Blood is composed of plasma, red blood cells, white blood cells, and platelets. Plasma is the liquid part of blood and contains water and many substances. Red blood cells contain hemoglobin, which carries oxygen from the lungs to tissues and returns to the lungs as waste. White blood cells help fight bacteria and viruses using enzymes, antibodies, and by engulfing pathogens. Platelets help form blood clots to seal wounds in damaged blood vessels.