Coagulation

What happens when you prick your finger with a pin or needle while sewing? You have red blood cells and platelets floating around in your bloodstream. When you prick your finger, this puts a hole in one or more small blood vessels and you start to bleed. When the platelets discover the holes, they shoot out fibrin strings. These strings cling to the red blood cells and quickly form a clump of red blood cells, fibrin, and platelets. This clump blocks the hole and seals it shut so no more red blood cells can escape. All, hopefully, before you get blood on your fabric.