Lymph is a milky (usually yellowish-white, but sometimes colorless) fluid similar in composition to blood plasma; it contains lymphocytes (about 8,000 per cubic millimeter), proteins, and fats. Lymph is derived from blood plasma as fluids pass through capillary walls at the arterial end. As the interstitial fluid begins to accumulate, it is picked up and removed by tiny lymphatic vessels and returned to the blood. As soon as the interstitial fluid enters the lymph capillaries, it is called lymph. Returning the fluid to the blood prevents edema and helps to maintain normal blood volume and pressure.