What is the term for making the internal diameter of the blood vessel smaller?

Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins. These vessels are often referred to as the "microcirculation" because they deliver oxygen and nutrients to all cells in the body, and remove carbon dioxide to be eliminated.

There are three primary types of capillaries, as well as a specialized type that makes up the blood-brain barrier. Certain medical conditions, such as macular degeneration, are caused by dysfunction in the capillaries.

This article explains capillary structure and function. It discusses their role in maintaining health, as well as how capillary function is evaluated and how that may lead to a medical diagnosis.

Capillaries are very thin, approximately 5 micrometers in diameter and composed of only two layers of cells. The inner layer is made up of endothelial cells with an outer layer of epithelial cells. They are so small that red blood cells need to flow through them single file.

It's been estimated that there are 40 billion capillaries in the average human body, and they'd stretch over 100,000 miles if placed in a single line. Surrounding this layer of cells is something called the basement membrane, a layer of protein surrounding the capillary.

Capillaries need to be small in order to facilitate oxygen and carbon dioxide gas exchange in cells. They also play a role in fluid and nutrient exchange, forming the connection between the body's smallest arteries (arterioles) and smallest veins (venules). They play a different role than veins, which return blood to the heart, and the arteries that pump blood out to the body.

Capillaries are found throughout the body and may be thought of as the central portion of circulation. Blood leaves the heart through the aorta and the pulmonary arteries, traveling to the rest of the body and to the lungs, respectively.

These large arteries become smaller arterioles and eventually narrow to form the capillary bed. From the capillaries, blood flows into the smaller venules and then into veins, flowing back to the heart.

This video has been medically reviewed by Anju Goel, MD, MPH.

The number of capillaries in a tissue can vary widely. Certainly, the lungs are packed with capillaries surrounding the alveoli to pick up oxygen and drop off carbon dioxide. Outside of the lungs, capillaries are more abundant in tissues that are more metabolically active.

Capillaries in the body include three primary types. These capillary types are called:

• Continuous, because these capillaries have no perforations and allow only small molecules to pass through. They are present in muscle, skin, fat, and nerve tissue.
• Fenestrated, due to small pores in the capillaries that allow small molecules to pass through. They are located in the intestines, kidneys, and endocrine glands.
• Sinusoidal or discontinuous, a type of capillary with large open pores—large enough to allow a blood cell through. They are present in bone marrow, lymph nodes, and the spleen, and are, in essence, the "leakiest" of the capillaries.

Capillaries in the central nervous system make up what is known as the blood-brain barrier. This barrier limits the ability of toxins (and many chemotherapy agents and other medications) to pass through into the brain.

Looking for drugs that can pass through the blood-brain barrier, and hence, treat conditions such as brain metastases from a number of cancers, is an active area of research.

The capillaries are responsible for facilitating the transport and exchange of gases, fluids, and nutrients in the body. While the arteries and arterioles act to transport these products to the capillaries, it is at the level of capillaries where the exchange takes place.

The capillaries also function to receive carbon dioxide and waste products that are then delivered to the kidneys and liver (for wastes) and the lungs (for exhalation of carbon dioxide).

In the lungs, oxygen diffuses from the alveoli into capillaries to be attached to hemoglobin and be carried throughout the body. Carbon dioxide (from deoxygenated blood) in turn flows from the capillaries back into alveoli to be exhaled into the environment.

Likewise, fluids and nutrients diffuse through selectively permeable capillaries into the tissues of the body, and waste products are picked up in the capillaries to be transported through veins to the kidneys and liver. There, they are processed and eliminated from the body.

Since the blood flow through capillaries plays such an important part in maintaining the body, you may wonder what happens when blood flow changes—for example, if your blood pressure would drop (hypotension).

Capillary beds are regulated through something called autoregulation, so that if blood pressure drops, blood continues to flow through the capillaries to provide oxygen and nutrients to the tissues of the body.

The flow of blood in the capillaries is controlled by precapillary sphincters. A precapillary sphincter is the muscular fibers that control the movement of blood between the arterioles and capillaries.

When you exercise, more capillary beds are recruited in the lungs to prepare for an increased need for oxygen in tissues of the body.

Capillaries regulate fluid movement between the capillaries and surrounding interstitial tissues. This process is determined by the balance of two forces: the hydrostatic pressure and osmotic pressure.

On the arterial side of the capillary, the hydrostatic pressure (the pressure that comes from the heart pumping blood and the elasticity of the arteries) is high. Since capillaries are "leaky" this pressure forces fluid and nutrients against the walls of the capillary and out into the interstitial space and tissues.

