Lungs

Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. To perform this function, the lungs need to be able to capture as much O2 as possible, a task that is easily achieved owing to their elastic morphology. Each lung is enclosed within the visceral pleura Pleura The pleura is a serous membrane that lines the walls of the thoracic cavity and the surface of the lungs. This structure of mesodermal origin covers both lungs, the mediastinum, the thoracic surface of the diaphragm, and the inner part of the thoracic cage. The pleura is divided into a visceral pleura and parietal pleura. Pleura and completely fills 1 of the non-symmetrical pleural cavities, which are situated on the left and right sides of the mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels. The lungs encase the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree and are divided functionally and anatomically into lobes.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Development

Development of the tracheobronchial tree and lungs occurs in 5 stages. The tracheobronchial tree originates from the foregut of the embryonic gut tube, beginning at week 4 of gestation and ending in childhood.

Table: Development of the tracheobronchial tree and lungs, and clinical relevance
Stage Description Clinical relevance
Embryonic period
  • Occurs during weeks 4–7
  • Respiratory diverticulum buds off the foregut.
  • Tracheoesophageal groove “pinches off” the bud → trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea and esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus
  • Trachea bifurcates into right and left bronchial buds.
Defects:
Pseudoglandular period
  • Occurs during weeks 5–16
  • Bronchial buds → secondary buds → tertiary buds
  • Continued branching → terminal bronchioles
  • Mesoderm → pulmonary vasculature/ capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries
  • Development of type II pneumocyte precursors → produce amniotic fluid
  • Defects:
    • Bronchogenic cyst
    • Bronchial atresia
  • Lung tissue is incapable of gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange at this stage.
  • Infants born at this stage cannot survive.
Canalicular period
  • Occurs during weeks 16–26
  • Terminal bronchioles → respiratory bronchioles → alveolar ducts → primitive alveoli
  • Prominent lung capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries
  • Surfactant production
  • Airway diameter ↑
  • Defects:
    • Pulmonary hypoplasia Pulmonary hypoplasia Pulmonary hypoplasia is the lack of normal fetal development of the pulmonary parenchyma. The condition is characterized by a decreased number of alveoli and bronchial generations. Oligohydramnios is a notable cause, but conditions that restrict lung development or lead to fetal lung compression can also result in pulmonary hypoplasia. Pulmonary Hypoplasia
    • Respiratory distress syndrome
  • Respiration is possible at 24 weeks.
  • Infants born at the end of this stage can survive with intensive care.
Saccular period
  • Occurs during week 26–birth
  • Alveolar ducts → terminal sacs
  • Gas-exchange surface area of the lungs expands.
  • Surfactant production increases.
  • Blood–air barrier develops fully (type I pneumocytes).
Infants born after 32 weeks or more have a higher survival rate.
Alveolar period
  • Occurs between 32 weeks’ gestation to 8 years of life
  • Mature type II pneumocytes
  • Terminal sacs septate → alveoli
  • Following birth, alveoli ↑ in number:
    • At birth: 50 million
    • By 8 years of age: 300 million
  • In utero: ↑ vascular resistance due to aspiration of amniotic fluid
  • Postpartum: Inspiration of air leads to a drop in pulmonary vascular resistance.
Bronchial buds development

Early stage of lung development:
During the early (embryonic and pseudoglandular) stages of lung development, the lung bud gradually transforms into the trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea and then bifurcates, forming the 2 mainstem bronchi. These bronchi further divide, giving rise to lobar and segmental bronchi.

Image: “Weeks 4–7” by Phil Schatz. License: CC BY 4.0

Gross Anatomy

General characteristics

  • The lungs are paired viscera composed of spongy tissue. 
  • Asymmetrical, with each lung having a slightly different morphology and weight: 
    • Left lung is slightly smaller than the right lung.
    • On average, lungs are heavier in men than in women.
  • Location and spatial relations:
    • Located in the thoracic cavity on either side of the mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels
    • Surround the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree
    • Enclosed within the visceral pleura Pleura The pleura is a serous membrane that lines the walls of the thoracic cavity and the surface of the lungs. This structure of mesodermal origin covers both lungs, the mediastinum, the thoracic surface of the diaphragm, and the inner part of the thoracic cage. The pleura is divided into a visceral pleura and parietal pleura. Pleura
    • Attached to the trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea via the main bronchi of the tracheobronchial tree
    • Attached to the heart via pulmonary vessels
  • Basic structure: 
    • Each lung has a base, apex, 2 surfaces, and 3 borders:
      • The base sits on the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm.
      • The apex projects above the 1st rib.
      • 1 surface faces the ribs (costal); the other faces medially (mediastinal) and contains the hilum
      • 3 borders: inferior, posterior, and anterior
  • Hilum:
    • Located between T5 and T7
    • Constitutes the roots of the lungs, a pleura Pleura The pleura is a serous membrane that lines the walls of the thoracic cavity and the surface of the lungs. This structure of mesodermal origin covers both lungs, the mediastinum, the thoracic surface of the diaphragm, and the inner part of the thoracic cage. The pleura is divided into a visceral pleura and parietal pleura. Pleura-encased passageway for the pulmonary neurovasculature
    • The following structures pass through the hilum of each lung:
      • Principal bronchus
      • Pulmonary artery
      • 2 pulmonary veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins
      • Bronchial vessels
      • Pulmonary autonomic plexus
      • Lymph nodes and vessels
      • Connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue
The basic structure of the lung

The basic structure of the lung:
The lung is composed of an apex, a base, 3 borders (inferior, anterior, and posterior), and 2 surfaces (mediastinal and costal).

