What is the definition or description of: blood loss?

Bleeding: Generally, blood loss is a term most commonly referring to bleeding. Blood loss can occur rapidly, due to trauma, or can occur slowly, as seen in diseases of the stomach or intestinal tract. Blood loss could refer to blood disorders that cause anemias, as seen in autoimmune disorders such as hemolytic anemia.

Blood loss: Is just that -- depletion of HGB due to bleeding. The only caveat is that if the bleeding is very slow then the source can be very difficult to isolate.