Hemorrhoids develop when the blood vessels in and around your anus swell under repeated or sustained pressure. Everyone has these blood vessels (called anal cushions), and they normally help with bowel control. But when pressure builds in the lower rectum, blood pools in those vessels, they stretch, and they become the painful, itchy, or bleeding bumps known as hemorrhoids. About half of adults older than 50 have them, making this one of the most common conditions people deal with.
The Basic Mechanism Behind Hemorrhoids
Your rectum and anus are lined with a network of small veins. Unlike veins in your legs, these vessels don’t have valves to keep blood flowing in one direction. When something increases pressure in the area or slows blood from draining back toward the heart, those veins engorge and bulge. Think of it like a balloon that gets inflated too many times: eventually the walls weaken, the tissue stretches, and it doesn’t snap back.
This is why hemorrhoids aren’t really something you “catch.” They result from mechanical forces, habits, and physical changes that put cumulative stress on a vulnerable area.
Straining During Bowel Movements
The single most common cause is straining to pass hard stools. A low-fiber diet produces small, firm stools that require more pushing. That pushing sharply increases pressure inside the abdomen, which gets transmitted directly to the veins around the rectum. Over time, repeated straining causes those veins to engorge and eventually stay swollen.
Constipation and diarrhea both contribute, though through slightly different routes. Constipation forces you to bear down harder and longer. Chronic diarrhea irritates the anal lining and keeps you on the toilet more often, which compounds the problem (more on that below). Either way, irregular bowel habits are the primary driver for most people who develop hemorrhoids.
Sitting on the Toilet Too Long
Spending extended time on the toilet, especially while scrolling your phone or reading, is a well-recognized risk factor. The toilet seat creates a kind of tourniquet effect: your body weight presses on the tissue around the anus while gravity pulls blood downward into the rectal veins. When you’re sitting in that position and hunched forward, the angle between the rectum and anus shifts in a way that increases pressure on those blood vessels. If that pressure is sustained day after day, it can push otherwise normal veins into symptomatic territory.
The fix sounds simple but is worth emphasizing: sit down, do your business, and get up. Save the reading for elsewhere.
Low Fiber and Poor Hydration
Fiber makes stools bulkier and softer, which means they pass with less effort. The current dietary guidelines recommend 14 grams of fiber for every 1,000 calories you eat, which works out to about 28 grams per day on a standard 2,000-calorie diet. Most people fall well short of that.
Good sources include beans, lentils, whole grains, fruits with skin, and vegetables. Fiber works best when paired with adequate water intake, because fiber absorbs fluid as it moves through the gut. Without enough water, high-fiber foods can actually make stools harder to pass.
Pregnancy
Pregnancy is one of the most common triggers, and several factors pile on at once. The growing uterus presses on the pelvic veins, making it harder for blood to flow out of the rectal area. Blood volume increases significantly during pregnancy, putting extra strain on veins throughout the body. Hormonal shifts slow digestion, leading to constipation. And the straining that comes with constipation adds yet more pressure to an already stressed system.
Labor and delivery compound the issue further. The intense pushing during vaginal delivery puts enormous force on the pelvic floor. Many people develop hemorrhoids for the first time during the third trimester or immediately after giving birth. These often improve in the weeks following delivery as pressure normalizes, though they don’t always resolve completely on their own.
Aging and Tissue Weakening
As you get older, the connective tissue that anchors those anal cushions in place gradually weakens. Microscopic studies of hemorrhoid tissue consistently show a fragmented, loosened connective tissue framework. Muscle mass in the area also declines over time, replaced by less supportive connective tissue. This is why hemorrhoids become increasingly common with age: the structural support that once held everything in place slowly deteriorates, making those cushions more prone to sliding downward and swelling with less provocation than it would have taken decades earlier.
Heavy Lifting
Weightlifting and other forms of heavy physical exertion can contribute to hemorrhoids through a mechanism similar to straining on the toilet. When you hold your breath and brace your core to lift something heavy (a maneuver called the Valsalva technique), pressure spikes inside your abdomen. That force pushes down on the pelvic floor and compresses the veins in the rectal area. People who regularly lift very heavy loads, whether in the gym or at work, face a higher risk if they consistently use breath-holding as a lifting strategy.
Exhaling during the exertion phase of a lift reduces this pressure buildup considerably. It won’t eliminate the risk entirely for someone lifting near their maximum, but it takes away a major part of the mechanism.
Excess Body Weight
Carrying extra weight, particularly around the midsection, adds sustained pressure to the pelvic floor. Research using genetic data to isolate the effect of body mass found a statistically significant link between higher BMI and hemorrhoid risk. The association appears in both men and women, though some studies have found it more pronounced in women. A BMI of 25 or above has been linked to increased rates of self-reported hemorrhoids, with the risk climbing further at a BMI of 30 and above.
The mechanism is straightforward: more abdominal weight means more constant downward pressure on the rectal veins, even when you’re not straining or sitting on the toilet. It’s a baseline load that makes all the other risk factors hit harder.
Other Contributing Factors
Prolonged sitting in general, not just on the toilet, plays a role. Desk jobs and long commutes keep pressure on the pelvic area for hours. People who sit for most of the day without regular movement are more likely to develop symptoms than those who alternate between sitting and standing.
Chronic coughing, from conditions like COPD or persistent allergies, repeatedly spikes abdominal pressure in the same way straining does. Anal intercourse can also irritate and stretch the tissues, contributing to hemorrhoid formation or worsening existing ones.
Why Some People Get Them and Others Don’t
Most people encounter several of these risk factors throughout their lives, yet not everyone develops symptomatic hemorrhoids. Genetics likely plays a role in how strong your connective tissue is and how your veins respond to pressure, though this is difficult to quantify. What’s clear is that hemorrhoids almost always result from a combination of factors rather than a single cause. Someone with a low-fiber diet who also sits for long periods, doesn’t drink enough water, and occasionally strains during bowel movements is accumulating risk from multiple directions at once.
The encouraging flip side: because the causes are largely behavioral and mechanical, most of them are modifiable. Increasing fiber intake to that 28-gram daily target, staying hydrated, limiting toilet time, maintaining a healthy weight, and avoiding unnecessary straining during lifting or bowel movements address the majority of what drives hemorrhoid development.