The process by which the cellular elements of the blood are formed. Blood contains many free-floating cells which are moved throughout the body within the blood vessels. The three main types of cells are the red cells (erythrocytes), which serve to carry oxygen, the white cells (leukocytes), which function in the prevention of and recovery from disease, and the thrombocytes, which function in blood clotting. In humans there is only one white cell in the blood for every 700 red cells.