Hemorrhoids form when the cushions of tissue that naturally line your anal canal slide out of position and their blood vessels become swollen. Everyone has these cushions from birth. They only become a problem when the supportive tissue holding them in place breaks down, allowing them to bulge, bleed, or protrude. Roughly one in four adults worldwide has hemorrhoids at any given time, making this one of the most common conditions in gastroenterology.
The Tissue That Becomes Hemorrhoids
Your anal canal contains three soft pads of tissue called anal cushions. These cushions are a spongy network of blood vessels, connective tissue, and muscle fibers. The vascular portion is made up of tiny arteries, veins, and direct connections between them, forming a dense web of blood-filled spaces similar in structure to erectile tissue. This is why they can swell so dramatically.
These cushions exist for good reason. Your internal sphincter muscle handles 60 to 80 percent of the pressure that keeps the anal canal closed, but it leaves a gap of about 7 to 8 millimeters. The anal cushions fill that gap, contributing 15 to 20 percent of your continence. When you’re at rest, the sphincter’s pressure actually slows blood drainage from the cushions, keeping them plump enough to seal the canal shut. They also contain sensory nerves that help you distinguish between gas, liquid, and solid stool before you decide to bear down.
Holding the cushions in place are small muscle fibers and a connective tissue ligament that anchor them to the sphincter and the surrounding muscle. When these anchoring structures weaken or fragment, the cushions lose their support. That’s when hemorrhoids begin.
How the Cushions Break Down
The most widely accepted explanation is called the sliding anal canal lining theory. It describes hemorrhoids as the result of the anchoring tissue deteriorating, allowing the cushions to slide downward from their normal position. As they descend, the blood vessels within them stretch and dilate, pooling with blood and swelling into what you feel as a hemorrhoid.
Straining during a bowel movement is the primary mechanical trigger. When you push hard, intra-abdominal pressure rises and compresses the veins that drain the cushions, trapping blood inside them. At the same time, passing hard stool creates a shearing force that drags the cushion tissue downward, gradually tearing it away from its anchors. Over months or years of repeated straining, the supportive muscle fibers and connective tissue fray and thin out, making each episode of straining more damaging than the last.
This is why constipation and hemorrhoids are so tightly linked. Hard, dry stool requires more force to pass and spends more time pressing against the anal canal walls. The combination of high pressure, mechanical drag, and prolonged sitting on the toilet creates exactly the conditions that pull cushions loose and engorge their blood vessels.
Internal vs. External Hemorrhoids
A ridge of tissue called the dentate line divides the upper and lower portions of the anal canal. Internal hemorrhoids form above this line, where the tissue has few pain-sensing nerves. That’s why they tend to bleed painlessly. You may notice bright red blood on toilet paper or in the bowl without feeling much discomfort at all.
External hemorrhoids form below the dentate line, under the skin around the anal opening. This area is rich in pain receptors, so external hemorrhoids can be intensely uncomfortable, especially when a blood clot forms inside one. A thrombosed external hemorrhoid appears as a firm, bluish lump near the anus and can cause sharp, constant pain that peaks within the first 48 to 72 hours.
Grades of Internal Hemorrhoids
Internal hemorrhoids are classified into four grades based on how far the cushion has slipped from its original position:
- Grade I: The cushions bulge into the anal canal during a bowel movement but don’t push outside the body. Bleeding is the most common symptom.
- Grade II: The tissue protrudes outside the anus during straining but slides back in on its own once you stop.
- Grade III: The tissue protrudes and stays out until you push it back in manually.
- Grade IV: The tissue is permanently prolapsed outside the anal canal and cannot be pushed back in.
This progression reflects increasing damage to the anchoring structures. Each grade represents more stretching, more vascular engorgement, and more loss of the connective tissue that once held the cushion firmly in place.
What Increases Your Risk
Low fiber intake is one of the most modifiable risk factors. Diets heavy in processed food, cheese, meat, fast food, and snack foods produce harder, drier stools that require more straining to pass. Increasing fiber softens stool and reduces the mechanical forces that damage anal cushions.
Prolonged sitting, whether on a toilet or at a desk, pools blood in the pelvic veins and keeps steady pressure on the anal cushions. Spending extra time on the toilet scrolling a phone extends the period of low-grade straining that many people don’t even notice they’re doing.
Pregnancy combines multiple risk factors at once. Hormonal changes relax the walls of blood vessels, making them more prone to dilation. Meanwhile, the growing uterus puts increasing pressure on pelvic veins, slowing blood return from the anal cushions. Vaginal delivery adds an acute burst of extreme straining. Together, these factors make hemorrhoids common during the second and third trimesters and the postpartum period.
Aging also plays a role. The connective tissue and muscle fibers anchoring the cushions naturally weaken over decades, which is why hemorrhoid prevalence rises steadily through middle age. Heavy lifting, chronic diarrhea, and obesity all increase intra-abdominal pressure in similar ways to straining, contributing to the same cycle of venous congestion and tissue descent.
Why Hemorrhoids Bleed
As a cushion stretches and slides downward, the thin layer of tissue covering it gets pulled taut. Hard stool scraping past this thinned surface can rupture the tiny blood vessels just beneath it. Because the hemorrhoidal network includes direct connections between small arteries and veins, even minor tears can produce surprisingly bright red, arterial-looking blood. This is why a small hemorrhoid can sometimes leave a startling amount of blood in the toilet, even though the injury itself is superficial.
How Thrombosed Hemorrhoids Form
When blood pools and stagnates inside a swollen hemorrhoidal vein, it can clot. This happens most often in external hemorrhoids. The clot stretches the overlying skin, which is densely packed with nerve endings, producing sudden, severe pain. The lump usually feels hard and tender, and the skin over it may look purplish. Thrombosis can follow a single episode of intense straining, a long period of sitting, or even vigorous physical activity. If left alone, the clot typically reabsorbs over one to three weeks, though the pain is worst in the first few days.