arterioles

Arterioles are small-diameter blood vessels that deliver blood from arteries to capillary networks. They are a key part of the microcirculation and contribute most of the peripheral resistance, helping to determine mean arterial pressure and regulate blood flow to individual tissues. In humans, arterioles typically range from about 10 to 100 micrometers in diameter.

Structure and organization: Arterioles have a thin tunica intima with an endothelial lining, a thick tunica media composed mainly of smooth muscle cells, and a relatively sparse adventitia. As vessels diminish in size toward the capillary bed, smooth muscle density increases and elastic tissue decreases. Some arterioles give rise to metarterioles and precapillary sphincters that can regulate flow into capillaries.

Function and regulation: The primary role of arterioles is to adjust tissue perfusion and systemic vascular resistance. They respond to autonomic nerve activity, circulating hormones such as norepinephrine, angiotensin II, and vasopressin, and local metabolic signals. Local factors released by the endothelium and surrounding tissue, including nitric oxide, adenosine, carbon dioxide, hydrogen ions, and potassium, influence dilation or constriction to match metabolic needs. Autoregulation maintains stable capillary perfusion across a range of arterial pressures through myogenic responses of the vascular smooth muscle.

Clinical relevance: Chronic hypertension can induce arteriolar remodeling, including hyaline arteriolosclerosis, increasing vascular resistance and impairing tissue perfusion. Arteriolar damage is also a feature of diabetic microangiopathy and other vascular diseases, contributing to reduced tissue oxygenation and organ dysfunction.

Overall, arterioles serve as the principal regulators of blood flow distribution and vascular resistance, integrating neural, hormonal, and local signals to meet tissue demands.