Blood is a red fluid which circulates in the body of humans and other vertebrates. It carries oxygen and nutrients to cells. It also carries carbon dioxide and impurities away from cells. Heart pumps blood throughout the body in specialised vessels called arteries and veins. Blood is mainly composed of red blood cells (RBCs), white blood cells (WBCs) and platelets. RBC contains haemoglobin, an iron containing protein. The characteristic red colour of blood is because of the presence of haemoglobin.