Lung

For other uses, see Lung (disambiguation).

"Lunged" redirects here. For other uses, see Lunge.

Lung
Detailed diagram of the lungs coloured for identification of the lobes
Details
Latin	pulmo
System	Respiratory system
Identifiers
MeSH	A04.411
TA
TH	{{#property:P1694}}
TE	{{#property:P1693}}
FMA	{{#property:P1402}}
Anatomical terminology [[[d:|edit on Wikidata]]]

The human lungs flank the heart and great vessels in the chest cavity

Air enters and leaves the lungs via a conduit of cartilaginous passageways—the bronchi and bronchioles. In this image, lung tissue has been dissected away to reveal the bronchioles

The lungs are the primary organs of respiration in humans and many other animals including a few fish and some snails. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their musculoskeletal systems to support and foster breathing. In early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. In humans, the primary muscle that drives breathing is the diaphragm.

Humans have two lungs, a right lung and a left lung. They are situated within the thoracic cavity of the chest. The right lung is bigger than the left lung, as the left lung shares space in the chest with the heart. The lungs together weigh approximately 1.3 kilograms (2.9 lb), with the right lung weighing more than the left. The lungs are part of the lower respiratory tract that begins at the trachea and branches into the bronchi and bronchioles and which receive air breathed in via the conducting zone. These divide until air reaches microscopic alveoli, which is where the process of gas exchange takes place. Together, the lungs contain approximately 2,400 kilometres (1,500 mi) of airways and 300 to 500 million alveoli. The lungs are enclosed within a sac called the pleural sac which allows the inner and outer walls to slide over each other whilst breathing takes place, without much friction. This sac encloses each lung and also divides each lung into sections called lobes. The right lung has three lobes, whilst the left lung only has two. The lobes are further divided into bronchopulmonary segments and lobules. The lungs have a unique blood supply, receiving deoxygenated blood sent from the heart for the purposes of receiving oxygen (the pulmonary circulation) and a separate blood supply of oxygenated blood (the bronchial circulation). The tissue of the lungs can be affected by a number of diseases, including pneumonia and lung cancer. Chronic diseases such as chronic obstructive pulmonary disease and emphysema can be related to smoking or exposure to harmful substances. Diseases such as bronchitis can also affect the respiratory tract.

In fetal life, the lungs develop as an outpouching of the foregut, a tube which goes on to form the upper part of the digestive system. At this time, the fetus is situated in the amniotic sac and so the lungs do not function to breathe. Blood is also diverted from the lungs through the ductus arteriosus. At birth however, air begins to pass through the lungs, and the diversionary duct closes, so that the lungs can begin to respire. The lungs only fully develop in early childhood.

Medical terms related to the lung often begin with pulmo-, from the Latin pulmonarius (of the lungs) as in pulmonology, or with pneumo- (from Greek πνεύμων "lung") as in pneumonia.

Structure

"Meet the lungs" from the Khan academy

Gross anatomy

The lungs are located in the chest on either side of the heart in the rib cage. They are conical in shape with a narrow rounded apex at the top and a broad base that rests on the diaphragm.The apex of the lung extends into the root of the neck, reaching shortly above the level of the sternal end of the first rib. The lungs stretch from close to the backbone in the rib cage to the front of the chest and downwards from the lower part of the trachea to the diaphragm. The left lung shares space with the heart, with an angular notch in its anterior border called the cardiac notch. In addition, on the mediastinal surface of each lung there is a concavity called the cardiac impression at the level of the heart. The cardiac impression on the left lung is wider and deeper than that of the right lung. Because of this, and the cardiac notch, the right lung is larger in volume, total capacity and weight, despite being about 5 cm shorter in height than the left.

Both lungs have a central recession called the hilum at the root of the lung, where the blood vessels and airways pass into the lungs. There are also bronchopulmonary lymph nodes on the hilum. The costal surface along the rib cage is smooth and convex, with slight grooves corresponding to indentations by the ribs.

