Arterioles are a part of the circulatory system. They connect the arteries with the capillaries as a part of the blood vessel system. Since they connect the larger arteries with the much smaller capillaries, arterioles are responsible for slowing down the flow of the blood. This means that they have muscular walls that can change the diameter of the blood vessel as needed by the body. Chemical messengers called hormones help to regulate the arteriole diameter and ultimately the blood flow. So if there is open bleeding, the body can constrict the arteriole walls via hormones to slow the flow of blood to help stop the bleeding.