochemistry
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 (OpenStax (Rice University), 2022); Venous Insufficiency (StatPearls (NCBI Bookshelf), 2025); Lymphedema (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.