The lungs (or pair of lungs, set of lungs, latin: pulmo, greek: pneumon) are a pair of essential organ of the respiratory system that are located in the chest and provide gas exchange between human blood and inhaled air.
The lungs are conical in shape with a rounded point at their apex and a flat base that rests on the convex surface of the diaphragm. The lungs are located in the thoracic cavity extending anteriorly and laterally from the heart to the ribs and posteriorly to the thoracic spine.
The main function of the lungs is to provide respiration by removing carbon dioxide from the blood and uptaking oxygen into the blood. Inspiration is an active process, where the diaphragm and muscles of inspiration contract, the diaphragm pulls the lower surfaces of the lungs downward, but the muscles of inspiration raise the rib cage. This results in increased intrathoracic volume which creates a negative pressure within the pleural space and sucks in air through the upper respiratory tract into the trachea, airways and thence into the alveoli, where gaseous exchange occurs. The expiration is mostly a passive process. During expiration, the diaphragm relaxes and the elastic recoil of the lungs, chest wall and abdominal structures compresses the lungs and expels the air.
The muscles of the inspiration that rise the rib cage are the external intercostal, sternocleidomastoid, anterior serrati and scaleni. The muscles of the expiration that pull the rib cage downward are the abdominal recti and the internal intercostals.
Gross anatomy of lungs
The lungs are conical in shape, each having an apex, base, three borders and two surfaces.
The apex is is a rounded upper tip of the superior end of a lung. The apex is situated above the superior thoracic aperture (thoracic inlet), more precisely, in the root of the neck, where it contacts the cervical pleura. The apex of each lung is covered by the suprapleural membrane.
The basal surface of each lung is broad, semilunar and concave that lies upon the superior surface of the diaphragm. The concavity is deeper on the base of the right lung because the diaphragm lies higher on the right side than on the left.
Each lung has two surfaces: costal and medial.
The costal surface of the lung (latin: facies costalis pulmonis) is smooth and curved and it lies in contact with the costal pleura, against the internal surface of the ribs.
The medial surface of the lung (also mediastinal surface of the lung; latin: facies mediastinalis, facies medialis pulmonis) is in contact with the mediastinum. The medial surface is divided into vertebral and anterior mediastinal parts. The vertebral part is in contact with the sides of the thoracic column. The anterior mediastinal part is deeply concave because it has a cardiac impression which is larger on the left lung where the heart projects.
The lungs have three borders: inferior, anterior and posterior.
The inferior border of the lung (latin: margo inferior) separates the base from the costal and mediastinal surfaces. It is thin and pointed where it divides the base from the costal surface, and more rounded medially where it separates the base from the mediastinal surface. The inferior border of the lung anteromedially passes from the lowest point of the anterior border to the sixth rib at about the midclavicular line, then to the eighth rib at about the midaxillary line and further continues posteriorly, medially to the tenth thoracic spine.
The anterior border of the lung (latin: margo anterior) is thin and sharp and corresponds to the anterior (costomediastinal) line of the pleural reflection on the right side, but on the left it corresponds to the same line only above the fourth costal cartilage. On the anterior border of the left lung lies the cardiac notch, a deep notch created by the apex of the heart.
The posterior border of the lung (latin: margo posterior) is thick and it divides the costal surface from the mediastinal surface.
The hilum (latin: hilum pulmonalis) is a large triangular depressed area that lies near the centre of the mediastinal surface of each lung. The hilum is the site where various structures enter and leave the lung via the lung root.
The root of lung (or pulmonary root, latin: radix pulmonis) is a shot broad pedicle formed by collection of structures that connect the medial surface of the lung to the heart and trachea. The structures that form the pulmonary root are surrounded by a sleeve of pleura, a two-layered membranous coating. These structures are the principal bronchus, pulmonary artery, two pulmonary veins, bronchial vessels, a pulmonary autonomic plexus, lymphatic vessels, bronchopulmonary lymph nodes and loose connective tissue.
