lungs

the sac-like, paired organs of RESPIRATION , situated with their base on the DIAPHRAGM and the top projecting into the neck. Each lung consists of fibrous, elastic sacs that are convoluted to provide a large surface area for gaseous exchange. Air enters the body through the windpipe or TRACHEA , which branches into the two BRONCHI , one to each lung. Further branching then occurs into numerous BRONCHIOLES . The bronchioles divide further and then end in alveoli (see ALVEOLUS ), which are tiny sac-like structures where the gaseous exchange occurs. The exchange of oxygen and carbon dioxide occurs between the many blood capillaries (see CAPILLARY ) on one side of the MEMBRANE and the air on the other. The lungs are served by the pulmonary arteries (see ARTERY ) and pulmonary veins. The total lung capacity of an adult male is five to six litres although only about half a litre (500 ml) is exchanged in normal breathing (called the tidal volume).