Final answer:
Compressing a pressure point on an extremity can aid in controlling bleeding by initiating vascular spasm and blood clotting.
Step-by-step explanation:
Compressing a pressure point on an extremity can aid in controlling bleeding. When an injury occurs, one of the body's responses is vascular spasm, where the smooth muscle in the vessel wall contracts near the injury point, reducing blood loss. Additionally, when there is excessive blood loss from a penetrating wound, the body initiates a cascade of events that leads to blood clotting and sealing off the damaged area, thus controlling bleeding.