I agree with cloud. If the heparin stayed in your body you'd never clot after needles were removed (for those on HD with fistula or graft). The whole purpose is to prevent clots in the lines, so they only need it there for during the treatment. During my hemo treatments of 5 hours they typically add heparin for the first 4 hours, and stop. By the end of treatment I clot like 2 minutes after the needle is taken out.