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How do arterioles control the amount of blood entering capillary networks?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

Arterioles act as control valves that regulate the amount of blood entering capillary networks by adjusting their diameter. Their walls contain a layer of smooth muscle that allows them to constrict or dilate.12

Vasoconstriction: When the smooth muscle contracts, the arteriole narrows, which decreases blood flow to the capillaries.1
Vasodilation: When the muscle relaxes, the arteriole widens, allowing blood flow to increase.1

This mechanism enables the body to direct blood to tissues and organs that require it most, such as skeletal muscles during exercise or the digestive system after a meal. Additionally, muscular gatekeepers known as sphincters located within the capillary unit itself help further direct blood flow to specific areas in greatest need.123