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Capillaries: Function & Definition

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Instructor: Marta Toran

Marta has taught high school and middle school Science and has a Master's degree in Science Education.

Capillaries, the thinnest blood vessels in the human body, play a crucial role in moving nutrients and oxygen throughout the body and removing waste. Learn more about the function and definition of capillaries and how differences in structure serve specific purposes. Updated: 08/25/2021


Capillaries are the smallest type of blood vessel in the body. Their job is to enable the exchange of substances between the blood and surrounding tissues. One place where they can be easily seen (no pun intended!) from the outside is on the whites of the eyes.

Eye Capillaries

Capillaries are found in every square inch of the body, from the skin to the deepest tissues in the body's cavities. There are between 60,000 to 100,000 miles of blood vessels stretching throughout the human body, depending on the size and weight of the person, and most of these are capillaries.

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Capillary Function and Structure

Capillaries deliver nutrients and oxygen to tissues and remove the byproducts of cellular reactions, such as carbon dioxide and water. With the exception of the lungs, where the opposite is true, capillaries bring oxygenated blood, blood-carrying oxygen, to organs and carry away deoxygenated blood, blood with the oxygen removed.

Their walls are very thin to allow substances to easily and quickly diffuse, or pass through them. Capillaries are much thinner than arteries and veins, because their walls are made up of only a single layer of endothelial cells, the flat cells that line all blood vessels.

Capillaries are selectively permeable, which means they allow some substances through but not others. Their permeability is what allows them to carry out their job, and how permeable they are varies depending on the organ or tissue they are found in.

Types of Capillaries

Capillaries are divided into three main types, according to their pore structure:

  • Fenestrated
  • Continuous
  • Discontinuous

Fenestrated capillaries have numerous pores of various sizes. Small intestine walls have fenestrated capillaries to allow digested food molecules to be carried into the blood.

Continuous capillaries feature tight junctions of the endothelium to make them highly impermeable to anything but the smallest molecules. Brain capillaries are continuous capillaries; their pores are so tiny that only water and small ions can pass through.

Discontinuous capillaries feature wide pores in their cell walls and large spaces between cell layers to allow large molecules to pass through. Discontinuous capillaries are found primarily in the liver, which produces a number of different proteins that need the larger space to pass through into the body.

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