Capillaries

What are capillaries?

Capillaries are delicate blood vessels that exist throughout your body. They transport blood, nutrients and oxygen to cells in your organs and body systems. Capillaries are the smallest blood vessels in your vascular system.

What do capillaries do?

Capillaries complete the circulatory system by connecting arteries to veins:

• Arteries carry oxygen-rich blood from the heart to your organs.
• Veins help the body remove low-oxygen blood and waste.

Read more about how blood flows through the body.

Do capillaries serve other functions?

Capillaries also support a variety of organs and systems. They support the:

• Bone marrow, by enabling new blood cells to enter your bloodstream.
• Brain, by forming the blood-brain barrier. This structure delivers nutrients to the brain while preventing toxins from passing through.
• Endocrine system, by delivering hormones to specific organs.
• Kidneys, where peritubular capillaries filter blood, produce urine and absorb water and sodium.
• Liver, by removing defective red blood cells and bacteria.
• Lungs, by releasing carbon dioxide and taking in oxygen.
• Lymphatic system, by collecting fluid from tissues and directing it to lymph nodes.
• Small intestine, by transporting digested nutrients so they can nourish your cells.

What is the anatomy of the capillaries?

Most capillaries are only about 8 to 10 micrometers in diameter (a micrometer is 0.001 mm). They’re so tiny that red blood cells have to pass through in a single file line.

Capillaries contain two layers of cells:

• Endothelial cells are inside the capillary. They control the flow of fluid, nutrients and gases.
• Epithelial cells form a protective layer around the endothelial cells.

What are the different types of capillaries?

Capillaries have three different shapes, which help them carry out various functions:

• Continuous fenestrated capillaries have small openings (fenestrae) that enable the rapid exchange of substances. This type of capillary is in your kidneys, small intestine and endocrine glands.
• Continuous nonfenestrated capillaries have a lining through which only small molecules can pass. This type of capillary exists in the nervous system as well as fat and muscle tissue.
• Sinusoidal capillaries have small fenestrae that allow certain substances to pass through. This type of capillary is in your liver and spleen.

What conditions affect the capillaries?

Broken capillaries are common. They can occur with severe coughing or vomiting. This causes little red dots on your skin that often heal on their own.

Other conditions affecting the capillaries include:

• Arteriovenous malformation (AVM): A tangle of arteries and veins in the brain or spinal cord that may bypass the capillaries. AV malformations aren’t just limited to your brain and spinal cord. They can occur in the limbs, trunk and organs.
• Capillary angiosarcoma: Cancer of the endothelial cells that can affect the capillaries.
• Capillary leak syndrome: A condition that causes a sudden drop in blood pressure. It sometimes requires emergency treatment.
• Hereditary hemorrhagic telangiectasia: An inherited blood vessel disorder that causes abnormal growths (telangiectases), which can burst. It’s also called Osler-Weber-Rendu syndrome.
• Macular degeneration: Damage to the inner eye due to capillary leaks.
• Microcephaly-capillary malformation syndrome: A condition causing wide capillaries in individuals with abnormally small heads.
• Spider nevus: Small blood vessels that branch from a central spot, usually on your face, neck or chest. It’s also called spider angioma or spider telangiectasia.
• Strawberry birthmark: A bright red cluster of blood vessels on the skin’s surface.
• Vasculitis: Blood vessel inflammation that can affect the capillaries. It can lead to complications that include rupture and blockages.

How do I care for my capillaries?

You can take care of your capillaries by maximizing your vascular health.

This includes:

• Living an active lifestyle.
• Limiting alcohol and caffeine consumption.
• Maintaining a healthy weight.
• Quitting smoking if you use tobacco.

You should also work with your healthcare provider to manage conditions that can affect vascular health.

These include:

• Diabetes.
• High cholesterol.
• High blood pressure.
• Stress.

When should I call a doctor about my capillaries?

It may not be necessary to contact your healthcare provider about capillary disease. They may detect problems before you experience symptoms. This makes it possible to start treatments to prevent them from getting worse. It is especially true if you stay current with preventive care, such as annual physical exams.

Contact your healthcare provider if you experience signs of vascular disease. These include:

• Broken capillaries.
• Enlarged blood vessels in your legs.
• Numbness or tingling on one side of your body.
• Pain in your legs after light exercise.
• Sudden changes in vision.

A note from Cleveland Clinic

Capillaries are delicate blood vessels that deliver blood, nutrients and oxygen to cells. It can be challenging to detect the early signs of capillary issues. Staying current with preventive care and chronic condition treatments can lower your risk.