The lungs are surrounded by the pulmonary pleurae. The pleurae are two serous membranes; the outer parietal pleura lines the inner wall of the rib cage and the inner visceral pleura directly lines the surface of the lungs. Between the pleurae is a potential space called the pleural cavity containing pleural fluid. Each lung is divided into lobes by the invaginations of the pleura as fissures. The fissures are double folds of pleura that section the lungs. These sections help in the expansion of the lungs.

The lobes of the lungs are further divided into bronchopulmonary segments based on the locations of bronchioles. Segments for the left and right lung are shown in the table. The segmental anatomy is useful clinically for localizing disease processes in the lungs.

Borders

Anterior border on costal surface of left lung

Posterior border on mediastinal surface of left lung

Each lung has three borders. The anterior border of the lung is thin and sharp, and overlaps the front of the pericardium. The anterior border of the right lung is almost vertical, and projects into the costodiaphragmatic recess; that of the left lung presents, below, an angular cardiac notch, in which the pericardium is exposed. Opposite this notch the anterior margin of the left lung is situated some little distance lateral to the line of reflection of the corresponding part of the pleura. The cardiac notch lies along the fifth and sixth intercostal space, and the spaces can be used to access the heart in pericardiocentesis to relieve cardiac tamponade.

The posterior border of the lung is broad and rounded, and is received into the deep concavity on either side of the vertebral column. It is much longer than the anterior border, and projects, below, into the costodiaphragmatic recess.

The inferior border of the lung is thin and sharp where it separates the base from the costal surface and extends into the costodiaphragmatic recess; medially where it divides the base from the mediastinal surface it is blunt and rounded.

Right lung

Lobes and bronchopulmonary segments

Right lung	Left lung
Upper Apical Anterior Posterior Middle Medial Lateral Lower Superior Anterior Posterior Medial Lateral	Upper Anterior Apicoposterior Lower Superior Anterior Posterior Medial Lateral Lingula Superior Inferior

The right lung has both more lobes and segments than the left. It is divided into three lobes, an upper, middle, and a lower, by two fissures, one oblique and one horizontal. The upper, horizontal fissure, separates the upper from the middle lobe. It begins in the lower oblique fissure near the posterior border of the lung, and, running horizontally forward, cuts the anterior border on a level with the sternal end of the fourth costal cartilage; on the mediastinal surface it may be traced backward to the hilum.

The lower, oblique fissure, separates the lower from the middle and upper lobes, and is closely aligned with the oblique fissure in the left lung. Its direction is, however, more vertical, and it cuts the lower (inferior) border about 7.5 cm. behind its anterior extremity.

It has a deep concavity on its inner surface called the cardiac impression at the level of the heart. This is not as pronounced as that on the left lung where the heart projects further.

On this inner surface, above the hilum, is an arched groove at the level of the azygos vein; while running superiorly, and then arching laterally some little distance below the apex, is a wide groove for the superior vena cava and right innominate vein; behind this, and proximal to the apex, is a groove for the innominate artery. Behind the hilum and the attachment of the pulmonary ligament is a vertical groove for the esophagus. This groove becomes less distinct below, owing to the inclination of the lower part of the esophagus to the left of midline.

In front and to the right of the lower part of the esophageal groove is a deep concavity for the extrapericardiac portion of the thoracic part of the inferior vena cava.

Left lung

The left lung is divided into two lobes, an upper and a lower, by the oblique fissure, which extends from the costal to the mediastinal surface of the lung both above and below the hilum. The left lung, unlike the right, does not have middle lobe, though it does have a homologous feature, a projection of the upper lobe termed the “lingula”. Its name means “little tongue”. The lingula on the left serves as an anatomic parallel to the right middle lobe, with both areas being predisposed to similar infections and anatomic complications. There are two bronchopulmonary segments of the lingula: superior and inferior. It is thought that the lingula of the left lung is the remnant of the middle lobe, which has been lost in the course of evolution.

