The lungs, intra thoracic organs, are part of the respiratory system. The role of both lungs is to carry out the gas exchanges that are vital for the body. Oxygen is necessary for the body's metabolism, and carbon dioxide must be evacuated. These exchanges take place in the alveoli, where the blood is enriched in oxygen and depleted in carbon dioxide. The human being has 2 lungs, a left and a right one, located inside the thorax in the pleural cavities. The right lung is divided into three lobes while the left lung is divided into two lobes because of the space taken up by the heart. Each lobe of the lungs is divided into lung segments. The trachea separates into two bronchi, each heading towards a lung. The bronchi divide successively and many times, forming bronchioles and finally alveolar ducts to the alveoli. The alveoli are made up of pneumocytes and macrophages.