Final answer:
The amount of external pressure required to stop bleeding is determined by the process of blood clotting. The body initiates a positive feedback loop to clot the blood and stop the bleeding. Substances released by the injured blood vessel walls begin the clotting process and platelets in the blood start to cling to the injured site and release chemicals to attract additional platelets.
Step-by-step explanation:
The amount of external pressure required to stop bleeding is determined by the process of blood clotting. When a wound causes bleeding, the body initiates a positive feedback loop to clot the blood and stop the bleeding. Substances released by the injured blood vessel walls begin the clotting process, and platelets in the blood start to cling to the injured site and release chemicals to attract additional platelets. This positive feedback accelerates the clotting process until the clot is large enough to stop the bleeding.