As seen on the surface, this fissure begins on the mediastinal surface of the lung at the upper and posterior part of the hilum, and runs backward and upward to the posterior border, which it crosses at a point about 6 cm. below the apex. It then extends downward and forward over the costal surface, and reaches the lower border a little behind its anterior extremity, and its further course can be followed upward and backward across the mediastinal surface as far as the lower part of the hilum. It has an angular indentation on the inner surface of its anterior border called the cardiac notch, and a concavity called the cardiac impression at the level of the heart. This is deeper and larger than that on the right, at which level the heart projects to the left.

On the same surface, immediately above the hilum, is a well-marked curved groove produced by the aortic arch. Running upward from this, toward the apex, is a groove at the level of the left subclavian artery. A slight impression in front of the latter, and close to the margin of the lung, lodges the left innominate vein. Behind the hilum and pulmonary ligament is a vertical groove produced by the descending aorta; in front of this, near the base of the lung, there is a shallow impression at the level of the lower esophagus.

The left lung (left) and right lung (right). The lobes of the lungs can be seen, and the central root of the lung is also present.

Respiratory system

Main article: Respiratory system

Alveoli with blood supply

The lung is part of the respiratory system, and contains the majority of the lower respiratory tract after the trachea. The trachea receives air from the pharynx and travels down to a place where it splits (the carina) into a right and left bronchus. These supply air to the right and left lungs, splitting progressively into the secondary and tertiary bronchi for the lobes of the lungs, and into smaller and smaller bronchioles until they become the respiratory bronchioles. These in turn supply air through alveolar ducts into the alveoli, where the exchange of gasses take place. Oxygen diffuses through the walls of the alveoli into the enveloping capillaries (small blood vessels).

Estimates of the total surface area of lungs vary from 30-50 square metres to up to 70-100 square metres (1076.39 sq ft) (8,4 x 8,4 m) in adults — which might be roughly the same area as one side of a tennis court. However, such estimates may be of limited use unless qualified by a statement of scale at which they are taken (see coastline paradox and Menger sponge). Furthermore, if all of the capillaries that surround the alveoli were unwound and laid end to end, they would extend for about 992 kilometres (616 mi).

The parts of the respiratory system that conduct oxygen to the alveoli is called the conducting zone, which is part of the respiratory tract, that conducts air into the lungs. This comprises the air passageways, with gas exchange only taking place in the alveoli of the respiratory system.

The bronchi in the conducting zone are reinforced with hyaline cartilage in order to hold open the airways. The bronchioles have no cartilage and are surrounded instead by smooth muscle. Air is warmed to 37 °C (99 °F), humidified and cleansed by the conduction zone; particles from the air being removed by the cilia on the respiratory epithelium lining the passageways.

Blood supply

Being an organ of respiration, the human lung has a dual blood supply.

The tissue of the lungs receive oxygenated blood via the bronchial circulation, a series of arteries that leave the aorta and are part of the systemic circulation.

The lungs also receive deoxygenated blood from the heart and supply it with oxygen, in a process known as respiration. In this process, venous blood in the body collects in the right atrium and is pumped from the right ventricle through the pulmonary trunk and the pulmonary arteries into the left and right lungs. Blood passes through small capillaries next to the alveoli in the lung, receives oxygen, and travels back to the heart. This is called the pulmonary circulation. The oxygenated blood is then pumped to the rest of the body. Blood that leaves the heart is then returned to supply the tissues of the lung via the bronchial circulation mentioned above.

Microanatomy

Lobule of lung

Cross-sectional detail

Underneath the serous visceral pleura surrounding the lung is a subserous layer of areolar tissue and beneath this is the lung parenchyma. Areolar tissue is a type of loose connective tissue characterised by a sparcity of fibroblasts with an abundance of interstitial fluid. This type of tissue acts as a reservoir of fluid, containing nutrients for the adjacent tissues which may be avascular. The parenchyma refers usually just to the alveolar tissue.

