The Cardiovascular System - Capillaries

By: Alere Staff

Most people are familiar with the two larger blood vessels of the Cardiovascular System: the arteries and the veins. In the junction where the arterial circulation ends and the venous system begins is another vessel of the Cardiovascular System called capillaries. Frequently referred to as capillary beds, capillaries consist of a network of small tubes needed to perform their job.

Capillaries are the smallest blood vessels in our bodies. So small that a microscope is needed to see them. Many are only one or two cells thick.1 They have the very important job of exchanging the oxygen and nutrients to our organs and tissues.1,2

This exchange is done by a process known as diffusion; oxygen and nutrients pass right through the walls of the capillary to equalize volume of these elements between cells and blood vessels.

Here is a simple example of how diffusion works:  when a perfume is sprayed on one side of a room, a few minutes later, it can be smelled on the other side of the room too.1

Capillaries also pick up waste products and C02 discarded by our tissues  and  transport them for removal from the body. The blood pressure in the capillariy beds is low, usually around 40-20 mm HG. This helps to prevent rupture of these fragile vessels but is still high enough to allow for the exchange of nuturients.2

Capillaries are also specialized depending on the area in the body which they are found. For example, capillaries in the kidneys are different than capillaries in the brain. They are customized to the tissues that they serve.2

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  1. Weber, C., M.D. Part #4: Capillaries. About Health. December 15, 2014. Retrieved from website:
  2. The Cardiovascular System: Blood Vessels. Retrieved from website: