Bloo substitutes, often called artificial blood, are used to fill fluid volume and/or carry oxygen and other blood gases in the cardiovascular system. Although commonly used, the term is not accurate since human blood performs many important functions. Red blood cells transport oxygen, white blood cells defend against disease, platelets promote clotting, and plasma proteins perform various functions. The preferred and more accurate are volume expanders for inert products, and oxygen therapeutics for oxygen-carrying products. Examples of these two "blood substitute" categories: