Stanford Cardiovascular Institute, Stanford, CA.
At the level of the capillaries, the systemic circulation loses about 2-4 liters of fluid and about 100g of protein into the interstitium daily. This ultrafiltrate of the systemic capillaries is returned to the circulatory system by the lymphatics. The lymphatic vasculature is highly specialized to perform this service, beginning with the blind-ended lymphatic capillaries. These vessels are highly permeable to protein, fluid and even cells, due to fenestrations in their basement membrane, and discontinuous button-like junctions rather than tight intercellular junctions as observed in the systemic capillaries(1). The lymphatic capillaries merge into collectors and larger lymphatic conduits that are invested with vascular smooth muscle (capable of contracting and propelling lymph forward) and valves for unidirectional flow. These conduits merge at lymph nodes, delivering antigens to the immune cells and serving as an early warning system of pathogen invasion. The lymph nodes drain into conduits that ultimately merge into the thoracic duct which empties into the left subclavian vein.