What Are Capillaries and What Is Their Function?

Capillaries are the smallest blood vessels in your body, thin enough that red blood cells squeeze through them in single file. Their walls are typically just one cell thick, and they serve as the site where your blood actually does its job: delivering oxygen and nutrients to tissues and picking up carbon dioxide and waste products for removal. Every other vessel in your circulatory system exists to move blood toward or away from capillaries. Arteries and veins are the highways; capillaries are the loading docks.

Why Capillaries Are So Small

A capillary’s diameter is roughly five to ten micrometers, barely wider than a single red blood cell. That extreme thinness is not a design limitation but a functional requirement. The thinner the wall separating blood from surrounding tissue, the easier it is for molecules like oxygen and carbon dioxide to cross. Diffusion across a thick barrier is slow; diffusion across a barrier one cell thick is fast enough to keep tissue alive. Your body contains an estimated 10 billion capillaries, and if you stretched them all end to end, they would cover a staggering distance. The sheer number means that nearly every cell in your body sits within a short walk of a capillary, keeping diffusion distances tiny.

Blood flow through capillaries is also much slower than through arteries or veins. That sluggish pace is another feature, not a bug. Slow transit gives molecules more time to cross the capillary wall in both directions. The combination of thin walls, enormous total surface area, and slow flow makes capillaries spectacularly efficient exchange platforms.

Three Main Types of Capillaries

Not all capillaries are built the same way. Different organs need different levels of permeability, so the body has evolved at least three structurally distinct capillary types. The classification hinges on how tightly sealed the capillary wall is and whether it has small openings called fenestrations.

• Continuous capillaries: These have an unbroken lining of endothelial cells joined by tight junctions. They are the most restrictive type and are found in muscle, skin, lungs, and the brain. In the brain specifically, these capillaries form the blood-brain barrier, where the junctions between cells are so tight that the upper limit for substances passing through is less than one nanometer.¹ Brain capillaries also carry a particularly dense coating of glycocalyx, a sugar-rich layer on the inner surface of endothelial cells, which adds a further barrier to molecular passage.²
• Fenestrated capillaries: These have tiny pores punched through the endothelial cells themselves. You find them in organs that need to move material across the vessel wall quickly, such as the kidneys, intestines, and endocrine glands. The pores in kidney glomerular capillaries, which lack a covering diaphragm, allow passage of molecules up to about 15 nanometers, while fenestrated capillaries in glands with diaphragmed pores top out between 6 and 12 nanometers.¹
• Sinusoidal capillaries: These are the leakiest type. They have large gaps between endothelial cells and an incomplete or absent basement membrane. The liver is the best-known example; its sinusoidal capillaries have intercellular gaps and a fragmented basement membrane that let macromolecules reach the metabolically demanding liver cells directly.³ The spleen and red bone marrow also use sinusoidal capillaries, and in the marrow, these vessels play an unusual additional role: newly formed blood cells physically squeeze through temporary pores in the endothelium to enter the circulation.⁴

The practical takeaway is that each organ gets the capillary type that matches its needs. Your brain requires an extremely tight barrier to protect neurons from toxins in the blood. Your kidneys need to filter blood rapidly, so their capillaries are perforated. Your liver has to process large molecules, so it opts for the wide-open sinusoidal design.

How Exchange Happens Across Capillary Walls

Small molecules like oxygen and carbon dioxide cross capillary walls by simple diffusion, driven by concentration differences between the blood and surrounding tissue. Oxygen is more concentrated in the blood arriving from the lungs than in the tissues that have been consuming it, so it flows outward. Carbon dioxide, built up in active tissue, flows inward for the return trip to the lungs. The exact dynamics of oxygen delivery in the brain are still not fully worked out; researchers have proposed that changes in hemoglobin’s shape as it releases oxygen help maintain a more even distribution of oxygen along the length of a capillary, rather than dumping it all near the vessel’s beginning.⁵

