Arterioles are the smallest of the visible arteries represented throughout the body’s vascular system. Here, they represent the transition of the arteries to the capillaries. However, they are not only there to connect the arteries with the capillaries, but they also regulate the blood pressure and the flow rate of the blood by their width. The comparable venous blood vessels are therefore also called venules.
What are arterioles?
Arterioles are the finest arteries in the human vascular system that are still visible to the naked eye. On the one hand, they have the task of establishing a connection between the arteries and the capillaries. On the other hand, due to their width (about 40 to 100 µm), they also regulate the speed of the blood and thus also the blood pressure. In emergencies, they can even virtually stop the flow of blood. In the vascular system, they emerge from the surrounding arteries through an invisible transition. The structure of arterioles is fundamentally similar to that of arteries. However, the layers and walls of the finer blood vessels are less strong and less developed. Nevertheless, a distinction is made here between the two types of arteries: the intima, media, and adventitia.
Anatomy and structure
In their structure, the fine arterioles are very much like the more powerful arteries into which they merge. In these, however, the vessel wall consists only of a very thin layer composed almost predominantly of smooth muscle cells. Also present, however, are fine reticular fibers and a non-fenestrated and lumenal endothelial lining that “sheaths” the arterioles. However, the inner membrane lining (membrana elastica interna) lies directly beneath the endothelial layer, in contrast to the arterioles. The outer membrane layer (membrana elastica externa), on the other hand, is completely absent, unlike in the arteries. In terms of structure, the arterioles always flow smoothly into the capillaries in the direction of blood flow. In this process, the muscle cells and the inner membrane layer of the arterioles, rather than those of the arteries, break down.
Function and tasks
The function of the arterioles is to regulate the blood flow to the following capillary vessel. Therefore, the diameter of the arterioles is controlled on the one hand by the nervous sympathetic nervous system and on the other hand by vasoactive hormones. However, even very small hormonal or even external fluctuations cause a change in resistance here that should not be underestimated, which means that the arterioles have a great influence on blood pressure. This is also due to the fact that a strong branching in the individual arterioles increases the frictional resistance of the blood, while it throttles the flow velocity and lowers the blood pressure. Of course, the exact opposite is true for less complex or wide arterioles. For this reason, arterioles are often referred to as resistance vessels. However, the arterioles also act “intelligently,” for example by constricting in the event of a major blood loss and thus reducing the loss. At the same time, the constriction in the periphery leads to a centralization of the blood volume and ensures that vital organs can continue to be supplied with blood as well as possible. Thus, in an emergency, less important organs are unceremoniously cut off from blood supply, or temporarily undersupplied, by constriction of the arterioles.
Diseases and ailments
Like the larger arteries, arterioles are particularly susceptible to vascular blockages and occlusions such as arteriosclerosis. Of course, the small width of the arterioles also plays a role here, increasing the risk. This is because vascular blockages, which often occur with elevated cholesterol levels, are caused by fat cells that are transported or wander around in the arteries and eventually stick to the vessel walls. If the passageway is now too narrow – as can be the case in a transition from the artery to the capillaries – an occlusion can occur even without direct adhesion. Such an occlusion can have numerous serious consequences. It can cause an undersupply of the surrounding organs, strokes or a heart attack. Another type of complaint is vasoconstriction, which can also have serious consequences in the arterioles. In most cases, this is also caused by fatty deposits on the fine walls of the arterioles.Other dangers are blood clots (thrombi), which can also lead to narrowing or occlusion. Corresponding constrictions of the arterioles can also be caused by malfunctions of the body, by diseases or by age. Therefore, it is incredibly important to consult a doctor at the first signs of narrowing or blockage of the blood vessels. Appropriate symptoms could be, for example, cold limbs, but also increased numbness or frequent tingling in certain parts of the body.
Typical and common arterial diseases
- Arterial circulatory disorders in the feet and legs.
- Arterial hypertension
- Arterial occlusive disease
- Peripheral arterial occlusive disease
