that losing blood cleans the blood is a logical and scientific fact. a portion of the solid matter and colloids are lost, yet the volume and pressure are immediately maintained and/or quickly re-established, mostly with water and the products of plasma. however, the blood will also pick up what is contaminating it in the first place in its travels through the body. so feeling a change may not occur. the total body burden, at least until it is replaced in diet and environmental contact, is lessened.
the biggest benefit to blood loss is increased circulation.