Arterioles (Latin: Arteriolae , Singular Arteriola ) are small arteries that lie behind the arteries and in front of the capillaries in the bloodstream . Their venous equivalents are called venules . Arterioles and venules are the finest blood vessels that can still be seen with the naked eye . You can recognize them, for example, on the sclera of the eye as fine vascular markings ( episcleral vessels ).
Layout and function
The wall structure of arterioles is similar to that of arteries, a distinction is made between intima , media and adventitia .
The media is only weak, but the smooth muscles in this middle layer are able to completely close the arteriole, which is not the case with larger arteries. This closure ensures that the adjoining capillary bed is not traversed by blood and that less stressed tissues and organs are not supplied with blood.
In the arterioles, the frictional resistance of the blood increases sharply, while the flow rate and blood pressure decrease. Functionally, arterioles are therefore called resistance vessels . Even small changes in the lumen width of arterioles in a body region have a major impact on blood pressure .
Arterioles have an inside diameter of less than 100 micrometers . The exact limit from which one speaks of an arteriole varies from author to author. Some authors define arterioles as those vessels that, in contrast to the upstream arteries, only have one continuous layer of smooth muscle cells, while others also include vessels lying in front with two to three layers of smooth muscle cells as arterioles.
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