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Respiratory system. Historical illustration of the lungs and the blood supply for respiration. At top a cross-section looks down through the body (front at top). Deoxygenated blood from the body reaches the heart (brown, upper centre) and is pumped to the lungs (orange, upper left and right) through the pulmonary arteries (blue). The blood passes through fine capillaries surrounding small sacs (alveoli, lower left), where gaseous exchange of oxygen for carbon dioxide oxygenates the blood (red). The diagram at lower right shows the nerves (white) of the respiratory system. Colour lithograph by Fairland from The Viscera of the Human Body, 1840. Based on drawings by Bagg. (KEYSTONE/SCIENCE PHOTO LIBRARY/SHEILA TERRY)