The lung is a pair of organs, consisting of the right and left lung (Pulmo dexter, Pulmo sinister). The right lung has three lobes, the left two. The lobes are further divided into segments (10 on the right, 9 on the left). Because of the heart the left lung is slightly smaller than the right one. The basis of both lungs rest on the diaphragm. The lung is covered by the visceral pleura which is a layer of simple squamous epithelium. The visceral pleura changes to the parietal pleura at the inner surface of the chest wall. Both the visceral pleura and parietal pleura play a key role in the respiration. One needs to understand their structures in order to diagnose and treat respiratory diseases (e.g. pneumothorax). The windpipe (trachea) provides ventilation of the lung by bifurcating into a right and left primary bronchus. The lung volume is about 5 to 6 liters. The alveoli mark the end of the bronchial tree. They are surrounded by a filigree network of blood capillaries. These capillaries are tightly close to the alveoli wall and together they form the blood-air barrier. This allows the gas exchange.
The primary bronchi in both lungs branch further and terminate in alveoli. Histologically, there are different types of epithelium cells within the bronchial tree: The inner surface of the trachea and both primary bronchi are lined by pseudostratified columnar epithelium. As the branching continues through the bronchial tree the epithelium flattens more and more. Finally the alveoli are lined by a single layer of pneumocytes. The thin wall between the alveoli (type I pneumocytes) and capillary endothelium cells of the surrounding blood vessels allows the passage of gases (e.g. oxygen, carbon dioxide).
The function of the lung is to supply the organism with oxygen and remove carbonic acid by breathing out carbon dioxide. Therefore the lung as an eliminating organ plays an important role in the acid-base homeostasis (as well as the kidney). The primary task of the lung is the gas exchange in the alveoli. Carbon dioxide produced by cell metabolism is transported in the blood as bicarbonate and released in the alveoli as carbon dioxide. On the other hand oxygen is transferred to the alveoli by inspiration where it diffuses into blood due to the concentration gradient.