This is a longitudinal section of a large vein with valves. It was stained with Masson's trichrome to show muscle fibers (red), collagen (blue), cytoplasm (light-red or pink), and nuclei (dark brown to black).
A valve consists of a fold (i.e., cusp or leaflet) of the tunica intima which extend into the lumen of the vessel. In this example, two open leaflets are shown along with a third leaflet whose opening is not within the plane of the section.
Blood pushes the the leaflets open as it moves towards the heart. If gravity or muscle contractions try to pull the blood backward, the cusps are pushed together preventing backward flow.