What Are Capillaries?
Reviewed: Dr. Gromatzky
3. Structure
As mentioned, capillaries are the smallest of all blood vessels. Indeed, their walls are only two cells thick and they are only around 5 micrometers in diameter. In order for blood cells to pass through, they have to do so in single file.
They may be tiny, but there’s a lot of them with an estimated 40 billion capillaries in each of our bodies. There are so many of them that if they could be removed from the body and laid out straight, they would measure approximately 100,000 miles long. That means that a single person’s capillaries would stretch around our planet approximately 2.5 times.
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