help blood clot following wounds, so that wounds stop bleeding. Most of blood is made of the yellow-colored liquid . blood cells are part of the immune system and fight off infections. Blood must reach every single cell in the body. In order to do this, blood flows through the , which are small blood vessels, often only 1 cell thick. Most carry oxygen-rich blood away from the heart and are coded red in anatomy diagrams. Blue-colored blood vessels indicate low-oxygen blood and are called . The is the main artery out of the heart, taking freshly-oxygenated blood from the left ventricle to deliver to the rest of the body. The right ventricle pumps blood to the lungs through the ; this is typically colored blue even though it carries blood away from the heart. is a waste gas that is carried by blood back to the lungs.