arteriola

An arteriola, plural arteriolas (arteriolae in some languages), is a small-diameter blood vessel that branches from a larger artery and delivers blood to capillary beds. In humans, arteriolas have diameters roughly 10 to 100 micrometers and are a key part of the microcirculation. They act as resistance vessels that regulate systemic and local blood flow, thereby influencing mean arterial pressure and tissue perfusion.

An arteriola's wall consists of an endothelial lining surrounded by a tunica media of smooth muscle cells, and a relatively thin adventitia. The smooth muscle layer can constrict or relax, changing the vessel radius in a process called vasomotion. In many tissues, pre-capillary sphincters at the arteriolar level further modulate entry of blood into capillary networks. Blood from arteries passes sequentially into arterioles before reaching capillaries.

Regulation of arteriolar tone is complex and involves local metabolic signals (such as hypoxia, hypercapnia, hydrogen ions), nitric oxide, endothelin, and autonomic nervous system input. Autoregulation allows certain organs (notably the kidneys and brain) to maintain stable blood flow despite fluctuations in systemic pressure. Arteriolar function is central to controlling peripheral resistance and distributing cardiac output.

Clinical relevance: Pathological thickening of arteriolar walls is called arteriolosclerosis, including hyaline arteriolosclerosis seen in diabetes and hypertension and hyperplastic arteriolosclerosis associated with malignant hypertension. Changes in arterioles can contribute to conditions such as hypertensive retinopathy and reduced renal filtration. Understanding arterioles is fundamental to cardiovascular physiology and pathology.