- Each lung lies in a pleural sac attached by the pleural vessels to the mediastinum. Each has an apex in the root of the neck and a base resting on the diaphragm. In the centre of the mediastinal surface the structures forming the root of the lung are surrounded by a collar of reflected pleura. The posterior border is rounded and lies in the paravertebral sulcus.
- The borders of the lungs closely follow the pleural reflection on the chest wall. The inferior border of the lung lies about two intercostal spaces above the inferior limit of the pleura. The base of the lung is separated by the diaphragm from (on the right side) the right lobe of the liver and (on the left side) the left lobe of the liver, the stomach and the spleen. Superiorly, the apex is covered by the dome of the pleura and the suprapleural membrane, above which lie the subclavian vessels. Posteriorly the apex is separated from the neck of the first rib by the anterior primary ramus of the first thoracic nerve, the superior intercostal artery, the sympathetic trunk and the pleura.
- On the left lung there is a concavity anteroinferiorly for the left ventricle, which is continuous superiorly with a groove for the aortic arch in front of and above the hilum. Above this groove there is the left brachiocephalic vein, left common carotid and left subclavian arteries and the oesophagus.
- On the right lung there is a concavity in front of the hilum for the right atrium, with grooves above and below for the superior and inferior vena cavae. Above the SVC groove there is the left brachiocephalic vein, the left common carotid and subclavian arteries and the oesophagus.
- The pulmonary trunk begins at the pulmonary valve, ascends posteriorly and bifurcates into the left and right pulmonary arteries below the concavity of the aortic arch.
- The right pulmonary artery passes to the hilus of the right lung behind ascending aorta and in front of oesophagus and right main bronchus.
- The left pulmonary artery passes to the left lung hilus in front of the left bronchus and the descending aorta.
- Both divide into branches which follow the segmented bronchi, ending in a capillary network within the alveolar walls.
- Bronchial arteries supply the lung tissue, and are small branches of the descending thoracic aorta.
- Bronchial veins drain into the azygos or hemiazygos veins.
- Pulmonary veins pass directly into the left atrium, two from each lung. They are formed from the union of smaller veins that drain oxygenated blood from the alveolar capillary beds.
Innervation is by the pulmonary plexus: sympathetic (bronchodilator) fibres from the upper four thoracic sympathetic ganglia, and parasympathetic (bronchoconstrictor) fibres from the vagus. Sensory fibres pass in the vagus nerves.
- Vascular reservoir/Role in fluid balance
- Filter for blood borne substances
- Defence against inhaled substances/Immune function
- Endocrine and metabolic functions
- Pulmonary drug metabolism
- Platelet formation
- The lung has an extremely distensible vasculature. The walls are thin and contain relatively little smooth muscle, making it compliant to increased blood volume. This enables it to cope with large variations in venous return, especially during postural changes, exercise, and increased intravascular volume. This is due to recruitment and distension of the vasculature. This helps control the preload delivered to the left heart, optimising cardiac output.
- Pulmonary capillaries have a diameter of 7micrometres, allowing them to act as a physical barrier to various blood borne substances. However, the lung can fail to filter substances much bigger than this. Also, almost 25% of the population have a patent foramen ovale. Blockage of major vessels or extensive micro-emboli can cause life-threatening V/Q mismatch or activation of the pathways leading to ARDS.
- The lung has an efficient fibrinolytic system which lyses clots in the pulmonary circulation. Pulmonary endothelium produces fibrinolysin activator, which converts plasminogen to fibrinolysin, which breaks down fibrin to fibrin degradation products. Also, the lung is a rich source of heparin (inhibits coagulation) and thromboplastin (promotes coagulation), so plays a role in the overall coagulability of blood.
- A highly viscous mucopolysaccharide gel forms a layer of defence against inhaled physical substances. The muco-ciliary escalator moves the mucous and entrapped particles from peripheral to central airways, from where it is expectorated or swallowed.
- Immune function within the lung is mediated by pulmonary alveolar macrophages (PAM) and a variety of immune mediators.
- PAMs engulf the particles that reach the alveoli and deposit them on the muco-ciliary escalator or remove them via blood or lymph. They also have a role in antigen presentation, T-cell activation and immunomodulation. PAMs release lysosomal products into the extracellular space, causing inflammation and eventually fibrosis. Epithelial cells also release a variety of substances which non-specifically shield the lung from attack. They produce a number of inflammatory mediators and immunoglobulins, such as IgA.
- The pulmonary neuroendocrine system (isolated pulmonary neuroendocrine cells and neuroepithelial bodies) secrete a wide variety of amines and peptides, and play a significant role in cell growth, differentiation and branching morphogenesis in the developing lung. Amines significantly reduce the levels of noradrenaline and serotonin from blood passing through the pulmonary circulation. More Angiotensin Converting Enzyme is found on pulmonary vascular endothelium than anywhere else in the body. Arachidonic acid metabolites and leukotrienes are metabolised extensively in the lungs. The purine AMP, ADP and ATP are metabolised to adenosine in the pulmonary circulation.