Hematology is the science or study of blood, blood-forming organs and blood diseases. In the medical field, hematology includes the treatment of blood disorders and malignancies, including types of hemophilia, leukemia, and lymphoma and sickle-cell anemia. Hematology is a branch of internal medicine that deals with the physiology, pathology, etiology, diagnosis, treatment, prognosis and prevention of blood-related disorders. Hematologists focus largely on lymphatic organs and bone marrow and may diagnose blood count irregularities or platelet irregularities. Hematologists treat organs that are fed by blood cells, including the lymph nodes, spleen, thymus and lymphoid tissue. Blood is made up of several parts, including red blood cells, white blood cells, platelets and plasma. Red blood cells, which make up about 45% of whole blood, carry oxygen from the lungs to the body’s tissue. They also carry carbon dioxide back to the lungs to be exhaled. They are disc-shaped, and produced in the bone marrow. White blood cells, which are also made in the marrow, help fight infection. Together with platelets, they make up less than 1% of whole blood. Platelets are small, colorless fragments that stick together and interact with clotting proteins to stop or prevent bleeding. They are also produced in bone marrow. Plasma is the fluid part of the blood. Composed of 92% water, it also contains vital proteins, mineral salts, sugars, fats, hormones and vitamins.