[4] It prevents blood from flowing backwards into the coronary sinus during contraction of the heart.

[5] It is formed by a semicircular fold of the lining membrane of the right atrium.

[citation needed] The valve may be completely absent;[6] it is present in 73-86% of individuals.

[citation needed] The valve prevents regurgitation of blood into the sinus during systole (i.e. the contraction of the atrium).

[4] This article incorporates text in the public domain from page 531 of the 20th edition of Gray's Anatomy (1918)