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The lymphatic system is a one-directional system wherein fluid is drained from tissue beginning under the skin and delivered to the subclavian veins for transport to the vena cava (major blood vessel which returns blood to the heart). Valves found in the lymph vessels prevent the fluid from reversing direction. The process begins when fluid from blood of arterial capillaries (the smallest vessels in the arterial system) seeps into the interstitial spaces (spaces between cells and tissue) throughout the body and provides the surrounding cells with nutrients. Changes in pressure in the surrounding tissue forces most of the fluid (80-90%) and waste products to be reabsorbed into the capillaries of the veins and transported back towards the heart. The remaining 10-20% is forced into the lymphatic capillaries where it is propelled by valves and vessel contractions through a series in increasingly larger-diameter vessels, through the lymph nodes, and ultimately into the large subclavian veins which carry all the blood back to the heart.

Thus, through a complex system of pressure created by filtration (removal of waste byproducts of cell metabolism) and reabsorption, the interstitial fluid levels in body tissue are kept at a steady level. The continual deposit of fluid from the arterial capillaries is balanced by the continual flow of interstitial fluid into venous and lymph capillaries and fluid buildup in the tissue (edema) is prevented. Interruption of this delicate balance can lead to collection of fluid in the tissue and is called lymphedema.