Image by Lecturio. License: CC BY-NC-SA 4.0

Right versus left lung

The 2 lungs are not symmetrical and vary both in weight and morphology. This difference is due to the size and location of the heart in the left hemithorax. 

Right lung:

  • 3 lobes: superior, middle, and inferior 
  • 2 fissures: oblique and horizontal 
  • Its medial or mediastinal surface is adjacent to:
    • Heart
    • Inferior vena cava
    • Superior vena cava
    • Azygos vein 
    • Esophagus

Left lung:

  • 2 lobes: superior and inferior
  • 1 fissure: oblique
  • Its medial or mediastinal surface is adjacent to:
    • Heart
    • Aortic arch Aortic arch The branchial arches, also known as pharyngeal or visceral arches, are embryonic structures seen in the development of vertebrates that serve as precursors for many structures of the face, neck, and head. These arches are composed of a central core of mesoderm, which is covered externally by ectoderm and internally by endoderm. Branchial Apparatus and Aortic Arches
    • Thoracic aorta
    • Esophagus 
  • The cardiac notch is located on the anteroinferior aspect of the superior lobe of the left lung on the anterior border to make room for the heart.
Lobes and fissures of the lungs

Lobes and fissures of the lungs

Image by Lecturio. License: CC BY-NC-SA 4.0

Bronchopulmonary segments

Beyond the lobes and following the branching of the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree, the lungs are divided into bronchopulmonary segments, which are the largest functional divisions of the pulmonary lobes.

  • Each segment has its own air and blood supplies.
    • Air supply: tertiary or segmental bronchi
    • Blood supply: branch of the bronchial arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
  • Asymmetrical distribution of segments between the right and left lungs:
    • Right lung: 10 segments
    • Left lung: 8–10 segments (some may fuse)
Table: Segments of the lungs
Left lung Right lung
Superior lobe:
  • Apico-posterior segment (I + II)
  • Anterior segment (III)
  • Superior lingular segment (IV)
  • Inferior lingular segment (V)
Superior lobe:
  • Apical segment (I)
  • Posterior segment (II)
  • Anterior segment (III)
Middle lobe:
  • Lateral segment (IV)
  • Medial segment (V)
Inferior lobe:
  • Superior segment (VI)
  • Anterior-basal segment (VII + VIII)
  • Lateral-basal segment (IX)
  • Posterior-basal segment (X)
Inferior lobe:
  • Superior segment (VI)
  • Medial-basal segment (VII)
  • Anterior-basal segment (VIII)
  • Lateral-basal segment (IX)
  • Posterior-basal segment (X)
The bronchopulmonary segments of the lungs (1)

The bronchopulmonary segments of the lungs

Image by Lecturio. License: CC BY-NC-SA 4.0

Microscopic Anatomy

The microscopic anatomy of the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree varies as the airways ramify. In general, the respiratory tract becomes smaller in diameter and has thinner walls with every ramification.

Table: Microscopic anatomy of the lungs
Bronchi Bronchioles Respiratory bronchioles Alveoli
Epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium Pseudostratified ciliated columnar epithelium Ciliated columnar to cuboidal epithelium Non-ciliated simple cuboidal epithelium Simple squamous epithelium
Special cells Goblet cells (mucus) Club (a component of surfactant) Club (a component of surfactant) Type I and II pneumocytes
Smooth muscle X
Cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage X X X

Alveoli

  • Hollow sacs that serve as the site of gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange
  • Found in respiratory bronchioles along the walls ducts, and in the alveolar sacs
  • Separated by interalveolar septa, which is made of elastic fibers and capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries
  • Lined by type I and II pneumocytes:
    • Type I: comprise 95% of the total alveolar area and form the blood–air barrier
    • Type II: comprise 5% of the total alveolar area and secrete surfactant
Alveolar stage

Representation of pulmonary histology in the alveolar stage:
1: Respiratory bronchiole
2: Primary septum
3: Alveolar sac
4: Capillaries
5: Type II pneumocyte
6: Type I pneumocyte
7: Alveolar duct

Image by Lecturio. License: CC BY-NC-SA 4.0

Neurovasculature

Blood supply

  • The vessels enter the lungs at the hilum.
  • The lung has a dual circulatory system:
    • Pulmonary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries:
      • Originate as the pulmonary trunk
      • Branch into left and right pulmonary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries and then into lobar and segmental branches according to the branching of the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree
      • Carry oxygen-poor blood from the right ventricle to be oxygenated in the lungs
    • Pulmonary veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins:
      • Originate from the millions of alveoli in the lungs
      • The tributaries merge following the branching of the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree.
      • Become the right and left, and superior and inferior pulmonary veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins
      • Drain into the left atrium as 4 veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins 
      • Carry oxygen-rich blood from the lungs to the left atrium to be distributed throughout the body

Innervation

The lung receives mixed innervation from the pulmonary plexus containing the parasympathetic, sympathetic, and visceral afferent fibers.

  • Parasympathetic fibers: 
    • Presynaptic fibers from the vagus nerve
    • These fibers synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission with parasympathetic ganglion cells.
    • Motor innervation to the smooth muscle of the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree (constrictor), pulmonary vessels (dilator), and glands of the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree (secretory-motor)
  • