Definition
When we talk about capillaries (hair vessels), we usually mean the blood capillaries, although we must not forget that there are also lymph capillaries. Blood capillaries are one of three types of vessels that can be distinguished in humans. There are the arteries that transport the blood away from the heart and the veins that bring the blood back to the heart.
At the transition between the arterial and venous systems are the capillaries. These are by far the smallest vessels. On average they are about 0.5 mm long and have a diameter of 5 to 10 μm. Since this is sometimes smaller than the red blood cells (erythrocytes), which are on average 7 μm in size, they usually have to deform to fit through the capillaries. Capillaries develop from the smallest arteries, the arterioles, and then form a net-like structure with the help of many branches, which is why it is sometimes called a capillary network, and then collect again to flow into the venules.
Classification
Depending on the classification, two or three forms of capillaries are distinguished. First of all there are the continuous capillaries. This means that the endothelium, the innermost cell layer of the vessels, is closed, which means that only very small molecules can pass through the vessel wall.
This type of capillaries can be found in the skin, skeletal muscles, heart, CNS and lungs, among others. Then there are the fenestrated (windowed) capillaries. These have pores (which are normally about 60 to 80 nm in size) in the endothelium, so that the lumen is only separated from its surroundings by the very thin basement membrane at these points.
Smaller proteins already fit through the pores. This type of capillaries is found in the kidney (where the pores are largest), endocrine glands and the gastrointestinal tract, among others. Finally, some people list sinusoids as an extra group of capillaries.
These are dilated capillaries that have pores not only in the endothelial cell layer but also in the basement membrane. These pores are much larger than those of the fenestrated capillaries, namely up to 40 μm in size, allowing passage of larger proteins and even blood cells. Sinusoids are found in the liver, spleen, lymph nodes, bone marrow and adrenal medulla, among others.
The capillary endothelium is a layer of epithelial cells that line the inside of a blood vessel. Endothelial cells are flat cells and represent the wall of a capillary. They lie on the so-called basement membrane.
Depending on the type of capillary, the endothelium can be continuous, fenestrated or discontinuous and can be passed by molecules of different sizes. In different tissues, one of the three capillary types mentioned above can be found, depending on the task of the capillary. The endothelium has a further task besides the barrier function for mass transfer.
The cells can produce nitrogen monoxide. If nitric oxide is released by the endothelial cells of the blood vessels, this has an expanding effect on the diameter of the vessel. By increasing the diameter, the tissue is better supplied with blood and receives, for example, more oxygen or nutrients. At the same time, the increased blood flow leads to an increased removal of waste products and carbon monoxide.
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