What causes hemorrhoids?

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Abstract
Conclusions There are two main types of hemorrhoids—vascular and mucosal. Vascular piles, occurring most commonly in young men, are caused by straining at defecation, when the anal sphincter relaxes, subjecting the anorectal tissues to a high pressure gradient. The superior hemorrhoidal plexus is filled initially by blood which is forced at very high pressure from the superior rectal venous radicles lying in the lower part of the ampulla of the rectum. Distention of the plexus is limited and is confined to the veins of the upper part of the anal canal by a tube-valve mechanism which also has a similar, but less direct, effect on the inferior rectal veins. Mucosal piles, usually found in pregnant women and the aged, are caused by deterioration of the function of the anal sphincter muscle. There is no evidence that any other etiologic factor is of significance in the causation of piles.

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