The lobule is the smallest microscopic unit of the lung. A lobule consists of a respiratory bronchiole, an alveolar duct, an alveolar sac, and alveoli. There are three types of alveolar cells. These are known as type I and type II alveolar cells (also known as pneumocytes), and alveolar macrophages. Types I and II make up the walls and septa of the alveoli. The macrophages are attached to the alveoli.

File:Anatomy and physiology of animals cs of an alveolus.jpg

Alveolus and gas exchange

Type I are squamous epithelial cells that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange. Their thinness is made possible by the clustering of cell organelles around the nucleus leaving a large area of free cytoplasm. There are pinocytic vesicles in the cytoplasm that hold particles of unwanted extracellular material. These fuse with the lysosomes for further breakdown of the material. These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associated basement membranes.

Type II are larger and they line the alveoli and produce and secrete the fluid lining the alveoli and also the pulmonary surfactant.

The macrophages have an important immunological role. Their secretions include nitric oxide, prostaglandins and cytokines some of which prevent inflammation.

Development

Lungs during development

The development of the human lungs arise from the laryngotracheal groove and develop to maturity over several weeks inside the foetus and for several months following birth. The larynx, trachea, bronchi and lungs begin to form during the fourth week of embryogenesis. At this time, the lung bud appears ventrally to the caudal portion of the foregut. The location of the lung bud along the gut tube is directed by various signals from the surrounding mesenchyme, including fibroblast growth factors. At the same time as the lung bud grows, the future trachea separates from the foregut through the formation of tracheoesophageal ridges, which fuse to form the tracheoesophageal septum.

The lung bud divides into two, the right and left primary bronchial buds. During the fifth week the right bud branches into three secondary bronchial buds and the left branches into two secondary bronchial buds. These give rise to the lobes of the lungs, three on the right and two on the left. Over the following week the secondary buds branch into tertiary buds, about ten on each side. From the sixth week to the sixteenth week, the major elements of the lungs appear except the alveoli. From week 16 to week 26, the bronchi enlarge and lung tissue becomes highly vascularised. Bronchioles and alveolar ducts also develop. During the period covering the 26th week until birth the important blood-air barrier is established. Specialised type I alveolar cells where gas exchange will take place, together with the type II alveolar cells that secrete pulmonary surfactant appear. The surfactant reduces the surface tension at the air-alveolar surface which allows expansion of the terminal saccules. These saccules form at the end of the bronchioles and their appearance marks the point at which limited respiration would be possible.

First breath

At birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. When the newborn is expelled from the birth canal, its central nervous system reacts to the sudden change in temperature and environment. This triggers it to take the first breath, within about 10 seconds after delivery. The newborn lung is far from being a miniaturized version of the adult lung. It has only about 20,000,000 to 50,000,000 alveoli or 6 to 15 percent of the full adult complement. Although it was previously thought that alveolar formation could continue to the age of eight years and beyond, it is now accepted that the bulk of alveolar formation is concluded much earlier, probably before the age of two years. The newly formed inter alveolar septa still contain a double capillary network instead of the single one of the adult lungs. This means that the pulmonary capillary bed must be completely reorganized during and after alveolar formation, it has to mature. Only after full microvascular maturation, which is terminated sometime between the ages of two and five years, is the lung development completed and the lung can enter a phase of normal growth.

Function

Respiration

Main article: Respiration (physiology)

Gas exchange

The respiratory system's alveoli are the sites of gas exchange with blood.