Larger molecules need more help. Proteins like albumin and hormones like insulin cross the capillary wall through a process called transcytosis, in which the molecule is engulfed in a tiny vesicle on one side of the endothelial cell, shuttled across, and released on the other side.⁶ In continuous capillaries, many of these vesicles originate from flask-shaped membrane structures called caveolae. Disrupting caveolae significantly reduces the transport of both insulin and albumin across endothelial cells.⁷ In the brain, where tight junctions are the primary gatekeeper, caveolae-mediated transcytosis has drawn attention as a secondary route that could be exploited for drug delivery.⁸

Fluid Balance and the Starling Principle

Capillaries do not just trade gases and nutrients; they also manage the balance of fluid between your blood and your tissues. The classic explanation for this is the Starling Principle, which describes how two competing pressures govern fluid movement. Blood pressure inside the capillary pushes fluid outward into tissue, while the concentration of proteins (especially albumin) in the blood draws fluid back in. The revised version of this principle recognizes that because capillary walls are not perfectly sealed to proteins, a simple pressure balance can never completely stop fluid from leaking out.⁹

This means a small amount of fluid always seeps out of capillaries into surrounding tissue. The lymphatic system picks up that excess and returns it to the bloodstream, maintaining the overall balance.¹⁰ When this system breaks down, whether because of inflammation, low blood protein levels, or lymphatic damage, fluid accumulates in tissues. The result is edema, the visible swelling you notice in conditions ranging from heart failure to severe infections.

Pericytes and Blood Flow Regulation

Capillaries were long thought to be passive tubes, with blood flow controlled entirely by the small arteries upstream. That picture has changed. Wrapped around capillaries are cells called pericytes, and research shows they actively regulate local blood flow.¹¹ In the brain, pericytes located at precapillary sphincters and first-order capillary branches respond strongly to signals from nearby neurons and from the vessel lining itself. When brain cells become more active, these pericytes relax to widen the capillary and increase blood flow to that region; when activity drops, they constrict.¹² Mathematical modeling of brain vasculature suggests precapillary sphincters are actually the dominant regulators of capillary blood flow and pressure, more so than the larger arterioles upstream.

Pericytes do more than regulate flow. They help maintain the structural stability of capillaries and contribute to blood-brain barrier integrity. When the communication between pericytes and endothelial cells breaks down, the consequences can include small vessel disease and neurodegeneration.¹³ Diabetes is one condition that disrupts this pericyte-endothelial cell partnership, leading to microvascular damage in the eyes, kidneys, and other organs.¹⁴

Aging also takes a toll. Studies in mice show that age-related declines in vascular responsiveness are most pronounced at precapillary sphincters, the very structures that play the biggest role in capillary blood flow control.¹⁵ This may help explain why cognitive function tends to decline with age even in the absence of major disease: the brain’s ability to fine-tune its own blood supply gradually weakens.

How New Capillaries Form

Your capillary network is not fixed at birth. The body can build new capillaries through a process called angiogenesis, which is triggered when tissues are not getting enough oxygen. Low oxygen levels prompt cells to release vascular endothelial growth factor (VEGF), one of the most powerful signals for new vessel growth. VEGF drives new vessel formation through two main mechanisms: sprouting, where an existing capillary extends a finger-like projection led by a “tip cell,” and intussusception, where an existing vessel splits internally to form two channels.¹⁶

These two mechanisms tend to happen in different places within a capillary network. Sprouting favors straight segments of vessels where blood flow is smooth, while intussusception occurs more at branch points where flow is turbulent. VEGF concentrations and gradients differ between these locations, steering each process to the spot where it works best. The distinction matters clinically because tumors hijack angiogenesis to feed their own growth. Drugs that block VEGF have become a standard part of cancer treatment, though the relationship between VEGF and new vessel formation is more complex than a simple on-off switch; blocking VEGF can halt the formation of mature vessels while still allowing endothelial sprouting to persist.¹⁷

When Capillaries Go Wrong

Because capillaries sit at the interface between blood and tissue, they are vulnerable to damage from systemic diseases. Diabetes is the most studied example. High blood sugar over time causes thickening of the capillary basement membrane, the thin structural sheet that supports the endothelial cells. Basement membrane thickening in the retina is one of the earliest visible changes in diabetic retinopathy and has been recognized for over half a century as a hallmark of diabetic microvascular disease.¹⁸ The thickening affects capillaries throughout the body, not just the eyes, and evidence suggests it contributes to the increased susceptibility to infections seen in people with diabetes.¹⁹

