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Hemorrhoids are dilated veins in the anus. Internal hemorrhoids: dilated veins of the plexuses of the superior and middle hemorrhoidal veins above the mucocutaneous junction. Internal hemorrhoids drain to the portal vein. External hemorrhoids: dilated veins of the inferior hemorrhoidal plexus below the mucocutaneious junciton the the tissues beneath the anal epithelium of the anal canal. External hemorrhoids drain into the systemic circulation. These veins become symptomatic when the venous pressures increase: such as in constipation, straining to defecate, pregnancy, tumors, and portal hypertension.

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