Vessels in mammals passively collect and direct one‑way flow of lymph.

The lymphatic system is a crucial partner to the circulatory system in mammals. Whereas the blood carries oxygen and other resources to tissues, the lymphatic system transports away excess liquid and other waste products. The lymphatic system is far more than a waste management system, and lymph is also important for the immune system and for proper absorption of fats, amongst other functions.

Unlike the circulatory system, which is a continuous loop operating at high pressure and with an active pumping system, flow of lymph is largely passive. The walls of capillaries (tiny vessels) in the circulatory system are leaky. Under pressure from the pumping heart, liquid plasma, platelets and other small components of blood are squeezed between the cells into the tissue. Having performed their function, this waste liquid and debris must be cleared away and recycled. Like blood vessels, lymph capillaries are found throughout almost all tissues, but unlike the circulatory system, lymphatic capillaries are blind-ended. The cells of lymphatic capillaries overlap. As the amount of liquid in the spaces between cells in a tissue increases, the pressure goes up. As pressure increases, it pushes down on the overlapping flaps of the cells of the lymphatic capillaries, causing them to open inwards. The waste material enters the vessel and when pressure eases, the flaps close and the liquid cannot escape.

Inside lymph vessels are a series of one-way flap valves. Some larger lymph vessels have smooth muscle cells on their outer walls which squeeze the vessels, causing the lymph to flow through the valves, however, much of the squeezing of the vessels that “pumps” the lymph past the valves occurs passively. As an animal moves, the surrounding muscles and tissues compress the lymph-collecting vessels, moving the contents along.

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Last Updated April 19, 2018