What Causes Internal Hemorrhoids?

Hemorrhoids—specifically internal hemorrhoids—are swollen, inflamed veins located in the upper two-thirds of the anal canal, above the dentate line. Unlike external hemorrhoids, they are covered by mucosa rather than skin and therefore typically do not cause pain, though they may lead to bright red rectal bleeding during bowel movements, prolapse, or a sensation of incomplete evacuation.

The primary underlying mechanism is increased venous pressure within the superior rectal venous plexus. This pressure elevation commonly results from chronic straining during defecation—often due to constipation or prolonged sitting on the toilet. Other well-established contributors include pregnancy (due to uterine compression of pelvic veins and hormonal relaxation of vascular smooth muscle), obesity, chronic coughing, heavy lifting, and conditions associated with portal hypertension, such as liver cirrhosis.

Structural weakening of the hemorrhoidal supporting tissue—including Treitz’s muscle and the submucosal connective tissue anchoring the anal cushions—also plays a critical role. With age, these supportive elements degenerate, allowing normal vascular cushions to descend and become symptomatic. Genetic predisposition and a low-fiber diet further exacerbate risk by promoting harder stools and increased intra-abdominal pressure.

It is important to distinguish internal hemorrhoids from other anorectal conditions presenting with similar symptoms—such as colorectal cancer, inflammatory bowel disease, or anal fissures—particularly when bleeding is new, persistent, or accompanied by weight loss, change in bowel habits, or iron-deficiency anemia. Clinical evaluation, including digital rectal examination and anoscopy, remains essential for accurate diagnosis and appropriate management.