The lungs make up the terminal portion of the respiratory apparatus and the largest portion of respiratory tissue. They are a pair of organs situated within the thoracic cavity and they are supplied with inhaled air that comes from the upper segments of the tract including the nasal cavity, the pharynx, the larynx, the trachea, the bronchi and the bronchioles. Their main function is breathing so that oxygen and carbon dioxide can be exchanged, with oxygen being inhaled and sent to the tissues and carbon dioxide being released via exhalation.
The lungs are a pair of organs, one being the right and the other being the left lung (Pulmo dexter, Pulmo sinister). The right lung has three lobes, while the left one has two. The lobes are further divided into segments (10 on the right, 9 on the left). Due to the position of the heart inside the thoracic cavity, the left lung is slightly smaller compared to the right one. The base of both lungs rest on the diaphragm.
The lungs are 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 respiration.
One needs to understand these structures in order to diagnose and treat respiratory diseases (e.g. pneumothorax). The windpipe (trachea) provides ventilation to the lungs by bifurcating into a right and left primary bronchi. 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, which allows gas exchange.
The primary bronchi in both lungs branch further and terminate in alveoli. Histologically, there are different types of epithelial 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 it out. Therefore the lung, as an eliminating organ, plays an important role in the acid-base homeostasis (together with 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 the blood due to a concentration gradient.
Out of the two lungs, the right one has three lobes, while the left one has two. The lobes are further divided into segments. The lungs receive air from the trachea, which bifurcates into the primary bronchi. These futher divide into small bronchi and bronchioles, finally ending up in the alveoli, where gas exchange takes place.
The trachea and primary bronchi are lined by pseudostratified columnar epithelium. In turn, the alveoli are lined by a single layer of pneumocytes.
The lungs have mostly two important functions: the main one, which is gas exchange, and a second one, which is the regulation of acid-base homeostasis, together with the kidneys.