Britannica Sources used in this answer

How do arterioles control the amount of blood reaching tissues?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Illustration of a human heart cross-section showing four chambers-left and right atria and ventricles-with labeled major blood vessels. An inset highlights cardiac muscle tissue, showing myocytes and intercalated disks. — human cardiovascular system The human heart is both a pump and a muscle. Its four chambers move blood efficiently, while cardiac muscle cells joined by intercalated disks synchronize each contraction.

Arterioles, which are small, muscular vessels that terminate the arterial tree, control the amount of blood entering capillary networks primarily through changes in their diameter. This regulation occurs via two main processes:12

Vasodilation: When arterioles widen, resistance decreases, allowing blood flow to increase to specific tissues or organs.1
Vasoconstriction: When arterioles narrow, resistance increases, which reduces blood flow to those areas.1

By adjusting their diameter, arterioles act as a control mechanism to direct blood toward tissues with the greatest metabolic demand, such as skeletal muscles during exercise or the digestive system following a meal. While arterioles are the primary regulators of blood flow into the capillaries, muscular gatekeepers known as sphincters within the capillary units further assist by directing blood to the specific areas in greatest need.13