Hemorrhoids represent abnormal dilatation of normal veins. Venous blood return from the anorectum has two paths. Above the dentate line, venous blood flows into the portal system through the superior rectal and inferior mesenteric veins. Below the dentate line, the external hemorrhoidal plexus drains into the internal iliac vein via the middle rectal vein, or the pudendal vein, which receives blood from the inferior rectal vein. Thus, the dentate line allows distinction between internal and external hemorrhoids. Most symptoms arise from enlarged internal hemorrhoids.
Abnormal swelling of the anal cushions causes dilatation and engorgement of the arteriovenous plexuses. This leads to stretching of the suspensory muscles and eventual prolapse of rectal tissue through the anal canal. The engorged anal mucosa is easily traumatized, leading to rectal bleeding that is typically bright red owing to high blood oxygen content within the arteriovenous anastomoses. Prolapse leads to soiling and mucus discharge (triggering pruritus) and predisposes to ulcers, incarceration, and strangulation.
Hemorrhoidal venous cushions are a normal part of the human anorectum and arise from subepithelial connective tissue within the anal canal. Internal hemorrhoids have three main cushions, which are situated in the left lateral, right posterior (most common), and right anterior areas of the anal canal. However, this combination is found in only 19% of patients; hemorrhoids can be found at any position within the rectum. Minor tufts can be found between the major cushions.
External hemorrhoids occur more commonly in young and middle-aged adults than in older adults. The prevalence of hemorrhoids increases with age, with a peak incidence in persons aged 45-65 years.
Portal hypertension may trigger hemorrhoids. However, hemorrhoidal symptoms do not occur more frequently in patients with portal hypertension than in those without it, and massive bleeding from hemorrhoids in these patients is unusual.
For more on hemorrhoids, read here.
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