arteriole

Arterioles are the small arteries just before the capillaries, and they act like adjustable faucets for blood flow. By tightening or relaxing the muscle in their walls, they decide how much blood is allowed into each capillary bed—turning the tap up for a working muscle, down for a resting one.

Although individually tiny, arterioles are the chief site of vascular resistance in the body. A small change in arteriole radius produces a large change in resistance, because resistance varies with the fourth power of radius. This gives the nervous system, hormones and local tissue signals a powerful lever: by adjusting arteriolar tone they control both blood pressure (resistance to outflow from the arteries) and the distribution of flow among organs.

Because they set systemic vascular resistance, arterioles are central to blood pressure regulation and are a major target of many antihypertensive drugs. A practical caveat: arterioles act locally and regionally, so different beds can constrict and dilate at the same time—shunting blood from the gut to muscle during exercise, for instance—without changing total flow much.