So when there is a build up of pressure here, it closes these overlapping regions so that fluid cannot exit that lymph node. It pushes against a valve that opens up and that allows the movement of lymph along our vessel as shown in the following diagram. And if there is a decrease in pressure and it basically wants to move back, it cannot move back because when the fluid tries to move back it forces these valves to actually close. 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.