On the vein side of the capillary, the hydrostatic pressure has dropped significantly. At this point, it is the osmotic pressure of the fluid within the capillary (due to the presence of salts and proteins in the blood) that draws fluids back into the capillary.

Osmotic pressure is also referred to as oncotic pressure. It is what pulls fluids and waste products out of the tissues and into the capillary to be returned to the bloodstream (and then delivered to the kidneys, among other sites).

Capillaries are important medically in many ways, and there are ways that you can actually indirectly observe these tiny blood vessels.

If you've ever wondered why your skin turns white when you put pressure on it, the answer is the capillaries. Pressure on the skin presses blood out of the capillaries and results in the blanching, or pale appearance, of skin when the pressure is removed.

If you develop a rash, a physician may push on your skin to see if the spots turn white. When capillaries are broken, the blood leaks into the skin, and the red spots will remain even with pressure. These are called petechiae and are associated with different conditions than rashes that do blanch with pressure.

Healthcare providers often check for "capillary refill." This is tested by observing how rapidly the skin becomes pink again after pressure is released and can give an idea of the health of the tissues.

An example of this use would be in people with burns. A second-degree burn may reveal capillary refill to be somewhat delayed, but in a third-degree burn, there would be no capillary refill at all.

Emergency responders often check capillary refill by pushing on a fingernail or toenail, then releasing pressure and waiting to see how long it takes for the nailbed to appear pink again. If color returns within two seconds (the amount of time it takes to say capillary refill), circulation to the arm or leg is probably OK.

If capillary refill takes more than two seconds, circulation in the limb may be compromised and it's considered an emergency. There are other settings in which capillary refill is delayed as well, such as in dehydration.

Capillary permeability refers to the ability of fluids to pass out of the capillaries into the surrounding tissues. You may hear healthcare providers talk about a phenomenon known as "third spacing."

Capillary permeability can be increased by cytokines (leukotrienes, histamines, and prostaglandins) released by cells of the immune system. The increased fluid (third spacing) locally can result in hives.

When someone is very ill, this third spacing due to leaky capillaries may be widespread, giving their body a swollen appearance.

Most of the time when you have your blood drawn, a technician will take blood from a vein in your arm. Capillary blood may also be used to do some blood tests, such as for those who monitor their blood sugar.

A lancet is used to cut the finger (cut capillaries) and can be used for testing blood sugar and blood pH.

Capillaries and their function play a role in several health conditions. Some of them are relatively common while others are considered more rare.

A small percentage of children are born with "birthmarks" consisting of an area of red or purple skin related to dilated capillaries. Most port-wine stains are a cosmetic problem rather than a medical concern, but they may bleed easily when irritated.

Capillary malformation may occur as part of an inherited syndrome present in roughly 1 in 100,000 people of European ancestry. In this syndrome, there is more blood flow than normal through the capillaries near the skin, which results in pink and red dots on the skin.

These may occur alone, or people may have other complications of this syndrome such as arteriovenous malformations (abnormal connections between arteries and veins) which, when in the brain, can cause headaches and seizures.

A rare disorder known as capillary leak syndrome involves leaky capillaries, which may result in constant nasal congestion and episodes of fainting due to rapid drops in blood pressure.

This syndrome also may occur due to adverse drug reactions during cancer treatment and can lead to severe and potentially life-threatening complications. It's also been noted in people diagnosed with COVID-19 infections, as well as some people who received COVID-19 vaccinations.

Macular degeneration, now the leading cause of blindness in the United States, occurs secondary to damage in the capillaries of the retina.

Capillaries, though tiny, play the key role in exchanging oxygen and carbon dioxide to all cells, while delivering nutrients and removing waste. They may offer diagnostic information about medical conditions, so contact your healthcare provider if you think you may have symptoms related to capillary function.

Frequently Asked Questions

• Why do I get broken capillaries on my face?

  Sun damage and rosacea are common reasons why capillaries break on the face. You may be able to avoid these flare-ups by protecting your skin from sunburn and avoiding excessive alcohol intake and smoking.

• How do I get rid of spider veins on my face?

  Laser treatments can make facial spider veins, also known as telangiectasias, disappear. A dermatologist can use a vascular laser to apply heat to the skin, which can collapse tiny blood vessels so they’re no longer visible. However, they may reopen, requiring additional treatments in the future.

• How do I keep my capillaries healthy?

  Capillaries are part of your cardiovascular system. The same lifestyle choices you'd make to protect heart health, like exercising and eating a healthy diet, will help to keep your capillaries healthy too.