Human lung

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This article is about the human lung. For lungs in general, see Lung.
Lung
Lungs diagram detailed.svg
Detailed diagram of the lungs coloured for identification of the lobes
Details
Latin pulmo
System Respiratory system
Identifiers
Gray's p.1093
MeSH A04.411
TA A06.5.01.001
FMA 7195
Anatomical terminology

The human lungs are the organs of respiration. Humans have two lungs, a right lung and a left lung. The right lung consists of three lobes while the left lung is slightly smaller consisting of only two lobes (the left lung has a "cardiac notch" allowing space for the heart within the chest).[1] Together, the lungs contain approximately 2,400 kilometres (1,500 mi) of airways and 300 to 500 million alveoli.

Estimates of the total surface area of lungs vary from 30-50 square metres[2] 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.[3] 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 lungs together weigh approximately 1.3 kilograms (2.9 lb), with the right lung weighing more than the left.

The pleural cavity is the potential space between the two serous membranes, the pulmonary pleurae; the parietal pleura, lining the inner wall of the rib cage, and the visceral pleura, lining the organs themselves–the lungs. The respiratory system includes the conducting zone, which is part of the respiratory tract, that conducts air into the lungs.

The parenchyma of the lung only relates to functional alveolar tissue, but the term is often used to refer to all lung tissue, including the respiratory bronchioles, alveolar ducts, terminal bronchioles, and all connecting tissues.[4]

Structure[edit]

"Meet the lungs" from the Khan academy

The lungs are located in the chest on either side of the heart close to the backbone, and are enclosed and protected by the rib cage. The left lung has a lateral indentation, the cardiac notch, where the heart is positioned in the left chest. Both lungs have broad bases enabling them to rest on the diaphragm without causing displacement, with the right lung base being elevated as compared to the left due to its shorter length. Each lung has a central recession called the root of the lung, or hilum, which anchors the lungs to the windpipe. The bronchi and pulmonary vessels extend from the heart and the trachea to connect each lung by way of the root.

Each lung is divided into lobes, then subdivided into segments, and ultimately lobules. The right lung has more lobes and segments than the left, with an upper, middle and lower lobe. These are then further delineated, with the upper lobe having an apical, anterior and posterior segment, the middle lobe having a medial and lateral segment, and the lower lobe having five segments, superior, anterior, posterior, medial and lateral.[5]

The left lung has fewer lobes and segments, with the upper lobe being divided into anterior and apicoposterior segments, and the lower lobe being divided into superior, anterior, posterior, medial and lateral segments, similar to the right, but also having superior and inferior lingular segments as an analogue to the right middle lobe. The segmental anatomy is useful clinically for localizing disease processes in the lungs.[5]

There are three lung surfaces: costal, mediastinal and diaphragmatic. The costal surface is the outer thoracic surface which is smooth and convex. This surface area is large and corresponds to the form of the thoracic cavity, being deeper at the back than the front. It is in contact with the ribs, and in lungs that have hardened in situ, grooves are visible reflecting rib indentations. The mediastinal surface of the lung is in contact with the mediastinal pleura and presents the cardiac impression. The diaphragmatic surface is the portion bordering the diaphragm.

The lungs sit within the thoracic cavity surrounded by the pleural cavity, which consists of two pulmonary pleurae. The parietal pleura lies against the rib cage, and the visceral pleura lies on the surface of the lungs. In between the pleurae is pleural fluid. The fluid in the pleural cavity helps to lubricate the lungs, as well as providing surface tension to keep the lungs in contact with the rib cage. Disruption of this surface tension, such as by trauma, infection or cancer, can cause the lung to collapse, a condition known as a pneumothorax. If there is positive pressure in this potential space, a more life threatening condition known as a tension pneumothorax results, in which the pressure prevents the return of deoxygenated blood to the right side of the heart, a medical emergency.[6]

The term lung parenchyma is strictly used to refer solely to alveolar tissue with respiratory bronchioles, alveolar ducts and terminal bronchioles.[4] However, it often includes any form of lung tissue, also including bronchioles, bronchi, blood vessels and interstices, as these structures can be difficult to distinguish from alveolar tissue with surgery or imaging.[4]

Respiratory system[edit]

Main article: Respiratory system

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 bronchi for the lobes of the lungs, then secondary and tertiary bronchi, which split into smaller bronchioles. Bronchioles continue to split into smaller and smaller vessels until the final the terminal bronchioles. These in turn supply air through alveolar ducts into the alveoli, which is where the exchange of gasses take place.

The conducting zone and the respiratory components, except the alveoli, comprise the air passageways, with gas exchange only taking place in the alveoli of the respiratory system.

The conducting zone is reinforced with cartilage in order to hold open the airways. Air is warmed to 37 °C (99 °F), humidified and cleansed by the conduction zone; particles from the air being removed by the cilia which are located on the walls of all the passageways. The lungs are surrounded and protected by the rib cage.

Gross anatomy[edit]

Right lung[edit]

Mediastinal surface of right lung.

The right lung is divided into three lobes, an upper, middle, and a lower, by two fissures, one oblique and one horizontal.

The right lung has a higher volume, total capacity and weight, than that of the left lung. Although the right lung is 5 cm shorter in vertical length, it is broader in diameter due to the presence of the cardiac notch on the left.

Lobes

The middle lobe is the smallest lobe of the right lung. It is wedge-shaped, and includes part of the anterior border, and the front part of the base of the lung. The upper and lower lobes are similar to those of the left lung (which lacks a middle lobe.)

