Why Do Hemorrhoids Happen?
Everyone has hemorrhoidal tissue — cushions of blood vessels, muscle, and connective tissue in the anal canal that help with continence, cushioning the area and helping seal the anus between bowel movements. “Having hemorrhoids,” in the sense people usually mean it, happens when this normal tissue becomes swollen, engorged, or displaced downward, causing symptoms.
- Hemorrhoidal tissue is a normal part of anatomy — the condition is this tissue becoming swollen or prolapsed, not something foreign forming.
- Increased pressure in this area — from straining, prolonged sitting, pregnancy, or chronic constipation or diarrhea — is the common thread across nearly every known risk factor.
- Weakening of the connective tissue that normally holds these cushions in place, which increases with age, is a second major contributing factor.
- Most individual risk factors (diet, sitting habits, pregnancy) work through one of these two mechanisms.
The basic mechanism
Hemorrhoidal cushions sit in the anal canal, internally (above the dentate line) and externally (below it), supported by connective tissue that keeps them in place and helps them return to position after each bowel movement. Two things tend to go wrong:
- Increased pressure and blood pooling — repeated or sustained pressure in the pelvic and anal veins causes these cushions to become engorged and swollen.
- Weakening of the supporting connective tissue — over time, or under repeated strain, the tissue that anchors these cushions in place can loosen, allowing them to slide downward (prolapse) rather than staying in their normal position.
Most of the well-known risk factors for hemorrhoids work through one or both of these mechanisms.
Why straining and pressure matter so much
Straining during a bowel movement directly increases pressure in the anal venous cushions. This is why constipation (which leads to more straining to pass harder stool) and — perhaps counterintuitively — prolonged sitting on the toilet even without straining (which allows blood to pool in the area over time) are both independently linked to hemorrhoid risk. See Constipation and straining and Sitting too long on the toilet.
Why pregnancy is such a common trigger
Pregnancy increases pressure on pelvic veins simply through the growing weight of the uterus, and hormonal changes during pregnancy also relax vein walls generally — a double contribution to hemorrhoid risk that explains why a large share of pregnant people experience hemorrhoids at some point. See Hemorrhoids in pregnancy: safe treatments.
Why age is a factor independent of anything you’re doing
Connective tissue throughout the body loses some elasticity and supportive strength with age, and the anal canal is no exception. This is part of why hemorrhoid prevalence rises with age even among people without other obvious risk factors — the structural support for hemorrhoidal cushions gradually weakens over decades. See Ageing and tissue changes.
Why genetics plays a role too
Connective tissue strength and vein wall characteristics have a hereditary component, which is part of why hemorrhoids can run in families independent of shared lifestyle factors. See Genetics: is it inherited?
The common thread across risk factors
Nearly every major hemorrhoid risk factor traces back to increased pressure in this area, weakened supporting tissue, or both:
| Factor | Primary mechanism |
|---|---|
| Constipation/straining | Increased pressure |
| Prolonged toilet sitting | Blood pooling / pressure |
| Pregnancy | Pressure + hormonal tissue relaxation |
| Low-fiber diet | Harder stool → more straining |
| Chronic diarrhea | Repeated irritation and pressure |
| Heavy lifting | Increased intra-abdominal pressure |
| Prolonged sitting (desk jobs) | Blood pooling |
| Obesity | Increased pelvic pressure |
| Aging | Weakened connective tissue |
| Genetics | Inherited tissue/vein characteristics |
Why this matters practically
Understanding the mechanism explains why the standard prevention advice (more fiber, less straining, less prolonged sitting, staying active) genuinely targets the root causes rather than just being generic health advice — each piece addresses either the pressure side or the tissue-support side of the equation.
This page is for general education about the mechanisms behind hemorrhoid formation.