What are Capillaries?

Tricia Christensen
Tricia Christensen

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.

Capillaries act as "communicators" between arteries and veins.
Capillaries act as "communicators" between arteries and veins.

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.

In general, most arteries carry oxygenated blood away from the heart, while most veins carry deoxygenated blood to the heart.
In general, most arteries carry oxygenated blood away from the heart, while most veins carry deoxygenated blood to the heart.

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.

Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent wiseGEEK contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

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Discussion Comments


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


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


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


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


@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!


the barrons capillaries are just massive.


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


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


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.

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