Capillaries are small blood vessels that help connect veins and arteries while also facilitating the exchange of different vital elements between tissues and blood. They represent core connections in the arterial system, which includes the blood vessels that carry blood in the direction away from the heart and toward the venous system. Therefore, active tissues, like the liver, kidneys, or muscles, have a large number of capillaries. Less metabolically active tissues, such as some types of connective tissues, do not have as many of them.
In the capillaries, the exchange occurs between nutrients, oxygens, and waste between blood and tissues. It happens with the help of two processes, such as passive diffusion and pinocytosis. Passive diffusion refers to the movement of a substance from a higher concentration area to that of a lower concentration (Eldridge, 2021). Pinocytosis is a process through which body cells take in small molecules actively, such as proteins and fats. It is also important to note that capillaries’ walls are composed of a thin layer of cells, endothelium, which is surrounded by another layer, the basement membrane (Eldridge, 2021). The composition of capillaries in the form of single-layered endothelium can range in different types of capillaries, while the surrounding membrane makes it more prone to ‘leak’ blood than other types of blood vessels.
There are three main types of capillaries, namely, continuous, fenestrated, and discontinuous, which can be found in different parts of the body, and specialized capillaries present in the brain and make up the blood-brain barrier (Eldridge, 2021). Continuous capillaries are not perforated, and such a structure allows only tiny molecules to pass through them. This type of capillaries is present in such body parts as muscles, skin, fat, and nerve tissues (Seladi-Schulman, 2019). The small gaps in the capillaries’ endothelial cells allow for the passing of gases, water, glucose, and some hormones. In the brain, continuous capillaries make up an exception because they are a part of the blood-brain barrier, facilitating the protection of the organ by allowing only the most essential nutrients to cross (Seladi-Schulman, 2019). This also explains why the continuous capillaries in the brain do not have gaps between endothelial cells while their basement membrane is more robust.
Fenestrated capillaries have small perforations that allow for the small molecules to get inside them to be transferred. This type of capillaries is present in the kidneys, intestines, and endocrine glands. Fenestrated capillaries’ function is concerned with the ongoing exchange of molecules between blood and tissue (Seladi-Schulman, 2019). For example, in the kidneys, the capillaries are essential for helping waste products get filtered out of the blood. In the small intestine, the capillaries allow nutrients to get absorbed from food. Finally, discontinuous or sinusoidal capillaries are different in structure from the other two types as they have large open pores, which size allows blood cells to pass through. This type of capillaries is present in the spleen, the bone marrow, and lymph nodes and is the ‘leakiest’ among all of them. For instance, in the bone marrow, the porous type of capillaries enable newly-produced blood cells to get into the bloodstream and start circulating within organs. Therefore, the type of capillaries has a direct influence on their functioning, and it is vital that they function correctly to facilitate the ongoing and efficient exchange of substances between tissues and the bloodstream.
References
Eldridge, L. (2021). Capillary structure and function in the body.
Seladi-Schulman, J. (2019). Capillaries and their functions.