Structure of capillaries | Capillary

Structure of capillaries

The structure of a capillary resembles a tube. The diameter of a capillary is about five to ten micrometers. Since the red blood cells (erythrocytes) that flow through the capillaries have a diameter of about seven micrometers, they must deform somewhat when they flow through the small blood vessels.

This minimizes the distance over which the exchange of substances between blood cells and tissue takes place. Since a constant exchange of substances between blood and tissue takes place via the wall of the capillaries, the wall must be as thin as possible (0.5 micrometers). The thickness of the wall of larger vessels, such as arteries or veins, through which no exchange of substances has to take place, is much greater.

Arteries and veins consist of three wall layers. The wall of capillaries, on the other hand, consists of only one layer. This layer is made up of so-called endothelial cells.

In addition, the so-called basement membrane reinforces the wall from the outside. The basement membrane is located everywhere in the body where epithelial cells are separated from the connective tissue. Furthermore, so-called pericytes participate in the construction of the capillary wall.

These are branched cells whose function is currently still disputed.There are three different types of capillaries, the continuous, the fenestrated and the discontinuous capillaries. Depending on the task of the individual capillaries, their structure can vary. The continuous capillaries are mainly found in the heart, lungs, skin, brain and muscles.

As the name suggests, they consist of a continuous layer of endothelial cells. These are strung together without gaps and lie completely on the basement membrane. This closed layer allows only very small molecules and gases to be exchanged through the wall.

The fenestrated capillaries have small gaps between the endothelial cells, which are about 60 to 80 nanometers in size, and lie only on a thin basement membrane. This type of capillaries is found in the gastrointestinal tract, in the kidneys and in hormone-producing glands. Through the pores present, larger molecules can be exchanged between the blood vessel and the tissue.

The third type of capillaries is characterized by gaps (up to 100 nanometers) in the wall, which affect not only the endothelial layer but also the basement membrane. These discontinuous capillaries are also called “sinusoids”. Much larger substances, such as proteins or blood components, can pass through these pores into the tissue. They are found in the liver, spleen, bone marrow and lymph nodes.