The term venule designates a vessel section in the blood vessel system of the body, which together with the arterioles and capillaries forms the end stream path of the vascular system.
The task of the venule involves the exchange between blood and tissue as well as the transport of the blood as part of the vascular system. It collects and directs the blood from the venous portion of the capillary bed and flows together with other venules, which eventually form a vein.
The venule differs both in its wall construction and in its function from the veins.
A special form of venules is located in the lymphatic tissue. These allow the lymphatic cells, which are passing in the blood, to migrate back into the lymphatic tissue. The venules have as a particularly permeable wall and thus ensure the exchange of tissue and blood.
The main transport vessels of the body consist of three wall layers, the tunica intima, the tunica media and the tunica adventitia. These in turn contain some sublayers, which vary in severity depending on the localization and function of the vessel.
The tunica intima consists mainly of so-called endothelium, which is responsible for the mass transfer. It also contains connective tissue.
In contrast, the tunica media consists of smooth, circular musculature and elastic fibers, which 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 additionally contain blood or lymphatic vessels as well as nerve tracts.
In contrast to the large vessels, the small venules have no or only a very thin tunica media. This layer gives the vessel wall stability. Since the main function of the venules is to exchange nutrients with the surrounding tissue, this wall layer is not needed. The part of the venole, which faces 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 very low, so that the wall layer of Tunica Intima and adventita is sufficient. The tunica media would merely be a barrier to mass transfer. In addition, the venules have no venous valves compared to the veins. Venous valves have a valve function in the large veins of the body and facilitate the return of the blood to the heart, by preventing a return of the blood.
An arteriole is also a component of the end stream of the vascular system and resembles an artery in its wall structure.
The arteries generally have a larger and more compact muscle layer than the veins. The arterioles form the resistance vessels in the systemic circulation and therefore have a much thicker muscle layer than the venules. The wall layer of smooth muscle cells serves to regulate the blood pressure in the following capillaries. The upstream arteries conduct the blood with a high blood pressure. This is strongly throttled in the area of the arterioles and individually adapted for each subsequent organ.
For example, if you have a lot of blood loss, you can throttle the blood flow to the end stream, causing centralization of the remaining blood. This function is essential for the maintenance of cardiovascular stabilization.
A shunt is an arteriovenous anastomosis, ie a direct transition from an arteriole to a venole, without an interposed capillary bed. By means of these shunt connections, the blood flow of the adjacent capillary beds can be regulated. Thus, some capillary districts can be completely switched off when needed. The circulation is also regulated by the wall musculature of the arterioles.
Shock may lead to a loss of capillary blood flow regulation. As a result, there is an increased amount of blood in the capillaries and the central vessels and the heart lacks this, which can lead to circulatory failure.