The lungs are a pair of sac-like respiratory organs which occupy the chest cavity. Gas exchange between inhaled air and the blood takes place in these organs. Thus, the function of the lungs is to oxygenated the blood and to expel out or exhale the carbon dioxide contained in the blood pumped into them by the heart right ventricle. The lungs incorporate the bronchial tree, which connects these organs to the trachea. Each lung is divided into lobes by deep clefts called interlobar fissures. The right lung has an upper, middle, and a lower lobe, separated by two fissures. The smaller left lung is divided into an upper and a lower lobe by a fissure which runs obliquely from the top at the back to the base in the front. The lungs have approximately 300 million alveoli, which are tiny, sac-like structures containing capillaries where the gas exchange takes place.