In humans, the trachea divides into the two main bronchi that enter the roots of the lungs. The bronchi continue to divide within the lung, and after multiple divisions, give rise to bronchioles. The bronchial tree continues branching until it reaches the level of terminal bronchioles, which lead to respiratory bronchioles and alveolar sacs. Alveolar sacs, are made up of clusters of alveoli, like individual grapes within a bunch. The individual alveoli are tightly wrapped in capillaries and it is here that gas exchange actually occurs. Deoxygenated blood from the heart is pumped through the pulmonary artery to the lungs, where oxygen diffuses into blood and is exchanged for carbon dioxide in the hemoglobin of the erythrocytes. The oxygen-rich blood returns to the heart via the pulmonary veins to be pumped back into systemic circulation.

• The sympathetic nervous system via noradrenaline acting on the beta receptors causes bronchodilation.
• The parasympathetic nervous system is through the vagus nerve, via acetylcholine, which acts on the M-3 muscarinic receptors, maintains the resting tone of the bronchiolar smooth muscle. This action is related, although considered distinct from bronchoconstriction.
• Many other non-autonomic nervous and biochemical stimuli, including carbon dioxide and oxygen (acting on the respiratory centers), are also involved in the regulation process.

There is also a relationship noted between the pressures in the lung, in the alveoli, in the arteries and in the veins. This is conceptualised into the lung being divided into three vertical regions called the zones of the lung.

Other

In addition to their function in respiration, the lungs also:

• Help in the regulation of blood pressure – as part of the renin-angiotensin system, the lungs convert angiotensin I to angiotensin II. In addition, they remove several blood-borne substances, such as a few types of prostaglandins, leukotrienes, serotonin and bradykinin.
• Balance the pH of blood by facilitating alterations in the partial pressure of carbon dioxide
• Filter out small blood clots (and gas micro-bubbles formed during decompression) from veins.
• Influence the concentration of some biologic substances and drugs used in medicine in blood
• May serve as a layer of soft, shock-absorbent protection for the heart, which the lungs flank and nearly enclose.
• Immunoglobulin-A is secreted in the bronchial secretion and protects against respiratory infections.
• Maintain sterility by producing mucus containing antimicrobial compounds. Mucus contains glycoproteins, e.g., mucins, lactoferrin, lysozyme, lactoperoxidase. We find also on the epithelium Dual oxidase 2 proteins generating hydrogen peroxide, useful for hypothiocyanite endogenous antimicrobial synthesis. Function not in place in cystic fibrosis patient lungs.
• Mucociliary clearance is an important defence system against air-borne infection. The dust particles and bacteria in the inhaled air are caught in the mucous layer present at the mucosal surface of respiratory passages and are moved up towards pharynx by the rhythmic upward beating action of the cilia.
• Provide airflow for the creation of vocal sounds.
• The lungs serve as a reservoir of blood in the body. The blood volume of the lungs is about 450 milliliters on average, about 9 percent of the total blood volume of the entire circulatory system. This quantity can easily fluctuate from between one-half and twice the normal volume. Loss of blood from the body due to hemorrhage can be partially compensated for by shunting blood from the lungs into the systemic circulatory vessels.

Clinical significance

Main articles: Respiratory disease and Pulmonology

Human lungs can be affected by a variety of diseases. The environment of the lung is moist which makes it very hospitable to microorganisms. Many respiratory illnesses are due to infections from bacteria or viruses. Tuberculosis is a serious infection as is bacterial pneumonia. Tuberculosis usually affects the lungs and infection spread through the bloodstream can result in a tubercular nodule in the apex of the lung, known as a Simon focus. Pneumonia is an inflammatory condition of the lungs caused by infection. Pleurisy is an inflammatory condition of the pleurae surrounding the lungs usually caused by a viral infection from the lungs.

Infarction of the lung due to a pulmonary embolism

Lung tissue affected by emphysema using H&E stain

A pulmonary embolism is a blood clot that becomes lodged in the lung. A ventilation/perfusion scan using scintigraphy may be used to detect this.

Lung diseases can arise suddenly, such as a pneumothorax or hemothorax, in which air or blood is trapped in the pleural cavity and compresses the lung. The accumulation of blood and other types of fluid in the pleural cavity can cause various types of pleural effusion which results in breathing difficulties.

