More tissue fluid leaves the capillaries at the arterial end than returns at the venous end. The excess must be drained away, or it would build up in the tissues. This is the job of the lymphatic system.Among the cells of most tissues are tiny lymph capillaries. These are blind-ended tubes with very thin, permeable walls. The endothelial cells overlap loosely, acting like one-way flaps: fluid can pass into the lymph capillary but cannot easily pass back out. Once the excess tissue fluid has entered these vessels it is called lymph.Lymph is similar to tissue fluid, but it also contains any plasma proteins that leaked out of the blood capillaries, which could not return by osmosis. Lymph from the small intestine also contains a lot of lipid absorbed from digested food, giving it a milky appearance. Lymph also carries lymphocytes.Lymph capillaries join to form larger lymph vessels. There is no pump in the lymphatic system. Instead, lymph is moved along by the squeezing of the vessels when the surrounding skeletal muscles contract, and by breathing movements. Like veins, lymph vessels contain valves that prevent backflow, so lymph can only move in one direction.
The lymph vessels eventually join into two large ducts that empty the lymph back into the blood, in the subclavian veins near the heart. In this way the fluid and proteins are returned to the circulation. If lymph vessels are blocked, tissue fluid builds up and causes oedema.
The lymph vessels eventually join into two large ducts that empty the lymph back into the blood, in the subclavian veins near the heart. In this way the fluid and proteins are returned to the circulation. If lymph vessels are blocked, tissue fluid builds up and causes oedema.