And that means we do not have a backflow of lymph inside our lymph system in the same way that we do not have a back flow inside our veins. Remember, the veins also contain this valve system that prevents the movement of our lymph back down that lymph vessel. So the walls of the lymph vessels consist of endothelial cells that overlap slightly when there is a build up of fluid in the interstitial tissue, in the tissue space, that creates fluid pressure that pushes on the cells, this opens up those overlapping regions, forces the fluid into that lymph vessel. Now, inside the lymph vessel we have a system of one wave valves. These valves, as well as the overlapping portion of the endothelial cells open only in one direction and not the other. And this keeps the lymph inside that lymph vessel and it keeps it moving along one direction.