Oedema is caused by mechanisms that interfere with normal fluid balance between plasma, interstitial fluid, and lymph flow. Fluid movement is controlled by opposing effects of vascular hydrostatic pressure and colloid osmotic pressure, where excess interstitial fluid is normally drained by lymphatics. Oedema can result from increased capillary hydrostatic pressure, reduced plasma oncotic pressure due to low albumin, lymphatic obstruction impairing drainage, sodium and water retention in the kidneys, or inflammation causing capillary leakage.

1. MECHANISM OF OEDEMA

2.  Oedema is caused by mechanisms that interfere
with normal fluid balance of plasma, interstitial
fluid and lymph flow.
Movement of fluid- controlled by opposing
effects of vascular hydrostatic pressure and colloid
osmotic pressure.
Exit of fluid from arteriolar end is balanced by
inflow at venular end of microcirculation.
Small residual amount of excess interstitial fluid is
drained by lymphatics.

3. 1. INCREASED CAPILLARY
HYDROSTATIC PRESSURE
Rise in hydrostatic pressure at the venular end of
capillaries to a level more than plasma oncotic
pressure
↓
Minimal/ No reabsorption of fluid at venular end
↓
OEDEMA

4. 2. REDUCED PLASMA OSMOTIC
PRESSURE
Reduced albumin synthesis in liver / protein
malnutrition
↓
Fall in plasma oncotic pressure
↓
Net movement of fluid into interstitial tissues
↓
OEDEMA

5. 3. LYMPHATIC OBSTRUCTION
Impaired lymphatic drainage
↓
Localized LYMPHOEDEMA
• Radical mastectomy for Ca breast
• Pressure on main lymph ducts
•Inflammation of lymphatics
• Occlusion of lymphatics by malignant cells
• Milroy’s disease

6. 4. SODIUM & WATER RETENTION
Hypovolaemia
↙ ↓ ↘
Renal vasoconstiction Renin ↑ADH
↓ ↓ ↓
↓GFR Aldosterone Reabsoption of water
↓ ↓ ↙
Renal retention of Na & water
↓
OEDEMA

7. 5. INFLAMMATION
Capillary endothelial injury by toxins/ histamine/
anoxia/ drugs
↓
Endothelial gap
↓
↑ Capillary permeability to plasma protiens
↓
↓Plasma oncotic pressure
↓
OEDEMA

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