Capillaries are the smallest blood vessels, visible only under a microscope. They link the arterioles to the venules (see circulation).
Capillaries were discovered by Marcello Malphigi in 1661.
There are 10 billion capillaries in your body.
The largest capillary is just 0.2 mm wide — thinner than a hair.
Each capillary is about 0.5 mm to 1 mm long.
Capillary walls are just one cell thick, so it is easy for chemicals to pass through them. It is through the capillary walls that your blood passes oxygen, food and waste to and from each one of your body cells.
There are many more capillaries in active tissues such as muscles, liver and kidneys than there are in tendons and ligaments.
Capillaries carry less or more blood according to need. They carry more to let more blood reach the surface when you are warm. They carry less to keep blood away from the surface and save heat when you are cold.