An introductory lesson into the nature of tissue fluid formation and lymphatic drainage. Includes a brief introduction into the functions of lymphatic drainage in health and disease.The lymphatic system The lymphatic system consists of the lymphatic drainage vessels and other structures which contain lymphatic tissue. Lymphatic tissue is also sometimes referred to as lymphoid tissue.
Lymphatic drainage structures Lymphatic capillaries drain the tissues. Most of the tissue fluid formed at the arterial end of the capillary is, as we have seen, reabsorbed back into the blood at the venous end of the capillary. However, along with tissue fluid, a few protein molecules also escape from the capillaries into the tissue spaces. If these large osmotic molecules were allowed to remain in the tissue fluid they would increase the osmotic potential, leading to an accumulation of water in the tissue spaces. An excess of fluid in the tissues is abnormal and is termed oedema. If excess tissue fluid was allowed to accumulate this would cause swelling and would increase the diffusional distance between the blood and tissue cells. This would reduce the speed at which oxygen could be delivered to tissue cells and reduce the rate at which waste products could be removed; both of these effects would reduce tissue viability. The mechanism which removes exuded proteins from the tissue fluid, preventing oedema, is the lymphatic drainage system. Once fluid has passed from the tissue spaces into a lymphatic vessel it is called lymph, or lymphatic fluid. Unlike blood capillaries the lymphatic capillaries are blind-ended. However, like blood capillaries they are located in the tissue spaces. Lymphatic capillaries are highly permeable and will absorb excess tissue fluid and infecting bacteria, as well as proteins. Endothelial cells which compose the lymphatic capillary walls are separated by pores which allow fluid to enter. The cells also overlap to form valves. This means fluid and other material is able to enter from the tissue spaces but may not escape back out from the lymphatic capillaries. As a lymphatic capillary fills with lymph the pressure in the lumen will close the valves in the wall of the capillary.

Its strange, the lymphatic system seems to be a less than popular subject, but is essential to the way we function as humans.