The structure and function of capillaries in the body.


Capillaries are the smallest blood vessels in the body and connect the smallest arteries to the smallest veins. These vessels are often called "microcirculation."

Only two layers of cells thick, capillaries play a central role in blood circulation, transporting oxygen from the blood to tissues and collecting carbon dioxide, which must be removed. They are also the place of supply of nutrients to nourish all the cells of the body.

There are three main types of capillaries: continuous, fenestrated, intermittent, or sinusoidal, which are found in different parts of the body, and specialized capillaries in the brain form the blood-brain barrier.

Tests that evaluate capillaries are important when evaluating people medically, and there are several diseases associated with these vessels .

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Capillaries are very thin, approximately 5 microns in diameter, and consist of only two layers of cells: the inner layer of endothelial cells and the outer layer of epithelial cells. They are so small that red blood cells must pass through them in one piece.

It has been estimated that the average human body has 40 billion capillaries. This layer of cells is surrounded by the so-called basement membrane, a protein layer that surrounds the capillary .

If all the capillaries in the human body were aligned, the line would stretch more than 100,000 miles.

Capillaries in the circulatory system.

Capillaries can be seen as the central part of the circulation. Blood leaves the heart through the aorta and pulmonary arteries, to the rest of the body and to the lungs, respectively.

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

The number of capillaries depends on the type of tissue.

The number of capillaries in the tissue can vary widely. Without a doubt, the lungs are filled with capillaries that surround the alveoli, which pick up oxygen and expel carbon dioxide. Outside of the lungs, there are more capillaries in the tissues that are more metabolically active .

Hair types

There are three main types of capillaries in the circulatory system:

  • Continuous : these capillaries do not have holes and only small molecules pass through. They are found in muscles, skin, adipose and nervous tissue.
  • Fenestrated : These capillaries have small pores through which small molecules pass and are located in the intestines, kidneys and endocrine glands.
  • Sinusoidal or intermittent : These capillaries have large open pores, large enough to allow the passage of blood cells. They are present in the bone marrow, lymph nodes and spleen and, in fact, they are the ones that escape the capillaries the most .

Blood-brain barrier

In the central nervous system, capillaries make up the so-called blood-brain barrier. This barrier limits the ability of toxins (and, unfortunately, many chemotherapy agents and other drugs) to enter the brain.

Finding drugs that can cross the blood-brain barrier and thus treat conditions such as brain metastases from various cancers is an active area of research .


Capillaries are responsible for facilitating the transport and exchange of gases, fluids and nutrients in the body. Although the arteries and arterioles transport these products to the capillaries, the exchange takes place at the capillary level.

Capillaries also receive carbon dioxide and waste products, which are then sent to the kidneys, liver (for waste), and lungs (to exhale carbon dioxide) .

Gas exchange

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

Exchange of fluids and nutrients

In addition, fluids and nutrients diffuse through selectively permeable capillaries into body tissues, and waste products are collected in capillaries and transported through veins to the kidneys and liver, where they are processed and excreted from the body. body .

Blood flow through capillaries.

Since blood flow through capillaries plays such an important role in maintaining the body, you may be wondering what happens if your blood flow changes, for example if your blood pressure drops ( hypotension ).

Capillary beds are regulated by what is called self-regulation, so that if blood pressure drops, the flow through the capillaries will continue to provide oxygen and nutrients to the body's tissues. When you exercise, more capillaries build up in your lungs to prepare for the increased oxygen demand from your body tissues.

The blood flow in the capillaries is controlled by the precapillary sphincters. The precapillary sphincter is a muscle fiber that controls the movement of blood between the arterioles and the capillaries .

Capillary microcirculation

The regulation of fluid movement between capillaries and the surrounding interstitial tissues is determined by the balance of two forces: hydrostatic pressure and osmotic pressure.

On the arterial side of the capillary, the hydrostatic pressure (the pressure that comes from the heart that pumps blood and the elasticity of the arteries) is high. Because capillaries "leak," this pressure forces fluid and nutrients to resist capillary walls and escape into the interstitial space and tissues.