Acute illness can also wreck capillary function. In severe acute pancreatitis, widespread inflammation causes capillaries to become abnormally leaky, allowing plasma proteins to escape into surrounding tissue. A model of this process predicts that the resulting protein loss drops the blood’s ability to hold onto fluid, leading to low blood pressure, kidney damage, and fluid accumulation in the lungs.²⁰ Sepsis causes a similar pattern: inflamed capillaries leak protein and fluid, dropping blood pressure and starving organs. This “capillary leak” is the bridge between systemic inflammation and multiorgan failure in many critical illnesses.

Exercise and Capillary Density

One of the most practical things to know about capillaries is that you can grow more of them. Regular exercise increases capillary density in skeletal muscle, meaning more vessels per unit of tissue. In animal studies, how often you exercise and for how long are the main factors that determine capillary growth in skeletal muscle, while exercise intensity matters more for capillary density in the heart.²¹ This distinction has real implications: for improving oxygen delivery to working muscles during everyday activity, frequent moderate exercise may be more effective than occasional hard sessions, while high-intensity work appears uniquely beneficial for the heart’s microcirculation.

The reverse is also true. Prolonged inactivity, bed rest, and certain chronic diseases lead to capillary rarefaction, a reduction in the number of capillaries per unit of tissue. Fewer capillaries mean longer diffusion distances, less efficient waste removal, and poorer oxygen delivery. Capillary rarefaction has been documented in conditions like hypertension and metabolic syndrome and is thought to be both a consequence and a contributor to disease progression.

Capillaries in Microgravity

Space travel provides a natural experiment in what happens when gravity no longer holds blood in its usual distribution. On Earth, gravity pulls blood toward your feet. In microgravity, that downward pull disappears, and fluid shifts headward. Astronauts develop puffy faces and thinner legs within hours of reaching orbit. Studies simulating this fluid shift by tilting volunteers head-down found that capillary pressures in the face rose significantly, from about 28 mmHg to 34 mmHg, while the protein concentration of blood plasma dropped, both of which favor fluid leaking out of facial capillaries into surrounding tissue.²² Meanwhile, capillary beds between the heart and feet shift from filtering fluid out to absorbing it, the exact reverse of their normal behavior.

These transcapillary fluid shifts in space may also have consequences beyond puffy faces. There is accumulating evidence that the redistribution of fluid affects bone density, potentially contributing to the bone loss astronauts experience during long missions.²³ The loss of tissue weight and external vessel compression in weightlessness may further alter how fluid moves across capillary walls throughout the body.²⁴

High-Altitude Adaptations in Animals

If microgravity shows what happens when normal capillary pressures are disrupted, high altitude shows what happens when oxygen is scarce and capillaries must compensate. Animals that live at extreme elevations have evolved denser capillary networks in their muscles and hearts, shortening the distance oxygen needs to travel from blood to working cells.²⁵ The vicuña, a wild relative of the llama that lives at over 4,000 meters in the Andes, has heart capillary densities averaging about 3,720 per square millimeter, a remarkably high number for a mammal of its body size.²⁶ Packing more capillaries into tissue is one of the simplest and most effective ways evolution has found to solve the problem of thin air, and it mirrors what exercise does for human muscles at a much faster timescale.

How Capillaries Were Discovered

The existence of capillaries was predicted before anyone could see them. In 1628, William Harvey described the circulation of blood but could not explain how blood got from arteries to veins. The missing link was found in 1661 by Marcello Malpighi, who used an early microscope to examine frog lungs and spotted the tiny vessels connecting arteries to veins.²⁷ Malpighi’s discovery was one of the first major findings enabled by microscopy and completed the picture of how blood circulates continuously. Before that, the gap between arteries and veins was a genuine mystery, and some physicians still doubted that blood circulated at all. It took a new technology, the microscope, to reveal a structure too small for the naked eye but essential to every tissue in the body.