The root of the right lung is located behind the superior vena cava and right atrium and beneath the azygos vein. The root of the left lung is situated beneath the aortic arch and anteriorly to the descending thoracic aorta.
The right lung is one of the two lungs situated on the right side of the heart and mediastinum. Vertically, the right lung is shorter than the left due to the liver lying beneath, that causes the right hemidiaphragm to be higher. The right lung is wider than the left lung due the impression of the heart on the left lung, therefore it has a greater capacity and weight than the left lung. Another difference between the right and left lungs is that the right main bronchus is longer, with larger caliber and more vertical than the left main bronchus.
The right lung is divided into three lobes by an oblique and a horizontal fissure:
- superior lobe
- middle lobe
- inferior lobe.
The oblique fissure (also the major fissure or great fissure) is the superior fissure that divides the inferior from the middle and superior lobes. The oblique fissure begins at the level of the spine of the fourth thoracic vertebra, crosses the fifth intercostal space and then follows the contour of the sixth rib to the sixth costochondral junction.
The horizontal fissure (also the minor fissure) is a short fissure that divides the superior and middle lobes. The horizontal fissure arises from the right oblique fissure close to the midaxillary line and follows the level of the sternal end of the fourth costal cartilage, further it passes posteriorly to the hilum on the mediastinal surface of the lung.
Each lobe is subdivided into bronchopulmonary segments based on the segmental bronchi. The segments of the lung are conical in shape, separated from each other by layers of connective tissue. Each bronchopulmonary segment is a discrete anatomical and functional unit, therefore surgical removement of a segment does not affect the function of the other segments. Overall, the right lung has ten bronchopulmonary segments.
The superior lobe of the right lung consists of three bronchopulmonary segments:
- apical segment,
- posterior segment,
- anterior segment.
The middle lobe has two bronchopulmonary segments:
- lateral segment,
- medial segment.
The inferior lobe of the right lung consists of five bronchopulmonary segments:
- superior segment,
- anterior segment,
- medial segment,
- lateral segment,
- posterior segment.
The left lung is situated in the left part of the mediastinum and on the left side of the heart with the heart creating an impression on the surface of this lung.
The left lung is divided into two lobes by an oblique fissure:
- superior lobe,
- inferior lobe.
The left oblique fissure (also the major or great fissure) is more vertical than the right oblique fissure. The left oblique fissure arises from the medial surface of the hilum, ascends obliquely backwards, crossing the posterior border of the lung. Further, the left oblique fissure passes anteriorly downwards across the costal surface and in the end it ascends on the medial surface to the hilum.
Each lobe is subdivided into bronchopulmonary segments based on the segmental bronchi. The segments of the lung are conical in shape, separated from each other by layers of connective tissue. Each bronchopulmonary segment is a discrete anatomical and functional unit, therefore surgical removement of a segment does not affect the function of the other segments. Overall, the left lung has eight bronchopulmonary segments.
The superior lobe of the left lung consists of four bronchopulmonary segments:
- apicoposterior segment,
- anterior segment,
- superior lingular segment,
- inferior lingular segment.
The inferior lobe of the left lung has four bronchopulmonary segments:
- superior segment,
- anteromedial segment,
- lateral segment,
- posterior segment.
The blood supply of the lungs include bronchial circulation and pulmonary circulation. The bronchial circulation is a part of the systemic circulation and includes the bronchial arteries and veins. The pulmonary circulation includes the pulmonary arteries and veins. The bronchial arteries provide oxygenated blood to the lungs, but the pulmonary arteries contain deoxygenated blood pumped from the right ventricle and provides gas exchange.
The left bronchial arteries (superior and inferior) arise from the thoracic aorta. The origin of the right bronchial artery is variable, it may arise from one of the following: the thoracic aorta, the superior bronchial artery on the left side, or the right intercostal arteries. The bronchial arteries run with and branch along with the bronchi, ending at the level of the respiratory bronchioles.
The bronchial veins are usually one on each side that are formed by the superficial bronchial veins and deep bronchial veins. The bronchial veins drain into the pulmonary veins.
