What causes the accumulation of fluids in tissues and how does this fluid become lymph?

Hydrostatic pressure in the capillaries forces nutritious plasma fluid into the extracellular spaces in cells. At the end of the capillary bed, the extracellular fluid which has a high metabolic waste reenters the capillaries through osmotic pressure.

When the two pressures are not balanced, however, fluid accumulates in tissues as tissue fluid. When the tissue fluid seeps into the lymph system, it becomes lymph.