Most animals, except some lower animals (especially insects), have red blood. Blood is usually composed of plasma and blood cells. These blood cells are again of three types—red blood cells, white blood cells and platelets. Blood is red mainly because of these red blood cells.
Red blood cells contain a protein called hemoglobin. Its color is bright red. Because iron is associated with hemoglobin. Hemoglobin mainly exchanges oxygen or carbon dioxide between the lungs and every cell in the body. Because the amount of this hemoglobin in the blood is the highest, the blood is red in color.
Normally, pure blood circulates in the arteries and contaminated blood circulates in the veins. This is why arterial blood looks more red. Venous blood is slightly bluish red.
1512.973 SRY