Diseases can also be chronic, such as bronchitis and asthma, both of which are inflammatory diseases; and emphysema, a common complication of smoking caused by inflammation and the progressive inability of alveoli to expand and contract with respiration. Pulmonary fibrosis can occur when the lung is inflamed for a long period of time. This can be due to an occupational disease such as Coalworker's pneumoconiosis or more rarely to a reaction to medication. Fibrosis in the lung is the replacement of lung tissue by fibrous connective tissue which causes irreversible lung scarring and can prove fatal.

Asthma, chronic bronchitis, bronchiectasis and chronic obstructive pulmonary disease (COPD) are all obstructive lung diseases characterised by airway obstruction. Bitter taste receptors have been identified in lung tissue, which cause airways to relax when a bitter substance is encountered. This mechanism is believed to be evolutionary adaptive since it helps to clear lung infections and could be exploited to treat asthma and COPD.

Lung cancer can either occur as a result of metastasis from another part of the body and are known as secondary tumors or they can arise from the epithelial tissue of the lung itself when they are known as primary tumors. There are two main types of primary tumor described as either small-cell or non-small-cell lung carcinomas and can often be incurable. Cancer in the lung can spread to other parts of the body and metastasise there, as also can cancers in other parts of the body spread via the bloodstream and end up in the lungs where the malignant cells can metastasise. Pancoast tumors form in the apex of a lung, and these can spreed to the ribs and backbone. Lung cancers are primarily caused by smoking.

A rare disease of the autoimmune system called Goodpasture syndrome usually affects both the lungs and the kidneys though sometimes only the lungs are affected.

Congenital disorders include pulmonary hypoplasia an incomplete development of the lungs, and infant respiratory distress syndrome caused by a deficiency in lung surfactant. An azygos lobe is a congenital anatomical variation which though usually without effect can cause problems in thoracoscopic procedures.

Lung volumes

Further information: Lung volumes and Pulmonary function testing

Lung volumes

Lungs have a tremendous reserve volume as compared to the oxygen exchange requirements when at rest. Such excess capacity is one of the reasons that individuals can smoke for years without having a noticeable decrease in lung function while still or moving slowly; in situations like these only a small portion of the lungs are actually perfused with blood for gas exchange. Destruction of too many alveoli over time leads to the condition emphysema, which is associated with extreme shortness of breath. As oxygen requirements increase due to exercise, a greater volume of the lungs is perfused, allowing the body to match its carbon dioxide-oxygen exchange requirements. Additionally, due to the excess capacity, it is possible for humans to live with only one lung, with the one compensating for the other's loss.

Lung volumes and lung capacities include inspiratory reserve volume, tidal volume, functional residual capacity, expiratory reserve volume, and residual volume. Pulmonary plethysmographs are used to measure functional residual capacity. The total lung capacity depends on the person's age, height, weight, and sex, and normally ranges between 4,000 and 6,000 cm³ (4 to 6 L). Females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Smokers have a lower capacity than nonsmokers. Lung capacity is also affected by altitude. People who are born and live at sea level will have a smaller lung capacity than people who spend their lives at a high altitude. In addition to the total lung capacity, the tidal volume is also measured, this is the volume breathed in with an average breath, which is about 500 cm³.

Another measure is the respiratory minute volume which is the volume of air which can be inhaled or exhaled from a person's lungs in one minute. This is normally registered when a person has a ventilator supporting breathing due to a sickness or injury. Other lung function tests include spirometry, measuring the amount (volume) and flow of air that can be inhaled and exhaled. The maximum volume of breath that can be exhaled is called the vital capacity. These tests can help to determine whether a disease is restrictive or obstructive.

Other animals

Birds

Main article: Bird anatomy § Respiratory system

The respiatory system in a bird.

On inspiration, air travels to air sacs near the back of a bird. These then pass through the lungs to air sacs near the front of the bird, where air is expelled.

