1. Increased capillary hydrostatic pressure: because the hydrostatic pressure increases and exceeds the osmotic pressure, the fluid is pushed to the interstitial compartment and out of the capillaries. When there's an excess of this fluid the tissue swells.

2. Loss of plasma proteins: results in a decrease in plasma osmotic pressure (loss of albumin, therefore loss of plasma), which allows the hydrostatic pressure to exceed the osmotic pressure and pushes fluid out.

3. Obstruction of lymphatic circulation: when the excess fluid and proteins isn't being returned to the general circulation as a result of an obstruction (tumors, damaged lymph nodes).

4. Increased capillary permeability: with increased permeability, more fluids leaks out of the capillary and into the interstitial compartment.

Learn: Pathophysiology, Chp. 2 - Fluid, Electrolyte and Acid – Base Imbalances