Lymphatic capillaries are larger than blood capillaries and provide for flow in just one direction. Lymph itself originates from blood plasma and returns to the blood after making its way through the lymphatic system.
Lymphatic vessels comprise three layers. The outer layer, known as tunica adventitia, consists of connective tissue and collagen. The tunica media is the middle layer and consists mainly of smooth muscle. The innermost layer is the tunica intima and contains valves that prevent lymph from flowing backwards through the lymphatic system.
The walls of lymph capillaries are very thin. This allows fluid to enter the capillaries from the blood. From the capillaries, lymph moves into larger lymph vessels that transport it to the lymph nodes for filtering. Lymph nodes contain high numbers of white blood cells that destroy pathogens and foreign bodies in the lymph.
After leaving the lymph nodes, the clean lymph enters the lymphatic trunks that merge to form lymph ducts. Lymph ducts empty lymph back into the blood to start the process anew.
Unlike the lymphatic system, the circulatory system allows for movement of fluid in two directions. Fluid in the lymphatic system moves via skeletal muscle and changes in pressure. In the circulatory system, the heart pushes blood to the rest of the body through the arteries and draws it back through the veins.