Introduction
The term venule refers to a section of blood vessels in the body’s vascular system which, together with the arterioles and capillaries, form the final flow path of the vascular system. The function of the venule includes the exchange between blood and tissue and the transport of blood as part of the vascular system. It collects and conveys the blood from the venous part of the capillary bed and flows together with other venules, which finally form a vein.
The venules differ from the veins both in their wall structure and in their function. A special type of venules is found in the lymphatic tissue. These enable the lymphatic cells, which are located passagally in the blood, to re-immigrate into the lymphatic tissue. The venules have a particularly permeable wall and thus ensure the exchange of tissue and blood.
Anatomy
The large transport vessels of the body consist in principle of three wall layers, the tunica intima, the tunica media and the tunica adventitia. These in turn contain several sublayers, which are differently pronounced depending on the localization and function of the vessel. The tunica intima consists mainly of so-called endothelium, which is responsible for the exchange of substances.
It also contains connective tissue. In contrast, the tunica media consists of smooth ring-shaped muscles and elastic fibers that serve as a muscle pump of the vessel and are essential for blood transport. The tunica adventitia is the outer layer of the vessel and is composed of loose connective tissue.
This layer stabilizes the vessel in the surrounding tissue and may also contain blood or lymph vessels and nerve tracts. In contrast to the large vessels, the small venules have no or only a very thin tunica media. This layer gives stability to the vessel wall.
Since the main function of the venules is to exchange nutrients with the surrounding tissue, this wall layer is not required. The part of the venule facing the capillary bed therefore contains no tunica media. In the course of the venule, a thin layer of smooth muscle develops.
The blood pressure in the venules is only very low, so the wall layer of tunica intima and adventita is sufficient. The tunica media would merely represent a barrier to the exchange of substances. In addition, in comparison to the veins, the venules do not have venous valves. Venous valves have a valve function in the large body veins and facilitate the return transport of blood to the heart by preventing blood from flowing back.
