Capillaries

What are capillaries?

Capillaries are delicate blood vessels (tubes that hold blood) throughout your body. They bring nutrients and oxygen to cells in your organs and body systems. In exchange, they also carry away tissue waste like carbon dioxide.

You can think of it like someone bringing groceries to your door and taking the trash with them as they leave. This happens constantly and in many different places in your body.

Types of capillaries

Capillaries have three different shapes that help them carry out various functions. Types of capillaries include:

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

What do capillaries do?

Capillary beds complete the circulatory system by connecting your arteries to your veins. Arteries carry oxygen-rich blood from your heart to your organs. Veins help your body remove low-oxygen blood and waste from your organs.

The role of the capillaries is to function as a place to exchange what arteries and veins carry. While arteries and veins carry gases and nutrients through your body, it’s the capillaries that get them to their destinations.

Capillaries serve a variety of organs and systems. They support your:

• Bone marrow by allowing 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 pee (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

Where are capillaries located?

Capillaries are nearly everywhere in your body — wherever arteries and veins connect. Capillary beds serve your tissues and organs.

What do they look like?

Capillaries are the smallest blood vessels in your vascular system. Researchers have to use a microscope to see capillaries, which are smaller than a human hair. A capillary is about 5 micrometers (a micrometer is 0.001 millimeter) around. It’s so tiny that red blood cells have to go through it lined up one behind the other. Capillaries are less than 1 mm long, or about the size of the tip of a pen.

Capillaries have thin walls (1 micrometer thick) that let nutrients, fluids and gases go in and out.

What are the parts of capillaries?

Capillaries contain two layers of cells:

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

What are the common conditions and disorders that affect capillaries?

Broken capillaries are common. Severe coughing or vomiting can cause them. The bleeding from broken capillaries creates little dots of discoloration on your skin (petechiae). These often heal on their own.

Other conditions that affect your capillaries include:

• Arteriovenous malformation (AVM). A tangle of arteries and veins in your brain or spinal cord that may bypass the capillaries. They also can occur in your limbs, trunk and organs.
• Capillary angiosarcoma. Rare 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) that can burst. It’s also called Osler-Weber-Rendu syndrome.
• Macular degeneration. Damage to your inner eye due to capillary leaks.
• Microcephaly-capillary malformation syndrome. A condition causing wide capillaries in people with heads that are smaller than average.
• Port wine stain. A birthmark that happens because of capillaries that don’t form correctly.
• Spider angioma. Small blood vessels that branch from a central spot, usually on your face, neck or chest. It’s also called spider nevus or spider telangiectasia.
• Strawberry birthmark. A bright red cluster of blood vessels on your skin’s surface.
• Vasculitis. Blood vessel inflammation that can affect your capillaries. It can lead to complications that include rupture and blockages.

Common signs or symptoms of capillary conditions

Symptoms of conditions that can affect your capillaries may include:

• Nausea
• Lightheadedness
• Headache
• Swelling
• Seizures
• Dizziness
• Fatigue
• Skin rash
• Weight loss
• Nosebleeds
• Skin discoloration
• Anemia
• Issues with your vision

Common tests to check the health of capillaries

Tests to check for conditions that affect your capillaries may include:

• Blood tests
• MRI
• CT
• Angiogram
• PET scan
• Eye exam
• OCT eye test

Common treatments for capillaries

Treatments for conditions that affect your capillaries may include:

• Medicines
• Surgery or a minor procedure
• Radiation therapy
• Chemotherapy
• Blood transfusion

How do I care for my capillaries?

You can take care of your capillaries the same way you care for your other blood vessels. This includes:

• Being physically active for 30 minutes five times a week (150 minutes total)
• Limiting the number of drinks you have that contain alcohol and caffeine
• Maintaining a weight that’s healthy for you
• Avoiding smoking or other tobacco products
• Visiting an eye doctor regularly for eye exams 

You can 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 healthcare provider?

Your provider may detect capillary disease before you experience symptoms. This makes it possible to start treatments to prevent them from getting worse. It’s especially true if you stay current with preventive care, like 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

What is the difference between a vein and a capillary?

A vein transports blood back to your heart after your cells have taken what they need from it. Veins carry blood that contains waste like carbon dioxide. A capillary is like an exchange center where things your cells need move toward them and the things your cells throw out move away from them.