Hemorrhoids are arterio-venous vessel cushions that form a ring above the Linea dentata and guarantee the fine continence of the rectum. Anatomically they are named the Corpus cavernosum recti. The blood supply comes from the superior rectal artery, which explains the predilection places for hemorrhoid formation at 3, 7 and 11 o'clock.
Contraction of the internal anal sphincter obstructs blood flow. Hemorrhoids fill with blood like a sponge and seal the rectum against fluids and gases.
Hemorrhoidal disease - normally called simply hemorrhoids - occurs when the blood cushion becomes enlarged and symptoms appear.
Picture: Armin Kübelbeck