Capillaries are tiny blood vessels that transport the blood from arteries to veins, irrigating all tissues of the human body. The network of capillaries is a lot denser in tissues and organs that play a greater biological role. There are for example many of them in internal organs and muscles, while connective tissues have lower needs and the network is smaller.

Blood follows a specific path inside the human body. It all starts in the heart, which acts like a pump. The blood then flows through the main arteries into smaller ones named arterioles, which eventually branch into small capillaries. The actual exchange of nutrients with cellular waste happens at this level. Capillaries later join into venules and eventually major veins, which take the blood back to the heart.

No capillary can work alone, they are connected in a vast network named the capillary bed, which takes the blood to every tissue and organ. Tissues that are more important in the metabolism require a larger amount of nutrients in order to function, so the network of capillaries must be larger in order to provide them and take away the waste. Usually, capillary beds consist of the so-called true capillaries, which start from the small arterioles and transport blood to the cells. However, in the mesenteric circulation there are also metarterioles, short vessels that provide a link between arterioles and venules.

For a long time, scientists believed that metarterioles were located in all capillary beds. Today, it is known they are only present in the mesenteric microcirculation. Also, it was erroneously considered that precapillary resistance was caused by precapillary sphincters not in the mesentery organ, but this is no longer the case.

Besides blood capillaries, the human body also has a network of lymphatic ones. These have a larger diameter and both of their ends are closed. By contrast, blood capillaries have open ends and communicate with arterioles and venules. The purpose of the design of lymphatic capillaries is to allow interstitial liquid to flow in but not to come out. They also have a higher internal pressure, because the lymphatic fluid is richer in plasma proteins than the blood.

Capillaries have an extremely small diameter, so tiny that a single red blood cell can pass at one time. Their diameter is not bigger than 5 or 10 microns. Their walls are also very fragile and are built of a simple type of squamous epithelial material named endothelium. This tissue is porous and allows the exchange of nutrients like oxygen, carbon dioxide or sugar with cellular waste.

The main role of capillaries in the biology of the body is microcirculation. This means the cycle of blood flow that starts and ends in the heart, going through arteries, arterioles, capillaries, venules and larger veins.

Precapillary sphincters are the control structures that are tasked with regulating the actual blood flow through the capillaries. They are normally placed at the connection of arterioles and capillaries and have a small muscle that allows them to open and close. Open sphincters allow red blood cells to flow into the capillaries, while closed ones prevent it for various reasons. The network known as the capillary bed is the place where the actual contact with the body tissues takes place.

Types of capillaries

Capillaries can be classified in three main types:

  • Continuous capillaries - These are located in the skin, fat, nervous and muscular tissues. Since their walls have no openings, only the smallest molecules can permeate them.
  • Fenestrated capillaries - Unlike the first type, these have small pores that allow larger compounds to pass through their walls. Can be found in the endocrine glands, intestines, as well as kidneys.
  • Sinusoidal or discontinuous capillaries - The third type of capillaries have sizeable pores, and even normal red blood cells can travel through them. These have the highest permeability of them all and are located in special locations like the bone marrow, spleen or lymphatic nodes.

A special type of capillaries can be found in the brain and is called the brain-blood barrier. The purpose of this structure is to prevent toxins in the blood to reach the central nervous system. However, the barrier also stops chemotherapy agents from fighting brain tumours.

Function of capillaries

Capillaries have a transport role and carry nutrients and gasses from organs to cells, exchanging them with waste and toxins that are then eliminated.

A very dense network of capillaries is located in the lungs. The oxygen processed by the lungs binds to the hemoglobin in the blood and is then transported to all parts of the body. Capillaries then return carbon dioxide back into the lungs, which release it in the air that we exhale back into the atmosphere.

Nutrients go through a similar cycle. All of the required nutrients and liquids are transported by the capillaries to every cell in the body. The waste generated by our metabolism is then taken to specialized organs like the liver and the kidneys, which have the ability to eliminate toxins.