On the venous side of the capillary, the hydrostatic pressure was significantly reduced. 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 causes the fluid to flow back into the capillary.

Osmotic pressure, also called oncotic pressure, draws fluids and waste products from the tissues into the capillary, which are then returned to the bloodstream (and then sent to the kidneys, among other things) .

Medical value

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

Pale skin

If you've ever wondered why your skin turns white when pressed, the answer is capillaries. Pressure on the skin causes blood to flow out of the capillaries, which causes the skin to turn pale or pale when the pressure is released .


If you develop a rash, your doctor may apply pressure to your skin to look for white patches. When capillaries break, blood seeps into the skin and the red spots remain even under pressure. They are called petechiae and are associated with conditions other than rashes, which turn pale with pressure .

Capillary filling

Doctors usually check for "capillary filling." This is tested by observing how quickly the skin turns pink again after pressure is released, and can provide an indication of the health of the tissues.

An example of such use would be people with burns . In a second-degree burn, capillary filling may take place with some delay, but in a third-degree burn, capillary filling does not occur at all.

Emergency workers often check capillary fill by pressing on the toenail or toenail, then releasing the pressure and waiting for how long it takes for the nail bed to turn pink again. If the color returns in two seconds (the amount of time it takes to say capillary fill), the circulation in the arm or leg is probably fine.

If it takes more than two seconds to fill the capillaries, the circulation in the limb is likely impaired and is considered an emergency. There are other parameters in which capillary replacement is also delayed, for example, during dehydration .

Third distance and capillary permeability

You can hear doctors talking about a phenomenon known as the "third interval." Capillary permeability refers to the ability of a liquid to pass from the capillaries to the surrounding tissue.

Capillary permeability can be increased by cytokines (leukotrienes, histamines, and prostaglandins) secreted by cells of the immune system. A higher amount of fluid (third distance) locally can cause hives . When someone is very ill, this third distance, due to the leakage of capillaries, can spread, giving their body a swollen appearance .

Capillary blood samples

In most cases, when blood is drawn, the technician will draw blood from a vein in your arm. Capillary blood can also be used for some blood tests, such as those that check blood sugar levels. The lancet is used to cut the finger (cut the capillaries) and can be used to control the blood sugar level and the pH of the blood .

Related conditions

There are several common and unusual conditions that affect the capillaries.

Port wine stain (mole)

A small percentage of babies are born with "moles," which consist of patches of red or purple skin associated with enlarged capillaries. Most port spots are more of a cosmetic problem than a medical problem, but they can bleed easily if irritated .

Capillary malformation

Capillary malformation (arteriovenous malformation syndrome) can occur as part of an inherited syndrome that occurs in about 1 in 100,000 people of European descent. In this syndrome, there is more blood flow than normal through the capillaries near the skin, resulting in pink and red patches on the skin.

They can occur on their own or people can have other complications of the syndrome, such as arteriovenous malformations (abnormal connections between arteries and veins) that, when released into the brain, can cause headaches and seizures .

Systemic capillary leak syndrome

A rare condition known as capillary leak syndrome involves leaky capillaries, leading to persistent nasal congestion and fainting spells due to a rapid drop in blood pressure .

Macular degeneration

Macular degeneration , now the leading cause of blindness in the United States, occurs as a result of damage to the retinal capillaries .

Frequently asked questions

Why do the capillaries on my face break?

Sun damage and rosacea are common causes of broken facial capillaries. You can avoid these breakouts by protecting your skin from sunburn and avoiding the things that cause rosacea, like alcohol and smoking.

How to get rid of spider veins on your face?

Laser treatment can reduce spider veins on the face, also known as telangiectasias . Exposing the skin to heat can destroy small blood vessels and become invisible. However, they can reopen and require additional treatment in the future.

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Capillaries, although they are the smallest blood vessels, play the most important role by being the site of exchange for oxygen and carbon dioxide in all tissues, delivering nutrients and removing waste from cells.

Capillaries are also very important for medical diagnosis and sometimes provide important information about a person's health. They were once thought to be primarily responsible for cosmetic conditions, but this changed when their role in macular degeneration was discovered .

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