Blood functions to deliver oxygen and nutrients to cells while removing waste. It is composed of plasma, red blood cells, white blood cells, and platelets. Plasma is 92% water and carries nutrients, gases, and waste. Red blood cells contain hemoglobin and carry oxygen to tissues. White blood cells help fight infection by ingesting pathogens and producing antibodies. Platelets aid in blood clotting when tissues are damaged. Certain diseases like leukemia and hemophilia involve abnormalities of blood components.