The science concerned with the study of blood and blood-producing organs is called Hematology. Blood is a fluid in the body the is required to deliver necessary substances such as nutrients and oxygen to the tissues and also transport metabolic wastes products away from the cells.Blood cells originate from specific blood-forming organs, notably the marrow of certain bones in the body. The bone marrow produces all of the red blood cells, 60 to 70 percent of the white blood cells and all of the platelets.