Continuous capillaries are the most common type of capillary in the human body. They’re found in the skin, lungs, heart, skeletal muscle, and the brain, among other locations. Most organs rely on this capillary type to control what passes between the bloodstream and surrounding tissue.
What Makes a Capillary “Continuous”
The name refers to the capillary wall itself. In a continuous capillary, the thin cells lining the vessel sit edge to edge without gaps or holes, forming an unbroken barrier. Beneath those cells is a complete basement membrane, a thin sheet of structural proteins that adds a second layer of filtration. Small molecules like oxygen and carbon dioxide can pass through easily, but larger molecules and cells are restricted.
Between the lining cells are tiny spaces called intercellular clefts. These clefts allow water and small dissolved substances (ions, glucose) to seep through in a controlled way. The result is selective permeability: the capillary lets your tissues receive nutrients and remove waste without flooding them with plasma proteins or blood cells.
Major Locations in the Body
Continuous capillaries supply the majority of your organs and tissues. The key locations include:
- Skeletal muscle: The muscles attached to your bones are packed with continuous capillaries that deliver oxygen during activity and carry away carbon dioxide.
- Skin: The dermis relies on continuous capillaries for temperature regulation, nutrient delivery, and immune surveillance.
- Lungs: A dense mesh of continuous capillaries wraps around every air sac (alveolus), creating the vast surface where oxygen enters the blood and carbon dioxide leaves it.
- Heart: The heart muscle itself is fed by continuous capillaries branching off the coronary arteries.
- GI tract: The walls of the digestive tract contain continuous capillaries (though the intestinal villi specifically use a fenestrated subtype, covered below).
- Brain and spinal cord: A highly specialized version of continuous capillaries forms the blood-brain barrier.
Continuous Capillaries in the Lungs
The lung capillary network is a good example of how continuous capillaries adapt to a specific job. Each alveolus is surrounded by a web of capillaries so dense it forms an almost sheet-like surface for gas exchange. Research published in Nature has shown that this network is more complex than previously thought: it’s actually a mosaic of two specialized cell types working together.
One cell type, called the aerocyte, is extremely thin and spread out, positioned in the thinnest parts of the air sac wall where oxygen can cross most efficiently. The other type, called a general capillary cell, handles blood flow regulation and acts as a stem cell that can repair damaged capillaries. Both are continuous capillary cells, but they divide the labor of gas exchange and vessel maintenance between them.
The Blood-Brain Barrier: A Special Case
The continuous capillaries in your brain are the tightest in the body. Unlike continuous capillaries elsewhere, brain capillaries have virtually no intercellular clefts. Their lining cells are sealed together by especially dense tight junctions, protein complexes that lock neighboring cells so firmly that almost nothing slips between them.
These capillaries also lack the small transport bubbles (vesicles) that other continuous capillaries use to shuttle molecules across the wall. The combined effect is a barrier that blocks most substances in the blood from reaching brain tissue. Only small, fat-soluble molecules like oxygen and carbon dioxide pass freely. Everything else, including glucose, needs a dedicated transport protein to get through. This extreme selectivity protects the brain from toxins and pathogens but also makes delivering medications to the brain notoriously difficult.
How They Differ From Other Capillary Types
Your body has three main capillary types, each matched to the needs of the tissue it serves.
Fenestrated capillaries look similar to continuous capillaries but have small pores (fenestrations) punched through the lining cells. These pores are covered by a thin membrane and allow faster, higher-volume exchange of fluids and small molecules. You find them in places that need rapid filtration or absorption: the intestinal villi (where nutrients enter the bloodstream), endocrine glands (where hormones need quick access to circulation), and the kidneys.
Sinusoidal capillaries (also called discontinuous capillaries) are the leakiest type. They have large gaps between lining cells and an incomplete basement membrane, allowing even whole cells and large proteins to pass through. The liver is the classic location. This makes sense because the liver needs to pull large molecules and old blood cells out of circulation for processing.
Continuous capillaries sit at the restrictive end of this spectrum. Their intact walls and complete basement membrane make them ideal for tissues that need a controlled, selective exchange, which is most of the body.
What Happens When They Break Down
Because continuous capillaries depend on intact cell-to-cell junctions and a complete basement membrane, damage to either structure causes fluid to leak into surrounding tissue. The visible result is edema, or swelling. This happens routinely in inflammation: immune signals deliberately loosen the junctions to let white blood cells and antibodies reach an infection or injury site.
In rare cases, capillary leakiness becomes a disease in itself. Systemic capillary leak syndrome (Clarkson’s disease) involves episodes of severe, unexplained capillary hyperpermeability throughout the body. Fluid pours out of the bloodstream into tissues, causing dangerous drops in blood pressure and widespread swelling. Electron microscopy of affected tissue shows direct breakdown of capillary walls and damage to the lining cells. A chronic form of this condition produces persistent, hard-to-treat edema rather than acute episodes, and recent genetic research has linked it to mutations in genes also associated with hereditary angioedema, another condition driven by vascular leakage.