Besides the blood vessels, the body contains a collateral system of channels, lined by endothelium which collect the tissue fluid and return into the blood stream.
The fluid in these vessels is lymph. Unlike blood, the lymph circulates in one direction only, from the periphery towards the heart.
The lymphatic capillaries starts blindly from the tissue collect the lymph and freely anastomose along their course and lymph passes through one or more lymph nodes in the course of lymph vessels before entering into the blood stream.
Lymph capillaries
These are like blood capillaries but are larger. Lymphatic capillaries are present in most tissues and organs except the central nervous system, bone narrow, eyeball, internal ear and fetal placenta.
Lymph vessels
These are like veins in structure but their walls are thinner than veins of corresponding caliber. They contain numerous valves, which are directed in the direction of lymph flow.
