An arteriole is the smallest arterial vessel in the human body, which in the further course of time immediately changes into a capillary. Arterioles are connected to the larger arteries and, together with the venules, are the smallest blood vessels that are still visible to the naked eye. The function of the arterioles is mainly to regulate the blood flow through the capillary bed and to control the blood pressure in the entire circulation in this way. In order to fulfil this function, arterioles are equipped with a muscular wall which they can contract or relax as required. Their counterpart in the venous vessels is called a venule, which is connected directly after the capillaries and later becomes the larger vein.
Like arteries, arterioles are characterized by their three-layer wall structure. This consists of the intima on the inside, the media in the middle with smooth muscle cells and the adventitia on the outside. However, these small vessels usually have only one or two layers of muscle cells, which are completely missing in the capillaries.
With the help of these muscle cells, arterioles can change their diameter and thus control the blood flow. Together with the small arteries, they are considered to be the resistance vessels of the human body, since they can interrupt the blood supply in the following tissue through the contraction of the muscle cells and thus raise the blood pressure in the circulation. In their totality they form about 50% of the total resistance. This mechanism can be life-saving, especially in situations of high blood loss, for the adequate blood supply to vital organs such as the brain and heart.
Differences to the Venole
In contrast to the arterioles, venules have only a very small media layer with almost no muscle cells. Only in the further course and after the union of several venules to so-called collecting venules do isolated muscle cells reappear in the wall structure. Venules thus represent the exact opposite of resistance vessels (arterioles) and also have a very permeable wall through which fluid exchange with the surrounding tissue can take place. Certain cells can also pass through the venous wall, for example in the course of infections, which physiologically should not be the case with arterioles.
A shunt is a connection between two usually separate hollow organs/bodies through which the passage of fluid is made possible. This can occur in the context of certain diseases or be artificially created for a medical indication. Examples of this would be, on the one hand, congenital heart defects and, on the other hand, the so-called dialysis shunt, in which a connection is created between the arterial and venous systems. For this purpose, an artery is connected to a vein, bypassing the arterioles, the capillary bed and the subsequent venules, in order to artificially create access to a large vessel through which, for example, haemodialysis can be performed.