What are small, muscular branches of arteries called, which when contracted, raise resistance to blood flow?

The correct choice, arterioles, refers to the small, muscular branches of arteries that play a crucial role in regulating blood flow. These vessels are primarily responsible for controlling the distribution of blood to various tissues and organs throughout the body. When the smooth muscle in the walls of arterioles contracts, the diameter of the vessel decreases. This contraction increases vascular resistance, thereby reducing blood flow to certain areas, which is an essential mechanism for regulating blood pressure and directing blood where it is needed most during different physiological demands.

In contrast, veins are larger vessels that carry blood back to the heart, and they do not typically have the muscular capacity to regulate resistance in the same manner as arterioles. Capillaries are the smallest blood vessels and are primarily involved in the exchange of oxygen, nutrients, and waste products between blood and tissues, without the muscular structure to significantly alter blood flow resistance. Ventricles are chambers of the heart that pump blood out to the lungs and the rest of the body, and are not involved in the regulation of blood flow resistance in the vascular system.