Microcirculation deals with the flow of blood from arterioles to cappillaries 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
What are capillaries?
Capillaries are extremely small vessels located within the tissues of the body that transport blood from the arteries to the veins. Capillary walls are thin and are composed of endothelium (a single layer of overlapping flat cells). Oxygen, carbon dioxide, nutrients and wastes are exchanged through the thin walls of the capillaries.
The flow of blood is controlled by structures called precapillary sphincters. These structures are located between arterioles and capillaries and contain muscle fibers that allow them to contract. When the sphincters are open, blood flows freely to the capillary beds of body tissue. When the sphincters are closed, blood is not allowed to flow through the capillary beds.
Capillary Size
Capillaries are so small that red blood cells can only travel through them in single file.
* 5-10 microns in diameter.
Sinusoids
What are sinusoids?
The liver, spleen and bone marrow contain vessel structures called sinusoids instead of capillaries. Similar to capillaries sinusoids are composed of endothelium. The individual endothelial cells however do not overlap as in capillaries and are spread out. Oxygen, carbon dioxide, nutrients, proteins and wastes are exchanged through the thin walls of the sinusoids.
Sinusoid Size
* 30-40 microns in diameter.
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:
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
What are capillaries?
Capillaries are extremely small vessels located within the tissues of the body that transport blood from the arteries to the veins. Capillary walls are thin and are composed of endothelium (a single layer of overlapping flat cells). Oxygen, carbon dioxide, nutrients and wastes are exchanged through the thin walls of the capillaries.
The flow of blood is controlled by structures called precapillary sphincters. These structures are located between arterioles and capillaries and contain muscle fibers that allow them to contract. When the sphincters are open, blood flows freely to the capillary beds of body tissue. When the sphincters are closed, blood is not allowed to flow through the capillary beds.
Capillary Size
Capillaries are so small that red blood cells can only travel through them in single file.
* 5-10 microns in diameter.
Sinusoids
What are sinusoids?
The liver, spleen and bone marrow contain vessel structures called sinusoids instead of capillaries. Similar to capillaries sinusoids are composed of endothelium. The individual endothelial cells however do not overlap as in capillaries and are spread out. Oxygen, carbon dioxide, nutrients, proteins and wastes are exchanged through the thin walls of the sinusoids.
Sinusoid Size
* 30-40 microns in diameter.
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 |