Capillaries do not have smooth muscle. They are the only blood vessels in the body that lack it entirely. While arteries, arterioles, and even the smallest veins all contain a muscular middle layer, capillaries are built from just a single layer of flat cells, a thin basement membrane, and a scattering of support cells called pericytes. This stripped-down design is not a limitation; it’s what makes capillaries work.
What Capillary Walls Are Made Of
Every other blood vessel in the body has three distinct layers: an inner lining, a muscular middle layer (the tunica media), and a supportive outer layer. Capillaries skip the middle and outer layers almost entirely. Their wall is roughly 1 micrometer thick, about one-hundredth the width of a human hair, and consists of just two components: endothelial cells that form the tube itself, and a basement membrane that holds them in place.
Wrapped around the outside of capillaries at irregular intervals are pericytes. These cells sit on the basement membrane and extend finger-like projections along the vessel. They are not smooth muscle cells, though the distinction between the two has been debated for over a century. Pericytes help stabilize the capillary wall and communicate with the endothelial cells beneath them, but they are far fewer in number and structurally different from the dense rings of smooth muscle that wrap arteries and arterioles.
Why Capillaries Can’t Afford a Muscle Layer
Capillaries exist for one purpose: exchange. Oxygen, carbon dioxide, nutrients, and waste products all pass through the capillary wall between blood and surrounding tissue. This transfer happens by diffusion, and diffusion works best over the shortest possible distance. Adding a muscular layer would thicken the wall and slow this process considerably.
Small gaps between the endothelial cells also allow certain molecules to pass through directly. These gaps would be sealed off or disrupted by additional layers of tissue. The entire design of a capillary is optimized for permeability, which is fundamentally at odds with the thick, contractile walls that characterize muscular vessels.
How Blood Flow Is Controlled Without Muscle
If capillaries have no smooth muscle, how does the body control how much blood flows through them? The answer lies upstream. Arterioles, the small vessels that feed into capillary beds, are rich in smooth muscle and can constrict or relax to increase or decrease blood flow to an entire region of tissue.
At the exact point where an arteriole transitions into a capillary, there is a structure called a precapillary sphincter. This is a specialized cell that wraps around the vessel opening and contains the contractile protein found in smooth muscle. When this sphincter constricts, it reduces both blood pressure and red blood cell flow into the downstream capillary bed. When it relaxes, the capillary fills with blood. These sphincters are strategically placed at branch points, giving the body fine-grained control over which capillary networks are actively perfused at any given moment.
This arrangement is efficient. Rather than building contractile machinery into every one of the billions of capillaries in the body, the circulatory system concentrates flow control at a few upstream chokepoints.
The Pericyte Debate
For decades, researchers assumed pericytes were purely structural. Studies consistently found that capillary pericytes contained little or no detectable contractile protein, while pericytes on slightly larger vessels like pre-capillary arterioles and post-capillary venules did. This supported the idea that blood flow regulation happens at the arteriole level, not within the capillary bed itself.
More recent work has complicated that picture. A 2018 study on mouse retinal capillaries found that pericytes actually do contain a small amount of the same contractile protein found in smooth muscle cells. The reason earlier studies missed it: the protein breaks down extremely rapidly during standard tissue preparation. When researchers used rapid-freeze techniques instead of conventional methods, they could detect it at much higher levels. When this protein was experimentally depleted from mouse retinas, capillary pericytes were more affected than cells on larger vessels, suggesting that pericytes depend on even their small amount of contractile machinery.
This has led to an ongoing classification debate. Some researchers have proposed reclassifying contractile pericytes as smooth muscle cells, while others argue they remain a distinct cell type that simply shares some molecular features. Capillaries in the brain and retina appear to change diameter in response to local neural activity, allowing more oxygen and nutrients to reach tissues with higher demand. Whether this makes pericytes “smooth muscle” is partly a question of semantics, but the functional takeaway is clear: some capillaries can actively adjust their diameter, even without a traditional smooth muscle layer.
How Capillaries Compare to Other Vessels
The difference in wall composition across vessel types follows a consistent pattern tied to function:
- Arteries and arterioles have thick walls with prominent smooth muscle layers. Arterioles in particular are described as “chiefly composed of smooth muscle” and serve as the primary regulators of blood pressure and flow distribution.
- Capillaries have no smooth muscle and no tunica media. Their single-cell-thick walls allow diffusion of gases and nutrients.
- Venules, the smallest veins that collect blood from capillaries, have very thin walls with minimal muscle. They are fragile and can rupture under excessive pressure, but they also participate in some exchange of oxygen and nutrients.
The transition from arteriole to capillary is not abrupt. There is a continuum of cell types along the vessel wall, gradually shifting from dense smooth muscle wrapping on arterioles to scattered pericytes on capillaries. The precapillary sphincter sits right at this transition point, marking the boundary where true smooth muscle coverage ends and the thin-walled capillary begins.
Three Types of Capillaries, None With Muscle
The body contains three structural types of capillaries, each with different degrees of permeability, but all share the same basic wall composition of endothelial cells, basement membrane, and pericytes.
- Continuous capillaries have the tightest junctions between cells. They are found in muscle, skin, lungs, and the brain, where selective barrier function is important.
- Fenestrated capillaries have small pores (fenestrations) in their endothelial cells, allowing faster exchange. They are found in the kidneys, intestines, and endocrine glands.
- Sinusoidal capillaries have large gaps and an incomplete basement membrane, allowing even whole cells to pass through. They are found in the liver, spleen, and bone marrow.
None of these types contain smooth muscle. The variations between them involve the tightness of cell junctions and the completeness of the basement membrane, not the addition of muscular layers. Even the most permeable sinusoidal capillaries rely on the same basic two-component wall structure.