The pulmonary artery arises from the infundibulum of the right ventricle of the heart, passes posteriorly and slightly upwards, and below the aortic arch divides into left and right pulmonary arteries. The right pulmonary artery passes posteriorly and near the hilum divides into upper, middle and lower branches, supplying the upper, middle and lower pulmonary lobes. The left pulmonary artery passes posterosuperiorly, enters the hilum of the left lung and divides into upper and lower branches which supply the upper and lower lobes. Further, these branches divide into pulmonary capillaries that form a network around the alveoli.
The pulmonary veins are two on each side and they carry oxygenated blood from the lungs to the heart. The pulmonary veins arise from the pulmonary capillaries that join together and form a single trunk. The pulmonary veins emerge from each lung at the hilum and drains into the left atrium of the heart. On its course, the pulmonary veins also receive the bronchial veins.
The lungs receive nerve supply through the vagus nerve (CN X) and the sympathetic trunk.
The branches innervating the lungs include the bronchial branches of the vagus nerve, sympathetic cervical cardiac nerves and the sympathetic cardiac branches from the second to fifth or sixth thoracic sympathetic ganglia form the anterior and posterior pulmonary plexuses. The pulmonary plexuses lie anterior and posterior to the hilar structures of the lungs.
The autonomic nervous system controls physiological functions of the lungs, including the regulation of airway smooth muscle tone, mucus secretion from bronchial glands and blood flow in the lungs.
Topography of lungs
Both lungs are located in the thoracic cavity on both sides of the mediastinum.
The apex of each lung (latin: apex pulmonis) is located behind the scalene muscles, anteriorly it lies one to two centimeters above the clavicle, posteriorly it project to the spinous process of the seventh cervical vertebrae.
The anterior border of the lungs passes from the apex obliquely downwards and anteriorly, projecting behind the sternoclavicular joint. Further, the anterior border of the lungs runs behind the sternum, then medially. The level around the second and fourth rib is the closest point between the right and left lungs. Reaching the level of the fourth rib, the anterior border of the lungs is different on each side. On the right lung the anterior border further descends oliquelly and on the right parasternal line reaches the upper margin of the sixth rib, after that the anterior border continues as the inferior border. On the left lung the anterior border at the level of the fourth rib turns horizontally, passes along the fourth rib reaching the parasternal line. Then the anterior border of the left lung descends and on the left midclavicular line reaches the inferior margin of the sixth rib, further the anterior border of the left lung continues as the inferior border that has similar course as on the right lung.
The inferior border of the right lung crosses the following lines:
- right midclavicular line at the level of the upper margin of the sixth rib;
- right anterior axillary line at the level of the seventh rib;
- right midaxillary line at the level of the eight rib;
- right posterior axillary line at the level of the ninth rib;
- right scapular line at the level of the tenth rib;
- right paravertebral line at the level of the spinous process of the eleventh thoracic vertebrae.
The posterior border of the right and left lungs passes upwards along the spine to reach the level of the second rib where it continues as the apex of the lungs.
The costal surface of each lung is in contact with the rib cage and the ribs form small grooves (latin: sulci costales) on this surface that follow the route of the ribs.
The medial surface of the lungs contact with organs of the mediastinum, therefore there are several impressions on the medial surface.
Both lungs feature the following impressions:
- cardiac impression (latin: impressio cardiaca pulmonis) - a depression that is produced by the presence of the heart and is more pronounced on the left lung;
- subclavian groove - a groove for the subclavian artery on the anterior surface of the pulmonary apex.
- groove for the brachiocephalic vein - runs adjacent to the medial surface of the apex of the lung.
Additionally, the right lung presents the following impressions:
- impression for the azygos vein - a groove for the azygos vein that passes over the root of the right lung;
- impression for the esophagus - a groove for the oesophagus, that is located superiorly and ascends vertically to the apex of the right lung.
- impression for the inferior vena cava - a groove for the inferior vena cava, located anteroinferiorly, descends vertically from the root.
Unlike the right lung, the left lung features a groove for the aortic arch that arises above the hilus and follows a curved route.