Where does blood come from?
You probably already know how important blood is. It keeps oxygen traveling through your body. Blood is made of white cells, red cells, and other nutrients that your body needs. Blood is liquid because the cells are carried in plasma, which is made of almost all water (about 90% of plasma is water).
The red and white cells are produced in your bones (in the marrow), the liver, spleen, and parts of your body called the thymus gland and lymph nodes.