The lungs are the centre of the respiratory system. The lungs consist of two lungs containing all the components of the bronchial tree beyond the primary bronchi and occupy the majority of the space in the thoracic cavity separated by the mediastinum, which contains the heart, the trachea, and the trachea, the oesophagus, the thymus and the lymph nodes. The lungs are pinkish and soft, and spongy because they are mostly air spaces surrounded by alveolar cells and elastic connective tissue. The right lung is made up of three lobes. It is shorter, broader and has a greater volume than the left lung. The left lung has only two lobes to make room for the apex of the heart in an indentation called the cardiac notch, which is located on the medial surface. The left lung is longer and narrower than the right lung. Each lungs attachment is at the hilum or root, on the medial side, where the bronchi, blood vessels, lymphatics and nerves enter the lungs. Each lobe is supplied by one of the secondary bronchi.
A double-layered serous membrane called visceral pleura covers the lungs. It is firmly attached to the surface of the lungs and protects and cushions the lungs. The visceral pleura protrudes between the fissures of the lobes. At the hilum of the lungs, the visceral pleura is continuous with the parietal pleural that lines the wall of the thorax. The thin space between the visceral and the parietal pleural is called the pleural cavity. Pleura produces a serous fluid that acts as a lubricant to reduces the friction between the parietal and visceral pleura and between the individual lobes as the lungs inflate and deflate.