Lymphatic vessels are closed at their distal end and serve simply to return fluid and plasma proteins to the blood. The smallest lymphatic vessels have walls which consist of a single endothelial cell layer, with relatively large holes separating the endothelial cells. The purpose of the vessels is to collect excess fluid and plasma proteins which leave the capillaries during filtration, and return these substances to the blood. The lymphatic system returns about 5 litres of fluid each day. The lymphatic system drains into the blood at the subclavian vein.
Lymphatic vessels comprise a series of segments, separated by valves.
Each segment is able to contract in a pulsatile manner. Both the valves and the peristalsis-like pumping action of the segments assist in returning plasma fluid and proteins to the circulatory system. This is the accumulation of excess fluid in the interstitial spaces. It may be caused by 2 factors. This may be caused by injury, inflammation, surgical removal of lymphatic vessels or filariasis.