The capillaries are the smallest blood vessels in the human body. These blood vessels are only 5 to 10 micrometers in diameter and their walls consist of only one layer of endothelial cells. This thin wall is necessary because the capillaries are the site of exchange of many substances between the bloodstream and the surrounding interstitial fluid. Substances that cross the capillary walls are:
- water;
- oxygen;
- carbon dioxide;
- uric acid;
- lactic acid;
- creatinine;
The capillaries form the connection between arteries and veins. They are the blood vessels that deliver the oxygen and nutrients to the cells themselves and receive the waste that is produced by the cell’s metabolism.