<P> In embryonic development, the lungs begin to develop as an outpouching of the foregut, a tube which goes on to form the upper part of the digestive system . When the lungs are formed the fetus is held in the fluid - filled amniotic sac and so they do not function to breathe . Blood is also diverted from the lungs through the ductus arteriosus . At birth however, air begins to pass through the lungs, and the diversionary duct closes, so that the lungs can begin to respire . The lungs only fully develop in early childhood . </P> <P> The lungs are located in the chest on either side of the heart in the rib cage . They are conical in shape with a narrow rounded apex at the top, and a broad concave base that rests on the convex surface of the diaphragm . The apex of the lung extends into the root of the neck, reaching shortly above the level of the sternal end of the first rib . The lungs stretch from close to the backbone in the rib cage to the front of the chest and downwards from the lower part of the trachea to the diaphragm . The left lung shares space with the heart, and has an indentation in its border called the cardiac notch of the left lung to accommodate this . The front and outer sides of the lungs face the ribs, which make light indentations on their surfaces . The medial surfaces of the lungs face towards the centre of the chest, and lie against the heart, great vessels, and the carina where the trachea divides into the two main bronchi . The cardiac impression is an indentation formed on the surfaces of the lungs where they rest against the heart . </P> <P> Both lungs have a central recession called the hilum at the root of the lung, where the blood vessels and airways pass into the lungs . There are also bronchopulmonary lymph nodes on the hilum . </P> <P> The lungs are surrounded by the pulmonary pleurae . The pleurae are two serous membranes; the outer parietal pleura lines the inner wall of the rib cage and the inner visceral pleura directly lines the surface of the lungs . Between the pleurae is a potential space called the pleural cavity containing a thin layer of lubricating pleural fluid . Each lung is divided into lobes by the infoldings of the pleura as fissures . The fissures are double folds of pleura that section the lungs and help in their expansion . </P>

Why are there three lobes in the right lung