Filtration forces water and dissolved substances from the capillaries into the interstitial fluid. Not all of this water is returned to the blood by osmosis, and excess fluid is picked up by lymph capillaries to become lymph. From lymph capillaries fluid flows into lymph veins (lymphatic vessels) which virtually parallel the circulatory veins and are structurally very similar to them, including the presence of semilunar valves. Lymph capillaries have flap-like minivalves which allow fluid to enter when pressure gradient is normal, but close to prevent backflow when pressure is higher in the lymph capillary.