Human blood is responsible for carrying oxygen from the lungs through the body. Oxygen dissolves and binds to red blood cells. The cells get their red color from tiny quantities of iron, which turns red when it combines with oxygen. As such, oxygenated blood is bright red and becomes darker red when oxygen levels are low.
The myth of the nobility having blue blood, or "sangre azul," may have started with the visible veins of Spanish aristocrats with fair complexions. Veins consist of bluish connective tissue, so when they are combined with a layer of skin cells and pigment, the eyes perceive the blood flowing through them as blue.Learn more about Blood