Capillary facts for kids

A capillary is a tiny blood vessel. It's so small that it's thinner than a single strand of hair! Capillaries are special because they have very thin walls, only one cell thick. This helps them move important things around your body.

Capillaries are the smallest blood vessels, about 5 to 10 micrometers wide. They connect arteries (which carry blood away from the heart) to venules (which carry blood back to the heart). Capillaries are where your blood swaps oxygen, water, and nutrients for carbon dioxide and waste products with the tissues of your body.

How Blood Flows Through Capillaries

Blood starts its journey from your heart in large arteries. These arteries branch into smaller ones, and then into even tinier vessels called capillaries. After your body's tissues have used the oxygen and nutrients, the capillaries join together. They become wider, forming small veins (venules), which then become larger veins. These veins carry the blood, now full of waste, back to your heart.

Imagine your body's organs, like your brain or muscles. Each organ has a network of capillaries called a "capillary bed." The more active a part of your body is, the more capillaries it needs. This is because active cells need more nutrients and produce more waste.

Special tiny arteries, called metarterioles, connect directly between arterioles and venules. They can sometimes let blood bypass the main capillary network. This helps control where blood flows. The true capillaries are where the real action happens, allowing things to move between your cells and your bloodstream. Red blood cells are about 8 micrometers wide, so they have to squeeze and fold to pass through capillaries in a single line!

Tiny smooth muscle rings, called precapillary muscles, are found at the start of true capillaries. These muscles act like little gates. They control how much blood flows into each capillary, helping to direct blood flow to different parts of your body as needed.

How Capillaries Work

How capillaries exchange substances with tissues.

The wall of a capillary is made of just one layer of cells. This wall is super thin, which is perfect for its job! Gases like oxygen and other tiny molecules such as water, proteins, and fats can easily pass through it. This movement happens because of pressure differences between the blood and the surrounding tissues.

At the same time, waste products like carbon dioxide and urea move from your body's tissues back into the blood. The blood then carries these wastes away to be removed from your body.

Usually, a capillary bed only uses about 25% of its full capacity for blood flow. But your body can increase this amount! It does this by relaxing the smooth muscles in the arterioles that lead to the capillary bed. This makes the arterioles wider, allowing more blood to flow into the capillaries.

Capillaries themselves don't have smooth muscle in their walls. So, they can't change their own width actively. Instead, they release special signaling molecules. These molecules tell the smooth muscle cells in nearby, larger vessels (like arterioles) to either tighten (constrict) or relax (dilate).

The ability of capillaries to let items pass through, called vascular permeability, can increase. This happens when your body releases certain cytokines. For example, during an immune response (when your body is fighting off germs), capillaries become more permeable to help your body defend itself.

Related pages

• Alveolar-capillary barrier
• Blood brain barrier
• Capillary action
• Hagen-Poiseuille equation

• Histology at BU 00903loa

Images for kids

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  Blood flows from the heart to arteries, which narrow into arterioles, and then narrow further still into capillaries. After the tissue has been perfused, capillaries widen to become venules and then widen more to become veins, which return blood to the heart.

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  Depiction of the three types of capillaries. The fenestrated type in the center shows small pores called fenestrations; the sinusoidal type on the right shows intercellular gaps and an incomplete basement membrane and is also known as a discontinuous capillary.

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  Scanning electron micrograph of a liver sinusoid with fenestrated endothelial cells. Fenestrae are approximately 100 nm in diameter.

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  Simplified image showing blood-flow through the body, passing through capillary networks in its path.

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  Depiction of the filtration and reabsorption present in capillaries.

See also

In Spanish: Capilar sanguíneo para niños