Blood is a fluid connective tissue that is composed of plasma and formed elements (blood cells). The fluid portion of blood is the plasma which is composed of mostly water plus blood proteins, ions, and salts. The remaining portion of the blood is composed of the formed elements, the cells of blood. Blood cells include red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes).
Red blood cells (rbc) transport oxygen to tissues of the body. Hemoglobin molecules bind oxygen allowing each red blood cell to bind billions of oxygen molecules. There are over 5 million red blood cells in a drop of blood.