The pleura, lobes and microstructure of the lungs

Abstract

The pleura is a double-walled serous lined sac. The outer layer clothes the chest wall and diaphragm, the inner (visceral) layer adheres to the lung: the two meet at the hilum. Between these two layers is the potential space of the pleural cavity, moistened by a film of serous fluid. The mediastinum is the space between the two pleural sacs. The right lung is larger than the left and comprises the upper, middle and lower lobes. The left lung has an upper and lower lobe separated by the oblique fissure. On the right an additional transverse fissure separates the upper and middle lobes. The hilum of the lung contains, within its pleural sheath, the main bronchus, pulmonary artery and vein, bronchial vessels, hilar lymph nodes and lymphatics, and autonomic sympathetic and parasympathetic (vagal) fibres. The epithelium comprises columnar ciliated cells and mucus secreting goblet cells down to the finer bronchi, while the alveoli are free from goblet cells and the epithelium comprises a thin membrane. The alveoli also contain large vacuolated cells that produce the phospholipid component of surfactant. Below the mucosa, the bronchial wall is made up of a basement membrane, submucous elastic tissue, non-striated muscle, and an outer fibrous coat containing cartilage. At each bifurcation there is a saddle-shaped piece of cartilage, which reinforces the two branches at their division.