This is a valve (guard, door) allowing blood to flow from the outlet of the right ventricle into the pulmonary artery and on into the lungs to be oxygenated (see figures 103-105B, 106B). It is semi-lunar and composed of three fibrous cusps attached to and suspended from the root of the pulmonary artery and circles the inside of the vessel root. The small space between the attachment of each adjacent cusp is called a commissure.