Hemorrhoids are filled with clusters of blood vessels, smooth muscle, and connective tissue. They are not hollow sacs containing fluid. Instead, they are solid cushions of vascular tissue where small arteries connect directly to veins, all held together by elastic fibers and a layer of smooth muscle. When hemorrhoids swell or become symptomatic, the blood vessels inside them engorge, the supporting tissue stretches, and in some cases a blood clot forms within the swollen vein.
What Makes Up a Hemorrhoid
Hemorrhoids are often described as “swollen veins,” but that’s an oversimplification. They are actually complex cushions made of several types of tissue, all packed into the lining of the anal canal. The main components include small arteries (arterioles), small veins (venules), and direct connections between the two. These vascular connections are surrounded by connective tissue containing both elastic and collagen fibers, which give the cushions their shape and flexibility.
A key structural component is a layer of smooth muscle that threads between the blood vessels and anchors the cushion to the wall of the anal canal. This muscle helps the cushions contract and shrink during bowel movements, then return to their resting position afterward. It also contributes significantly to the overall bulk of the cushion, meaning much of what you’d feel in a hemorrhoid is not just blood-filled vessels but muscle and connective tissue as well.
Everyone has these cushions. They exist in three main clusters and serve a real purpose: they help seal the anal canal and contribute to continence. Roughly one in four adults worldwide has symptomatic hemorrhoids at any given time. The cushions only become a problem when the tissue swells, slides out of position, or develops a clot.
Why Hemorrhoids Swell
When hemorrhoids become symptomatic, the blood vessels inside them dilate and fill with more blood than usual. At the same time, the connective tissue and smooth muscle that normally hold the cushion in place begin to deteriorate. The muscle fibers that anchor the cushion to the anal wall weaken, allowing the engorged tissue to slide downward. This combination of vascular engorgement and structural collapse is what produces the familiar bulge.
Inflammatory changes also add volume. When the tissue is irritated, whether from straining, prolonged sitting, or chronic pressure, the body releases chemical signals that increase blood flow to the area and cause fluid to leak into the surrounding tissue. This edema, or fluid swelling, adds to the size and tenderness of the hemorrhoid beyond what the blood vessels alone would produce.
What Fills a Thrombosed Hemorrhoid
A thrombosed hemorrhoid contains a blood clot. This happens when blood inside the swollen vein pools and solidifies into a firm lump, usually in an external hemorrhoid near the surface of the skin. The clot is what creates the sudden, intense pain and the hard, bluish lump that many people feel.
The clot stays localized in the anal area and does not travel to other parts of the body. Left alone, your body will gradually reabsorb the clot over one to three weeks, with pain improving a little each day. If the pain is severe, a doctor can make a small incision to remove the clot, which provides almost immediate relief.
Internal vs. External: Different Tissue Coverings
The contents of internal and external hemorrhoids are similar, but their outer coverings differ. Internal hemorrhoids sit higher in the anal canal, above a boundary called the dentate line. They are covered by a thinner type of lining (columnar cells) and supplied by nerves that don’t register sharp pain, which is why internal hemorrhoids can bleed without hurting.
External hemorrhoids sit lower, near the anal opening. They are covered by skin-like tissue (squamous epithelium) and connected to the same type of nerves found in your skin. This is why external hemorrhoids, especially thrombosed ones, can be extremely painful to the touch. The underlying vascular and muscular contents are structurally similar in both types, but the difference in nerve supply and covering explains why they feel so different.
The Role of Structural Support
One reason hemorrhoids become problematic is the breakdown of their internal scaffolding. The smooth muscle fibers that weave between the blood vessels have two jobs: they compress the cushions to help them shrink, and they anchor the entire structure to the muscular wall of the anal canal. When these fibers weaken, whether from aging, repeated straining, or chronic pressure, the cushion loses its ability to spring back into place. The blood vessels inside remain dilated, the tissue slides downward, and what was once a firm, well-contained cushion becomes a loose, swollen mass.
This is why hemorrhoids tend to worsen progressively. The structural damage is cumulative. Early-stage internal hemorrhoids may bleed but return to their normal position on their own. As the supporting muscle and connective tissue continue to break down, the hemorrhoid begins to protrude during bowel movements, and eventually it may stay protruded permanently. The contents haven’t fundamentally changed; it’s the loss of the framework holding them together that drives the progression.