The lymphatic system is a one-way system, in which fluid is pumped from the tissues starting under the skin and ending in the subclavian veins for transport to the superior vena cava (large vena cava that returns blood to the heart). Valves present in the lymphatic vessels prevent fluid from moving in the opposite direction. The process begins when fluid from the blood in the arterial capillaries (the smallest vessels in the arterial system) passes into the interstitial space (space between cells and tissues) throughout the body and provides nutrients to the cells. Changes in pressure in the surrounding tissues cause most of the fluid (80-90%) and waste products to be reabsorbed into the capillaries of the veins and transported back to the heart. The remaining 10-20% is forced into the lymphatic capillaries where it is pumped through valves and vasoconstriction into a series of vessels of increasing diameter, to the lymph nodes and finally to the large subclavian veins that carry blood back to the heart.
In this way, through a complex system of pressure created by filtration (removal of useless by-products of cellular metabolism) and reabsorption, the level of intercellular fluid in the tissues of the body remains at a constant level. The continuous deposition of fluid from the arterial capillaries is compensated by the continuous flow of fluid to the venous and lymphatic capillaries and thus the accumulation of fluid in the tissues (edema) is prevented. Disruption of this balance can lead to the collection of fluid in the tissues called lymphedema .