Fissures

A lung fissure is formed from a double fold of visceral pleura that penetrates the lung parenchyma and separates the lung into lobes.[7]

  • 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.[8] Its direction is, however, more vertical, and it cuts the lower border about 7.5 cm. behind its anterior extremity.
  • 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.
Impressions

There is a deep concavity on the mediastinal surface called the cardiac impression, which accommodates the pericardium; this is not as pronounced as that on the left lung where the heart projects further. On the same surface, immediately above the hilum, is an arched furrow which accommodates 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 furrow 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 the middle line.

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[edit]

Diagram showing the left lung, showing (1) Oblique fissure, (2) Vertebral part (3) Hilum of lung, (4) Cardiac impression, and (5) Diaphragmatic surface
Left lung showing root

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 a middle lobe, though it does have a homologous feature, a projection of the upper lobe termed the “lingula”. Its name means “little tongue”. 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.

Surfaces

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.

Impressions

There is a large and deep concavity called the cardiac impression, on the mediastinal surface to accommodate the pericardium. On the same surface, immediately above the hilum, is a well-marked curved furrow produced by the aortic arch, and running upward from this toward the apex is a groove accommodating 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 furrow produced by the descending aorta, and in front of this, near the base of the lung, the lower part of the esophagus causes a shallow impression.

Microscopic anatomy[edit]

Alveolus diagram.svg

Underneath the serous visceral pleura surrounding the lung is a subserous layer of areolar tissue and beneath this is the the lung parenchyma.[9]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 sac, an alveolus, and an alveolar duct.

Cross-section and external view of alveoli

Development[edit]

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.[10] The larynx, trachea, bronchi and lungs begin to form during the fourth week of embryogenesis.[11] 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.[12] 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.[13] From the sixth week to the sixteenth week, the major elements of the lungs appear except the alveoli, which makes survival, if born, impossible.[14] 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 alveolar cells where gas exchange will take place, together with the 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.[15]

First breath[edit]

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.[16] 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 compliment. 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.[17]

Function[edit]

Respiration[edit]

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 alveolar sacs. Alveolar sacs, are made up of clusters of alveoli, like individual grapes within a bunch. The individual alveoli are tightly wrapped in blood vessels 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 haemoglobin of the erythrocytes. The oxygen-rich blood returns to the heart via the pulmonary veins to be pumped back into systemic circulation.[18]

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.[20]

Volumes[edit]

Further information: Lung volumes

Regarding lung volumes and lung capacities include inspiratory reserve volume, tidal volume, expiratory reserve volume, and residual volume.[21] The total lung capacity depends on the person's age, height, weight, sex, and normally ranges between 4,000 and 6,000 cm3 (4 to 6 L). For example, 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, one also measures the tidal volume, the volume breathed in with an average breath, which is about 500 cm3.[22]

Lung function tests include spirometry, measuring the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled.

Other[edit]

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

  • Alter the pH of blood by facilitating alterations in the partial pressure of carbon dioxide
  • Filter out small blood clots formed in veins
  • Filter out gas micro-bubbles occurring in the venous blood stream such as those created during decompression after underwater diving.[23]
  • Influence the concentration of some biologic substances and drugs used in medicine in blood
  • Convert angiotensin I to angiotensin II by the action of angiotensin-converting enzyme
  • 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.[24] Mucus contains glycoproteins, e.g., mucins, lactoferrin,[25] lysozyme, lactoperoxidase.[26][27] We find also on the epithelium Dual oxidase 2[28][29][30] proteins generating hydrogen peroxide, useful for hypothiocyanite endogenous antimicrobial synthesis. Function not in place in cystic fibrosis patient lungs.[31][32]
  • Ciliary escalator action 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 systemic circulation by hemorrhage can be partially compensated for by shunting blood from the lungs into the systemic vessels[33]
  • In 2010, researchers found bitter taste receptors in lung tissue, which cause airways to relax when a bitter substance is encountered. They believe this mechanism is evolutionarily adaptive because it helps clear lung infections, but could also be exploited to treat asthma and chronic obstructive pulmonary disease.[34]

Clinical significance[edit]

Main article: Respiratory disease

Human lungs can be affected by a variety of diseases. The environment of the lung is moist which makes it very hospitable for bacteria. Many respiratory illnesses are due to infectious diseases arising from bacteria or viruses. Tuberculosis is a serious infection as is bacterial pneumonia. Inflammation of the lungs is known as pneumonia; inflammation of the pleurae surrounding the lungs is known as pleurisy.

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 emphysema, a common complication of smoking caused by inflammation and the progressive inability of alveoli to expand and contract with respiration. Fibrotic diseases of the lung occur when the lung is inflamed for a long period of time, whether because of a person's occupation (such as Coalworker's pneumoconiosis) or rarer causes, such as a person's medication.

Pulmonary fibrosis is a condition that can prove fatal. The lung tissue is replaced by fibrous connective tissue which causes irreversible lung scarring.

Lung cancer can often be incurable. Also cancers in other parts of the body can be spread via the bloodstream and end up in the lungs where the malignant cells can metastasise.

A pulmonary embolism is a blood clot that becomes lodged in the lung.

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 CO2/O2 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.

An azygos lobe is a congenital anatomical variation which though usually without effect can cause problems in thoracoscopic procedures.[36]

Measuring lung function[edit]

The function of human lungs is often measured by lung function tests such as spirometry. These measure how much a person is able to inhale (total lung capacity) or exhale (vital capacity). How well and how quickly a person's lungs expel air helps indicate the health of their lungs and whether, if sick, the disease is obstructive (caused by a difficulty getting air to the alveoli, such as in asthma or choking) or restrictive.

Vital capacity is the maximum volume of air that a person can exhale after maximum inhalation; it can be measured with a spirometer. In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease.

See also[edit]

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

Additional images[edit]

References[edit]

This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

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External links[edit]