The lungs of birds are relatively small, but are connected to 8–9 air sacs that extend through much of the body, and are in turn connected to air spaces within the bones. On inspiration, air travels through the trachea of a bird into the 8-9 air sacs. Air then travels continuously from the air sacs at the back of the bird, through the lungs of the bird, and to the air sacs at the front of the bird. From here, air is expelled. This type of lung construction is called a circulatory lung, as distinct from the bellows lung possessed by other animals. This means that they are able to extract a greater concentration of oxygen from inhaled air. Birds are thus equipped to fly at altitudes at which mammals would succumb to hypoxia. This also allows them to sustain a higher metabolic rate than most equivalent weight mammals.

The lungs of birds are honey-comb-like and contain millions of tiny passages called parabronchi. Small sacs called called atria radiate from the walls of the tiny passages, and are the site of gas exchange. Gas exchange occurs by diffusion in these walls, as gas travels between the lumen of each parabronchus and blood vessels.

The air sacs, which hold air, do not contribute much to gas exchange, despite being thin-walled, as they are poorly vascularized. The air sacs expand and contract due to changes in the volume in a bird's thorax and abdomen. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronized with movement of the flight muscles.

This typical system is one of two types of parabronchi found in birds, called paleopulmonic parabronchi and is found in all birds. Some bird species also have a lung structure where the air flow is bidirectional, called neopulmonic parabronchi.

Reptiles

Main article: Reptile anatomy § Respiration

The lung of most reptiles has a single bronchus running down the centre, from which numerous branches reach out to individual pockets throughout the lungs. These pockets are similar to alveoli in mammals, but much larger and fewer in number. These give the lung a sponge-like texture. In tuataras, snakes, and some lizards, the lungs are simpler in structure, similar to that of typical amphibians.

Snakes and limbless lizards typically possess only the right lung as a major respiratory organ; the left lung is greatly reduced, or even absent. Amphisbaenians, however, have the opposite arrangement, with a major left lung, and a reduced or absent right lung.

Both crocodilians and monitor lizards have developed lungs similar to those of birds, providing an unidirectional airflow and even possessing air sacs. The now extinct pterosaurs have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case of Pteranodontia, the hindlimbs.

Reptilian lungs typically receive air via expansion and contraction of the ribs driven by axial muscles and buccal pumping. Crocodilians also rely on the hepatic piston method, in which the liver is pulled back by a muscle anchored to the pubic bone (part of the pelvis), which in turn pulls the bottom of the lungs backward, expanding them. Turtles, which are unable to move their ribs, instead use their forelimbs and pectoral girdle to force air in and out of the lungs.

Amphibians

The axolotl (Ambystoma mexicanum) retains its larval form with gills into adulthood

The lungs of most frogs and other amphibians are simple and balloon-like, with gas exchange limited to the outer surface of the lung. This is not very efficient, but amphibians have low metabolic demands and can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Amphibians employ a positive pressure system to get air to their lungs, forcing air down into the lungs by buccal pumping. This is distinct from most higher vertebrates, who use a breathing system driven by negative pressure where the lungs are inflated by expanding the rib cage. In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. The throat muscles then presses the throat against the underside of the skull, forcing the air into the lungs.

Due to the possibility of respiration across the skin combined with small size, all known lungless tetrapods are amphibians. The majority of salamander species are lungless salamanders, which respirate through their skin and tissues lining their mouth. This necessarily restrict their size: all are small and rather thread-like in appearance, maximizing skin surface relative to body volume. The only other known lungless tetrapods are the Bornean flat-headed frog (Barbourula kalimantanensis) and Atretochoana eiselti, a caecilian.

The lungs of amphibians typically have a few narrow internal walls (septa) of soft tissue around the outer walls, increasing the respiratory surface area and giving the lung a honey-comb appearance. In some salamanders even these are lacking, and the lung has a smooth wall. In caecilians, as in snakes, only the right lung attains any size or development.

