This is the study of "heme" - blood; specifically the cells within our blood. These include white cells, red cells, and platelets. These are also known as formed elements. The RBCs carry oxygen from the lungs to the tissues and carbon dioxide from the tissues back to the lungs where it is exhaled. The oxygen is carried by hemoglobin molecules. The quantity of hemoglobin can be measured spectrophotometrically by manual or automated methods.