Blood is a fluid tissue composed of two parts: liquid and solid. Plasma is the liquid part, and blood cells like red blood cells and white blood cells are the solid part of the blood. Blood cells float in plasma. Plasma is a straw-coloured liquid. Blood cells are made in the bone marrow. Blood carries oxygen to the body tissues. The oxygen carried by the blood is bound to haemoglobin, an iron containing protein. Blood also transports salts and carbon dioxide. It can be said that blood is a vehicle that runs throughout the body. Blood cells are formed in bone marrow, lymph nodes, tonsils and spleen.