Lungfish

The lungs of lungfish are similar to those of amphibians, with few, if any, internal septa. In the Australian lungfish, there is only a single lung, albeit divided into two lobes. Other lungfish and Polypterus, however, have two lungs, which are located in the upper part of the body, with the connecting duct curving round and above the esophagus. The blood supply also twists around the esophagus, suggesting that the lungs originally evolved in the ventral part of the body, as in other vertebrates.

Invertebrates

Further information: Respiratory system of gastropods

Book lungs of spider (shown in pink)

Some invertebrates have "lungs" that serve a similar respiratory purpose as, but are not evolutionarily related to, vertebrate lungs. Some arachnids, the spiders and the scorpions have structures called "book lungs" used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs. The scorpion has spiracles on its body for the entrance of air to the book lungs.

The coconut crab is terrestrial and uses structures called branchiostegal lungs to breathe air. They cannot swim and would drown in water. They can breathe on land and hold their breath underwater. The branchiostegal lungs are seen as a developmental adaptive stage from water-living to enable land-living, or from fish to amphibian.The crabs also have a rudimentary set of gills.

Pulmonates are mostly land snails and slugs that have developed a simple lung from the mantle cavity. An externally located opening called the pneumostome allows air to be taken into the mantle cavity lung.

Function

Lungs of a raccoon being manually inflated during a dissection

The lungs of mammals including those of humans, have a soft, spongelike texture and are honeycombed with epithelium, having a much larger surface area in total than the outer surface area of the lung itself.

Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward, increasing volume and thus decreasing pressure, causing air to flow into the airways. Air enters through the oral and nasal cavities; it flows through the pharynx, then the larynx and into the trachea, which branches out into the main bronchi and then subsequent divisions. During normal breathing, exhalation is passive and no muscles are contracted (the diaphragm relaxes). The rib cage itself is also able to expand and contract to some degree through the use of the intercostal muscles, together with the action of other respiratory and accessory respiratory muscles. As a result, air is transported into or expelled out of the lungs. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows.

Thermoregulation via panting is seen in some animals, but not humans.

Evolutionary origins

The lungs of today's terrestrial vertebrates and the gas bladders of today's fish are believed to have evolved from simple sacs, as outpocketings of the esophagus, that allowed early fish to gulp air under oxygen-poor conditions. These outpocketings first arose in the bony fish. In most of the ray-finned fish the sacs evolved into closed off gas bladders, while a number of carps, trouts, herrings, catfish, eels have retained the physostome condition with the sack being open to the esophagus. In more basal bony fish, such as the gar, bichir, bowfin and the lobe-finned fish, the bladders have evolved to primarily function as lungs. The lobe-finned fish gave rise to the land-based tetrapods. Thus, the lungs of vertebrates are homologous to the gas bladders of fish (but not to their gills). This is reflected by the fact that the lungs of a fetus also develop from an outpocketing of the esophagus and in the case of the physostome gas bladders, which can serve as both buoyancy organ and with the pneumatic duct to the gut also serve as lungs. This condition is found in more "primitive" teleosts, and is lost in the higher orders. (This is an instance of correlation between ontogeny and phylogeny.) No known animals have both a gas bladder and lungs.

See also

This article uses anatomical terminology; for an overview, see Anatomical terminology.

• Drowning
• Mechanical ventilation
• Pulmonary contusion
• Liquid breathing

Additional images

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  Diagram of the respiratory system

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  Anatomy of lungs

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  Front view of heart and lungs

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  Transverse section of thorax, showing relations of pulmonary artery

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  Human embryo, 38 mm, 8–9 weeks.

Further reading

• Dr D.R. Johnson: Introductory anatomy, respiratory system, leeds.ac.uk
• Franlink Institute Online: The Respiratory System, sln.fi.edu
• Avian lungs and respiration, people.eku.edu