arterioli

Arterioles are small-diameter blood vessels that branch from arteries and feed into capillary networks. They typically measure about 10 to 100 micrometers in diameter and are the primary site of vascular resistance, regulating blood flow to tissues and contributing to overall blood pressure.

Structure and layers: Arterioles have a thin tunica intima lined by endothelial cells, a tunica media composed mainly of smooth muscle cells arranged in concentric layers, and a relatively sparse adventitia. In larger arterioles, an internal elastic lamina may be present; as they narrow, smooth muscle dominates and tight control of tone becomes the dominant function.

Function and regulation: Arterioles respond to neural, hormonal, and local metabolic signals to dilate or constrict, thereby adjusting tissue perfusion. Endothelium-derived factors such as nitric oxide promote vasodilation, while endothelin-1 promotes constriction. Autonomic nerves can induce vasoconstriction via norepinephrine, and local metabolites like carbon dioxide and adenosine promote dilation during increased activity. In the kidney, the afferent and efferent arterioles help regulate glomerular filtration rate through angiotensin II–mediated constriction and other mechanisms.

Clinical significance: Chronic hypertension and diabetes can cause arteriolar remodeling, a process known as arteriolosclerosis, which may be hyaline or hyperplastic. These changes increase vascular resistance and can contribute to end-organ damage. Arteriolar dysfunction is also a feature of various microvascular diseases and can affect tissue perfusion in multiple organs.

Related vessels and concepts: Terminal arterioles connect to metarterioles and capillaries, with some beds featuring precapillary sphincters that regulate flow. The exact arrangement varies among tissues, reflecting local regulatory needs.