Histology, Lung

Excerpt

The lungs are a pair of primary organs of respiration, present in the thoracic cavity beside the mediastinum. They are covered by a thin double-layered serous membrane called the pleura.

The respiratory system consists of two components, the conducting portion, and the respiratory portion. The conducting portion brings the air from outside to the site of the respiration. The respiratory portion helps in the exchange of gases and oxygenation of the blood.

The conducting portion of the respiratory system includes the nose, nasopharynx, larynx, trachea, and a whole series of successive narrowing segments of bronchi and bronchioles. The conducting portion end at the terminal bronchiole. The respiratory portion begins from the respiratory bronchiole and continues with the alveolar ducts, alveolar sacs, and finally ends at the alveoli where the significant exchange of gases takes place. The branching pattern of these conducting passages looks like the branching of a tree and hence called the tracheobronchial tree .

The right lung has three, and the left lung has two lobes. Each is aeriated by a secondary (lobar) bronchus. The lobes are further divided into smaller pyramidal shaped sections called the bronchopulmonary segments. There are ten bronchopulmonary segments in each lung with their apex directed towards the hilum, and each is aeriated by a tertiary (segmental) bronchus .

The alveoli are the structural and functional units of the respiratory system. There are around 300 million alveoli in an adult human amounting to approximates 80 square meters of surface area for the gaseous exchange .

The lungs are an essential component of the pulmonary circulation where the deoxygenated blood pumped by the right ventricle of the heart is gushed through the pulmonary arteries to alveolar-capillary beds of the lung for gaseous exchange. The oxygenated blood from the capillaries of the lungs is returned to the left atrium by the four pulmonary veins.