What makes a wound stop bleeding?

A wound stops bleeding due to the process of clot formation called coagulation. Coagulation is from the Latin coagulatus meaning "to cause to curdle".

Blood contains an enzyme called Protease 34 kD or thrombin for short. Thrombin is made in the liver. The only time thrombin seems to become active in the body is when there is an open wound.

Blood is circulating tissue composed of a fluid portion (plasma) with suspended formed elements (platelets, red blood cells, and white blood cells). Arterial blood is the means by which oxygen and nutrients are transported to body tissues. Venus blood is the means by which carbon dioxide and metabolic by-products are transported from body tissues to exit the body.

In blood plasma there is a specific protein called fibrinogen. In an open wound the enzyme thrombin unites with the fibrinogen to form needlelike crystals called fibrin. This union forms a biochemical alliance that catches blood cells called corpuscles as they try to flood out of the body through the wound. Corpuscle is from the Latin "corpusculum" meaning "any small particle or body".

All this chemical action creates a plug called a blood clot. After a time, moisture is squeezed out of the clot and it contracts. This process is called syneresis. It's not widely known but the same chemical process of syneresis also happens in the formation of jams and jellies.