The pleura is a layer of connective tissue covered by a simple squamous epithelium. The visceral pleura covers the surface of the lung, lines the interlobar fissures, and is continuous at the hilum with the parietal pleura, which lines the inside of the hemithorax. At the hilum the visceral pleura continues alongside the branching bronchial tree for some distance before reflecting back to join the parietal pleura. The pleurae are in apposition apart from a small quantity of lubricating fluid, so the pleural cavity is only a potential space.
The diaphragm is lined by parietal pleura and peritoneum. Its muscle fibres arise from the lower ribs and insert into the central tendon. Motor and sensory nerve fibres go separately to each half of the diaphragm via the phrenic nerves. Fifty per cent of the muscle fibres are of the slow-twitch type with a low glycolytic capacity that are relatively resistant to fatigue.
• Pulmonary vasculature and lymphatics
The pulmonary artery divides to accompany the bronchi. The arterioles accompanying the respiratory bronchioles are thin-walled and contain little smooth muscle. The pulmonary venules drain laterally to the periphery of the lobules, pass centrally in the interlobular and intersegmental septa, and eventually join to form the four main pulmonary veins.
In addition, a further bronchial circulation arises from the descending aorta. These bronchial arteries supply tissues down to the level of the respiratory bronchiole. The bronchial veins drain into the pulmonary vein, forming part of the physiological shunt observed in normal individuals. Lymphatic channels lie in the potential interstitial space between the alveolar cells and the capillary endothelium of the pulmonary arterioles.
Nerve supply to the lung
The innervation of the lung remains incompletely understood. The parasympathetic supply is from the vagus and the sympathetic from the adjacent sympathetic chain. The nerve supplies entwine in a plexus at the nerve root and branches accompany the pulmonary arteries and the airways. Airway smooth muscle is innervated by vagal afferents, postganglionic cholinergic vagal efferents and vagally derived non-adrenergic non-cholinergic (NAN C) fibres. The parietal pleura is innervated from intercostal and phrenic nerves whilst the visceral pleura has no innervation.