Microcirculation deals with the circulation of blood from the heart to arterioles (small arteries), to capillaries, to venules (small veins) and back to the heart. The liver, spleen and bone marrow contain vessel structures called sinusoids instead of capillaries. In these structures, blood flows from arterioles to sinusoids to venules. A vessel called a thoroughfare channel allows blood to flow freely between an arteriole and a venule. Capillaries extend from this channel and structures called precapillary sphincters control the flow of blood between the arteriole and capillaries. Fluid exchange between the capillaries and the body tissues takes place at the capillary bed.

The precapillary sphincters contain muscle fibers that allow them to contract. When the sphincters are open, blood flows freely to the capillary bed where fluids, gasses, nutrients, and wastes are exchanged between the blood and body cells. When the sphincters are closed, blood is not allowed to flow through the capillary bed 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 to aid in digestion and nutrient absorption.