Blood is a liquid substance that circulates continuously throughout the body. Its most important function is the transport of oxygen (bound to hemoglobin in red blood cells) from lungs to body tissues. Blood also moves nutrients and hormones throughout the body, and carries away waste materials such as carbon dioxide. Blood also contains white blood cells (called leukocytes) which produce antibodies to combat disease. Another important component of blood is platelets, which repair tissue injuries and prevent bleeding.