Lymphatic vessels are endothelial tubes within the tissues. They carry fluid, serum proteins, lipids, and foreign substances from the interstitial spaces back to the circulatory system.
Lymphatic vessels begin as blind-ended lymphatic bulbs. The wall of a lymph capillary is constructed of endothelial cells that overlap one another. The pressure in the interstitial fluid surrounding the capillary pushes open the overlapping cells of the lymphatic bulbs. The movement of fluid from the surrounding tissue into the lymphatic vessel lumen is passive. That is, there are no active pumps in the lymphatic system. Once the interstitial fluid enters a lymph capillary, it is referred to as lymph.
A compression/relaxation cycle causes the lymphatic vessels to "suck up" lymph. Lymph valves in lymphatic vessels make this possible. A volume of fluid equal to the total plasma volume in a single human is filtered from the blood to the tissues each day. It is critical that this fluid is returned to the venous system by lymph flow each day. The lymphatic vasculature collects approximately 3 L/day of excess interstitial fluid and returns it to the blood. The lymph system is responsible for returning plasma proteins to the blood.
Lymphatic pressures are only a few mmHg in the bulbs and the smallest lymphatic vessels, but can be as high as 10 to 20 mmHg during the contraction of larger lymphatic vessels. Lymph valves make possible this apparent progression or flow from low pressure toward high pressure.
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