Capillaries are very tiny blood vessels — so small that a single red blood cell can barely fit through them.

They help to connect your arteries and veins in addition to facilitating the exchange of certain elements between your blood and tissues.

This is why tissues that are very active, such as your muscles, liver, and kidneys, have an abundance of capillaries. Less metabolically active tissues, such as certain types of connective tissue, don’t have as many.

Read on to learn more about the function of capillaries and the conditions that can affect them.

Capillaries connect the arterial system — which includes the blood vessels that carry blood away from your heart — to your venous system. Your venous system includes the blood vessels that carry blood back to your heart.

The exchange of oxygen, nutrients, and waste between your blood and tissues also happens in your capillaries. This happens through two processes:

  • Passive diffusion. This is the movement of a substance from an area of higher concentration to an area of lower concentration.
  • Pinocytosis. This refers to the process through which your body’s cells actively take in small molecules, such as fats and proteins.

The walls of capillaries are made up of a thin cell layer called endothelium that’s surrounded by another thin layer called a basement membrane.

Their single-layer endothelium composition, which varies among the different types of capillaries, and surrounding basement membrane makes capillaries a bit “leakier” than other types of blood vessels. This allows oxygen and other molecules to reach your body’s cells with greater ease.

Additionally, white blood cells from your immune system can use capillaries to reach sites of infection or other inflammatory damage.

There are three types of capillaries. Each has a slightly different structure that allows to function in a unique way.

Continuous capillaries

These are the most common types of capillaries. They contain small gaps in between their endothelial cells that allow for things like gases, water, sugar (glucose), and some hormones to pass through.

The continuous capillaries in the brain are an exception, however.

These capillaries are part of the blood-brain barrier, which helps to protect your brain by only allowing the most essential nutrients to cross.

That’s why the continuous capillaries in this area don’t have any gaps between endothelial cells, and their surrounding basement membrane is also thicker.

Fenestrated capillaries

Fenestrated capillaries are “leakier” than continuous capillaries. They contain small pores, in addition to small gaps between cells, in their walls that allow for the exchange of larger molecules.

This type of capillary is found in areas that require a lot of exchange between your blood and tissues. Examples of these areas include:

  • the small intestine, where nutrients are absorbed from food
  • the kidneys, where waste products are filtered out of the blood

Sinusoid capillaries

These are the rarest and “leakiest” type of capillary. Sinusoid capillaries allow for the exchange of large molecules, even cells. They’re able to do this because they have many larger gaps in their capillary wall, in addition to pores and small gaps. The surrounding basement membrane is also incomplete with openings in many places.

These types of capillaries are found in certain tissues, including those of your liver, spleen, and bone marrow.

For example, in your bone marrow, these capillaries allow newly produced blood cells to enter into the bloodstream and begin circulation.

While capillaries are very small, anything unusual in their functioning can cause visible symptoms or even potentially serious medical conditions.

Port wine stains

Port wine stains are a type of birthmark caused by the widening of capillaries located in your skin. This widening causes the skin to appear pink or dark red in color, giving the condition its name. Over time, they can darken in color and thicken.

While they don’t go away on their own, port wine stains also don’t spread to other areas.

Port wine stains typically don’t require treatment, although laser treatment can help to make them lighter in color.


Petechiae are small, round spots that appear on the skin. They’re typically about the size of a pinhead, can be red or purple in color, and are flat in the skin. They happen when capillaries leak blood into the skin. They don’t lighten in color when pressure is applied over them.

Petechiae are typically a symptom of an underlying condition, including:

Some medications, including penicillin, can also cause petechiae as a side effect.

Systemic capillary leak syndrome

Systemic capillary leak syndrome (SCLS) is a rare condition that doesn’t have a clear cause. But experts think it may be related to a substance in the blood that damages capillary walls.

People with SCLS have recurring attacks during which their blood pressure drops very quickly. These attacks can be severe and require emergency medical attention.

These attacks are usually accompanied by some initial warning signs, including:

  • nasal congestion
  • cough
  • nausea
  • headache
  • abdominal pain
  • lightheadedness
  • swelling in arms and legs
  • fainting

SCLS is usually treated with medications that help to prevent these attacks from occurring.

Arteriovenous malformation syndrome

People with arteriovenous malformation syndrome (AVM) have an abnormal tangle of arteries and veins that are connected to each other without capillaries in between. These tangles can occur anywhere in the body, but are most often found in the brain and spinal cord.

This can cause lesions that interfere with blood flow and oxygen delivery. These lesions may also cause bleeding into the surrounding tissue.

AVM usually doesn’t cause symptoms, so it’s usually only discovered while trying to diagnose another condition. However, in some cases, it can cause:

  • headaches
  • pain
  • weakness
  • issues with vision, speech, or movement
  • seizures

AVM is a rare condition that’s often present at the time of birth. Treatment usually involves surgically removing or closing the AVM lesion. Medication can also help to manage symptoms, such as pain or headaches.

Microcephaly-capillary malformation syndrome

Microcephaly-capillary malformation syndrome is a rare genetic condition that starts before birth.

People with this condition have smaller heads and brains. They also have widened capillaries that increase the flow of blood near the skin surface, which can cause pinkish red spots on the skin.

Additional symptoms can include:

  • severe developmental delays
  • seizures
  • difficulty eating
  • unusual movements
  • distinct facial features, which can include a sloped forehead, round face, and unusual hair growth
  • slower growth
  • shorter or smaller stature
  • finger and toe abnormalities, including really small or absent nails

Microcephaly-capillary malformation syndrome is caused by a mutation in a specific gene called the STAMBP gene. Mutations to this gene can result in cells dying during development, affecting the entire development process.

Treatment for this condition can involve stimulation — particularly through sound and touch — bracing to maintain posture, and anticonvulsant medication therapy for management of seizures.

Capillaries are tiny blood vessels that play a big role in facilitating the exchange of various substances between your bloodstream and tissues. There are several types of capillaries, each with a slightly different structure and function.