The fluid circulates around cells and returns to the capillaries near their venule ends. There are a number of forces acting to force fluid through the endothelial cell of the capillary. Of course, small amounts of protein do leak through the epithelium or are given off by parenchymal cells. This protein could not get back to the blood stream if it were not for lymphatic drainage. Whenever fluid or pressure build up outside the circulatory system, blind-ended lymph vessels have a valve that flaps inward and allows fluid entrance and even materials as large as cells.
Valves prevent what is trapped within the lymph vessels to return to interstitial fluid.
Local control Just before an arteriole splits into many capillaries, one last smooth muscle cell surrounds it. This is the precapillary sphincter . All of these tend to make the sphincter relax and allow blood flow through the capillary bed. There opposites cause contraction and cut off capillary blood flow. They may open briefly several times a minute.