A heart valve was a section of the heart responsible for regulating blood flow in and out.
In 2267, Doctor Leonard McCoy noted that Khan Noonien Singh's heart valve action was twice that of normal Humans. (TOS: "Space Seed")
In 2268, McCoy diagnosed Ambassador Sarek with a malfunctioning heart valve following the Vulcan's near collapse. (TOS: "Journey to Babel")
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