What are Capillaries?

What Role Do Capillaries Play in the Circulatory System?

diagram of capillaries surrounding alveoli
What are capillaries, what types are there, and what is their function?. istockphoto.com

Definition: Capillaries

Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins. These vessels are often termed "microcirculation."

Structure of Capillaries

Capillaries are very thin, approximately 5 micrometers in diameter, and are composed of only 2 layers of cells; an inner layer of endothelial cells and outer layer of epithelial cells. They are so small that red blood cells need to flow through them single file.

If all the capillaries in the human body were lined up in single file, the line would stretch over 100,000 miles. It's been estimated that there are 40 billion capillaries in the average human body. Surrounding this layer of cells is something called the basement membrane, a layer essentially of protein surrounding the capillary.

Function of Capillaries

The capillaries are responsible for facilitating the transport of gasses, fluids, and nutrients in the body.

Gas Exchange

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.

Fluid and Nutrient Exchange

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 where they are thus processed and eliminated from the body.

Types of Capillaries

There are 3 primary types of capillaries:

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

Blood Flow Through Capillaries

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. Capillary beds are regulated through something called autoregulation, so that if blood pressure would drop, flow through the capillaries will continue to provide oxygen and nutrients to the tissues of the body. With exercise, more capillary beds are recruited in the lungs to prepare for an increased need for oxygen in tissues of the body.

Conditions Involving the Capillaries

There are several both common and uncommon conditions which involve the capillaries. A few of these include:

Port wine stain - "birth mark"  - Around 1 in 300 children are born with "birth marks" 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 - arteriovenous malformation syndrome - 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. The 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.

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

Examples: Carl's doctor told him that due to fluid build-up in the alveoli in his lungs, less oxygen could be transferred to the capillaries in his blood and distributed to the rest of his body.


Genetics Home Reference. Capillary malformation – arteriovenous malformation syndrome. Accessed 02/27/16. https://ghr.nlm.nih.gov/condition/capillary-malformation-arteriovenous-malformation-syndrome

U.S. National Library of Medicine. Capillaries. Accessed 02/27/16. http://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0022018/

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