The yellow fluid that remains after blood clots is called serum. The molecule contains serotonin. Serotonin is released into the serum as the molecule breaks down. It is like plasma. It contains serotonin which causes constriction of blood vessels. The branch of biology that deals with serum is called Serology. Serum does not contain fibrinogen, prothrombin, labile and antihemophilic factors.