There are three main types of capillaries: continuous, fenestrated, and sinusoidal. Each type has a different wall structure that controls what can pass through it, and each shows up in different organs based on how much filtration that tissue needs. All capillaries share a basic design: a single layer of endothelial cells surrounded by a basement membrane, with no muscular layers. This makes them the thinnest-walled vessels in your circulatory system, with diameters as small as 2 to 5 micrometers in the brain.
How Capillary Walls Differ From Other Blood Vessels
Arteries and veins have three distinct layers in their walls: an inner lining, a muscular middle layer, and a tough outer layer. Capillaries have only the inner lining, called the tunica intima. This consists of flat endothelial cells sitting on a thin basement membrane. The extreme thinness of capillary walls is the whole point: it allows oxygen, nutrients, and waste products to move between blood and surrounding tissue.
Wrapped around the outside of capillaries are cells called pericytes. These cells sit embedded in the basement membrane and connect to the endothelial cells through specialized contact points. Pericytes stabilize the vessel wall and can contract or relax to adjust blood flow, functioning like miniature versions of the smooth muscle cells found in larger vessels. They respond to oxygen levels: when oxygen is low, pericytes relax and the vessel widens to deliver more blood. When oxygen is sufficient, they contract. This couples blood flow directly to how much oxygen the surrounding tissue needs.
Continuous Capillaries
Continuous capillaries are the most common type in the body. Their endothelial cells sit tightly side by side with no holes punched through them. Small gaps between adjacent cells, called intercellular clefts, allow water and small molecules to squeeze through, but larger molecules like proteins are mostly kept out. You find continuous capillaries in your skin, muscles, lungs, and nervous system.
The continuous capillaries in the brain deserve special attention because they form the blood-brain barrier. Brain capillary endothelial cells are sealed together by exceptionally tight junctions that restrict nearly all paracellular transport, meaning almost nothing slips between the cells. These capillaries also lack the small vesicles that other capillaries use to shuttle molecules across their walls. The result is a highly selective barrier that prevents most blood-borne substances from reaching brain tissue, protecting neurons from toxins and pathogens while still allowing essential nutrients through dedicated transport channels.
Fenestrated Capillaries
Fenestrated capillaries look like continuous capillaries that have been hole-punched. Their endothelial cells contain small circular openings, called fenestrations, typically 60 to 80 nanometers in diameter. These pores dramatically increase how quickly substances can cross the capillary wall, which is why fenestrated capillaries appear in organs that need rapid filtration or absorption.
Most fenestrated capillaries have a thin diaphragm stretching across each pore, like a screen door. This diaphragm still allows faster exchange than a continuous capillary wall, but it provides some selectivity. Capillaries with diaphragmed fenestrations are found in the intestinal lining, endocrine glands (like the pancreas and adrenal cortex), exocrine glands, and the peritubular capillaries surrounding kidney tubules.
The kidney’s glomerular capillaries are a notable exception. These fenestrated capillaries lack diaphragms over their pores, making them more permeable. They still sit on a basement membrane, which, along with specialized cells on the other side, forms the kidney’s filtration barrier. This open-pore design allows the glomerulus to filter large volumes of blood rapidly, producing the fluid that eventually becomes urine. The physiologic upper limit of what can pass through is about 15 nanometers in effective pore size, which is large enough for water and small solutes but blocks most proteins.
Sinusoidal Capillaries
Sinusoidal capillaries, or sinusoids, are the leakiest type. They are slightly larger in diameter than other capillaries and have large gaps between their endothelial cells. Their basement membrane is either incomplete or absent entirely. This combination means that even large molecules, including proteins, can pass freely between the blood and the surrounding tissue.
You find sinusoids in the liver, spleen, bone marrow, and adrenal glands. The liver’s sinusoidal capillaries are especially well studied. Their fenestrations average about 105 nanometers in diameter in humans and can range from 50 to 180 nanometers, substantially larger than those in other fenestrated capillaries. This allows lipoproteins and small fat-carrying particles to move freely from the blood into liver tissue for processing. Hepatic sinusoids also contain immune cells (macrophages) interspersed among their endothelial cells, adding a built-in defense system that filters bacteria and debris from the blood.
Bone marrow sinusoids serve a different purpose. Their gaps are large enough for newly formed blood cells to squeeze through the vessel wall and enter the bloodstream for the first time. Splenic sinusoids similarly allow blood cells to pass in and out of the surrounding tissue, which is part of how the spleen filters old or damaged red blood cells.
Comparing the Three Types
- Continuous: Tightest walls. Small intercellular clefts only. Found in skin, muscle, lungs, and brain. Best for tissues that need strict control over what enters and exits.
- Fenestrated: Small pores (60 to 80 nm) through endothelial cells, often covered by diaphragms. Found in kidneys, intestines, and glands. Built for rapid filtration and hormone secretion into the blood.
- Sinusoidal: Large gaps between cells, incomplete or absent basement membrane. Found in liver, spleen, and bone marrow. Allows free passage of large molecules and even whole cells.
The pattern across these three types is straightforward: as you move from continuous to fenestrated to sinusoidal, the wall becomes progressively more permeable. Each organ gets the capillary type that matches its job. Tissues that need tight control, like the brain, use continuous capillaries with reinforced junctions. Organs that filter or secrete at high rates use fenestrated capillaries. And organs that need to exchange large molecules or release cells into the bloodstream use sinusoids.