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What are Capillaries?

Tricia Christensen
Updated Mar 03, 2024
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Capillaries are tiny blood vessels that pass blood from the arteries into the veins. They are very small, the largest being about 10 micrometers in diameter. Their walls are thin which allows materials to pass into them. Different types of capillaries exist and perform different functions for the body. Primarily, however, they are able to profuse the tissues of the body with needed oxygen and important nutrients supplied by blood.

There are three types: continuous, fenestrated, and sinusoidal. They vary in construction and in the degree to which they will allow things outside the capillaries to get into them. All vessels have an endothelial wall with a differing degree of permeability depending upon type.

Continuous capillaries have the thickest endothelial wall. They allow only water, and ions into their pathways. Fenestrated capillaries have “windows” that lets larger molecules in and out. Sinusoidal capillaries have the greatest amount of permeability, letting red blood cells and proteins in through the endothelial walls.

While capillaries function in one respect as the “communicators” between arteries and veins, they also are the tiny blood vessels that supply blood to organs. Those supplying blood to an organ, when taken in whole, are called a capillary bed. They are numerous, and feed the organ with amino acids, proteins, and most importantly oxygen, without which organ cells could not survive.

In addition to being the transporters of blood products, capillaries allow for waste products to enter. In this way they perform an important function because waste is ultimately transported out of the body through this interchange.

The amount of capillaries in the human body is quite amazing. If one could count and measure all in the average human adult, one would find about 25,000 miles (40,233.6 km) of them. The extensive supply in the body indicates their extreme importance to our existence and health.

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Tricia Christensen
By Tricia Christensen , Writer
With a Literature degree from Sonoma State University and years of experience as a TheHealthBoard contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.

Discussion Comments

By anon331475 — On Apr 23, 2013

By what process does the majority of tissue fluid return to the capillaries?

By anon289474 — On Sep 04, 2012

How can a person survive if they are born without any capillaries? What is the name of this disorder/disease?

By anon261265 — On Apr 14, 2012

Does anyone know what the glands found on top of each kidney that are not part of the excretory system is?

By anon261264 — On Apr 14, 2012

I still don't get it. What is a capillary bed?

By SkittisH — On May 30, 2011

@anon25952 - No, a "capillary bed" isn't a type of capillary. It's just the name for a big bunch of capillaries as a group. Capillary beds are divided by which organ they supply blood to, so you could say that all of the capillaries that supply the liver, for example, are referred to as the liver's capillary bed. Hope this helps!

By anon62698 — On Jan 28, 2010

the barrons capillaries are just massive.

By anon44147 — On Sep 05, 2009

"What are mayofocapillaries" ? or something that sounds like that? please help!

By anon25952 — On Feb 05, 2009

Is a 'capillary bed' classified as type of capillaries, or not?

By somerset — On Oct 05, 2008

The capillaries are so small that blood cells move through the capillaries one cell at a time. The walls are equally thin, and create a mesh of vessels throughout the whole body.

Capillaries also play a role in body's temperature. During workout the extra heat is delivered to the capillaries and then distributed to the tissue.

Tricia Christensen

Tricia Christensen


With a Literature degree from Sonoma State University and years of experience as a TheHealthBoard contributor, Tricia...
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