And because these endothelial cells open only one way and not the other way, when our hydrostatic pressure pushes on our cells this way, the fluid cannot escape back because these overlapping regions between the cells only open this way and not the other way. And we also have a system of one way valves found along the lymph vessel. So that means these valves also open one way and not the other way. 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.