The lymphatic system is present throughout the body and carries a colourless fluid called lymph. Lymph is similar in composition to blood plasma and is derived from blood plasma as fluids pass through the capillary walls at the arterial end.

When interstitial fluid accumulates, it is transported and removed by very small 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 oedema (when an abnormal quantity of fluid accumulates beneath the skin) and helps to maintain normal blood volume and pressure.