Microcirculation deals with the flow of blood from arterioles to capillaries or sinusoids to venules. Blood flows freely between an arteriole and a venule through a vessel channel called a thoroughfare channel. Capillaries extend from this channel and structures called precapillary sphincters control the flow of blood between the arteriole and capillaries.

The precapillary sphincters contain muscle fibers that allow them to contract. When the sphincters are open, blood flows freely to the capillary beds where gases and waste can be exchanged with body tissue. When the sphincters are closed, blood is not allowed to flow through the capillary beds and must flow directly from the arteriole to the venule through the thoroughfare channel.

It is important to note that blood is supplied to all parts of the body at all times but all capillary beds do not contain blood at all times. Blood is diverted to the parts of the body that need it most at a particular time. For instance when you eat a meal blood is diverted from other parts of your body to the digestive tract.

Vessel Sizes

Vessel Diameter in Microns
Arterioles 20-50
Capillaries 5-10
Sinusoids 30-40
Venules 30-40