SEM of normal circulating human blood. One can see red blood cells, several white blood cells (including lymphocytes, a monocyte, a neutrophil, and many small disc-shaped platelets. Blood circulates through the heart, arteries, capillaries, and veins. It is composed of red blood cells, white blood cells, and platelets suspended in plasma, which is a straw-colored fluid composed of proteins, water, and salts. Erythrocytes are made in the bone marrow. Mature red blood cells are non-nucleated. In place of a nucleus, they contain hemoglobin. The iron in hemoglobin is an important protein which allows the cell to carry oxygen to other parts of the body. Erythrocytes also carry away carbon dioxide from the body cells to the lungs where it is expelled through the process of breathing. The infection-fighting leukocytes are classified in two main groups: granular and agranular. Granulocytes are formed in bone marrow, and include eosinophils, basophils, and neutrophils. Agranulocytes are produced by lymph nodes, and the spleen. There are two types of agranulocytes: lymphocytes, which fight disease by producing antibodies which destroy foreign material, and monocytes. Thrombocytes are tiny cells formed in the bone marrow, and necessary for blood clotting.