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.