Capillaries them highly impermeable; and discontinuous, with wide

Capillaries are very thin blood vessels that were first discovered in frog lungs in 1661. They bring nutrients and oxygen to tissues and remove waste products. In this lesson, you will learn more about their structure and function.


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.

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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.

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.Discontinuous capillaries in the liver are also called sinusoidal capillaries because of their wide diameter and because they resemble sinus cavities. They are also found in the spleen and lymph nodes, as well as in bone marrow and some endocrine glands. Sinusoidal capillaries found outside the liver may be either fenestrated or continuous capillaries, depending on their wall structure and their function.

Capillary Beds

Humans, like other mammals, have a closed circulatory system. This means that blood stays inside the vessels while being transported around the body.

Capillaries do not end or empty out at tissues; they form capillary beds , net-like structures where substances get exchanged with tissue cells as part of tissue fluid, the fluid between cells. Capillary beds connect arteries and veins so that blood can be taken back to the heart and lungs to be re-oxygenated and pumped around the body again.

Lesson Summary

Capillaries are the thinnest blood vessels in the body. These vessels are found throughout the body and bring oxygen and necessary nutrients to every cell in the body and remove cell waste. Capillary walls are selectively permeable, allowing some, but not all substances through. They are divided into three types: fenestrated, with numerous pores of various sizes; continuous, with tight junctions of the surrounding endothelium to make them highly impermeable; and discontinuous, with wide pores and large spaces between cell layers.

Key Terms

capillary system
  • Capillaries: Capillaries are the thinnest blood vessels found throughout the body.
  • Selectively Permeable: This term refers to capillary walls that allow some substances to pass through them.
  • Closed Circulatory System: In this type of system, blood stays inside the vessel while being transported throughout the body.

Learning Outcomes

View the lesson on capillaries and study its transcript so that you can subsequently:

  • Indicate what capillaries are and where they are in the body
  • Analyze the purpose of capillaries in various parts of the body
  • Recognize the three types of capillary walls
  • Emphasize the role of capillary beds

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