Human blood consists of about 22 percent solids and 78 percent water. Blood cells are made in the bone marrow. The bone marrow is the spongy material in the center of the bones that produces about 95 percent of the body's blood cells. There are other organs and systems in our bodies that help regulate blood cells. The production and development of new cells is a process called hematopoiesis.
Blood cells formed in the bone marrow start out as a stem cell.
As the stem cell matures, several distinct cells evolve such as the red blood cells, white blood cells, and platelets. Immature blood cells are also called blasts. Some blasts stay in the marrow to mature and others travel to other parts of the body to develop into mature, functioning blood cells. The primary function of red blood cells, or erythrocytes, is to carry oxygen and carbon dioxide. The primary function of white blood cells, or leukocytes, is to fight infection.