Why is the blood thick in Wet Cupping / Hijama
We had a demonstration of blood coagulation/clotting in a biology lesson with the school nurse. She just took half a test-tube of blood sample from a class pupil and we waited a moment, and the blood became a solid lump inside the test-tube.
The process is there to protect you from leaking blood if you cut yourself, the process uses fibers and platelets and complex proteins.
https://www.youtube.com/results?search_query=blood+clotting
wiki: Coagulation
Blood letting is a very ancient practice that used all sorts of methods like annelids/leeches, which has not been proven to have any benefits. The blood in your body does not seperate into good blood and bad blood by sitting in a jar.