The lungs are the organs of respiration. They are located in the thorax, either side of the mediastinum and are responsible for oxygenating the blood. In this article, we will be looking at the anatomy of the lungs, their anatomical relations, neurovascular supply and clinical correlations.
The lungs lie either side of the mediastinum, within the thoracic cavity. Each lung is surrounded by a pleural cavity, which is formed by the visceral and parietal pleura. The lungs have a roughly cone-shaped structure, with an apex, base, three surfaces and three borders. The left lung is typically slightly smaller than the right, due to the presence of the heart.
The right lung has three lobes; superior, middle and inferior. The lobes are divided from each other by two fissures:
The left lung contains superior and inferior lobes, which are separated by a similar oblique fissure.
Each lung has three surfaces, each corresponding to an area of the thorax. The mediastinal surface of the lung faces the lateral aspect of the middle mediastinum, and is where the lung hilum is located (where structures enter and leave the lung). The base of the lung is formed by the diaphragmatic surface. It rests on the dome of the diaphragm and has a concave shape. This concavity is deeper on the right lung due to the higher position of the right dome overlying the liver. The costal surface is smooth and convex and faces the internal surface of the chest wall. It is related to the costal pleura, which separates it from the ribs and innermost intercostal muscles.
The anterior border of the lung is formed by the convergence of the mediastinal and costal surfaces. On the left lung, the anterior border is marked by a deep notch, created by the apex of the heart - known as the cardiac notch. The inferior border separates the base of the lung from the costal and mediastinal surfaces. The posterior border is smooth and rounded (in contrast to the anterior and inferior borders, which are sharp). It is formed by the costal and mediastinal surfaces meeting posteriorly.
The lung root is a collection of structures that suspends the lung from the mediastinum. Structures that enter or leave the lungs do so via the hilum – a wedge-shaped area on its mediastinal surface. Inside the lung, each of the structures divide up to form lobar bronchi – each supplying a lobe. Each lobar bronchus then further divides into several tertiary segmental bronchi, which each supply a bronchopulmonary segment – which are the functional units of the lungs.The segmental bronchi then give rise to conducting bronchioles, which eventually lead into terminal bronchioles. Each terminal bronchiole gives off respiratory bronchioles, which feature thin walled outpocketings (alveoli) that extend from their lumens – the site of gaseous exchange. The lungs are supplied with deoxygenated blood by the paired pulmonary arteries. Once the blood has been oxygenated, it leaves the lungs via four pulmonary veins (two for each lung). The bronchi, lung roots, visceral pleura and supporting lung tissues require an additional nutritive blood supply.
The lungs are located in the thoracic cavity, either side of the mediastinum. They are surrounded by a pleural cavity, formed by the visceral and parietal pleura. The right lung is slightly larger than the left, due to the presence of the heart. Each lung has three surfaces (corresponding with the area of the thorax that they face) and three borders, as well as an apex and base. The right lung has three lobes; superior, middle and inferior - the left lung is divided into a superior and inferior lobe, with an oblique fissure separating them. Suspending the lungs from the mediastinum is the lung root, a collection of structures that enter and leave the lungs via the hilum - a wedge shaped area on the mediastinal surface.
The bronchial tree is also present within the lungs. Starting with the trachea, it divides to form left and right bronchi. These further divide to form lobar bronchi - one for each lobe of the lung - which then subdivide into several tertiary segmental bronchi. Each of these provide air to a bronchopulmonary segment - the functional units of the lungs. The segmental bronchi then branch off into many conducting bronchioles, before finally ending in terminal bronchioles. From these, respiratory bronchioles extend, featuring thin walls and outpocketings - these are the alveoli, the site of gaseous exchange.
The lungs are supplied with deoxygenated blood from the paired pulmonary arteries. Once oxygenated, the blood leaves the lungs via four pulmonary veins (two for each lung). The bronchi, lung roots, visceral pleura and supporting lung tissues also require a nutritive blood supply. This is delivered by the bronchial arteries, which arise from the descending aorta. Furthermore, the bronchial veins provide venous drainage. The right bronchial vein drains into the azygos vein, with the left draining into the accessory hemiazygos vein.
The nerves of the lungs are derived from the pulmonary plexuses. These consist of sympathetic, parasympathetic and visceral afferent fibres, which have the following functions:
The lungs have two lymphatic plexuses, responsible for draining the lung parenchyma and structures of the lung root. These plexuses empty into the trachebronchial nodes, located around the bifurcation of the trachea and the main bronchi. From here, lymph passes into the right and left bronchomediastinal trunks.
The lungs sit either side of the mediastinum, in the thoracic cavity, surrounded by a pleural cavity. The right lung is larger due to the presence of the heart, and each contains an apex, base, three surfaces and three borders. The right has three lobes; superior, middle and inferior - the left lung consists of a superior and inferior lobe, separated by an oblique fissure. The bronchial tree and vascular system supply the lungs with necessary blood and air. Additionally, nerves and lymphatic drainage are crucial for the proper functioning of the lungs.
The anatomy of the lungs can be divided into several components, each with distinct clinical correlations. The lungs are situated within the thoracic cavity in the anatomical position. The right lung has three lobes, and the left lung two. Each lobe has a distinct surface, border, root, and hilum, and are subdivided into bronchi and bronchioles. The vasculature of the lungs includes the pulmonary arteries, pulmonary veins, bronchial arteries, and bronchial veins. The nerve supply and lymphatic drainage of the lungs is also important.
The nerve supply is relevant in terms of the clearance of the lungs. Afferent nerves project from the anterior and posterior lungs to the respiratory centre in the medulla. These nerves activate the reflex responses associated with respiration. The efferent nerves travel in the vagus nerve to the respiratory muscles. The lymphatic drainage of the lungs can be divided into two categories. The apical plexus is comprised of small vessels that drain the lymph from the apex of the lung, and the basal plexus that drains from the basal and subpleural areas.
Both plexuses empty into the trachebronchial nodes, which are located around the bifurcation of the trachea and the main bronchi. Lymph then passes into the right and left bronchomediastinal trunks. This anatomy has clinical relevance, particularly in the context of pulmonary embolism.
A pulmonary embolism refers to the obstruction of a pulmonary artery by a substance that has travelled from elsewhere in the body. The most common emboli are thrombus, which is responsible for the majority of cases and usually arises in a distant vein, fat following a bone fracture or orthopaedic surgery, and air following cannulation in the neck.
The effect of a pulmonary embolism is a reduction in lung perfusion, resulting in decreased blood oxygenation and the accumulation of blood in the right ventricle of the heart. Clinical features include dyspnoea, chest pain, cough, haemoptysis and tachypnoea. In clinical medicine, the Wells' score is used to assess the probability of PE.
Definitive treatment involves anticoagulation and thrombolytic therapy. This helps reduce the size of the embolus, and prevents further clotting. In addition, for thrombolytic therapy, anisoylated plasminogen streptokinase activator complex (APSAC) is sometimes used. This is a haemophilic drug that activates the fibrinolytic system, which reduces the clot.
The anatomy of the lungs and their associated nerve supply, vasculature and lymphatic drainage play an important role in the management of pulmonary embolism. In addition to anticoagulation and thrombolytic therapy, the anatomy of the lungs can provide insights into the cause and possible consequences of pulmonary embolism. As such, it is important to recognize the anatomy of the lungs and their relevance to clinical medicine.
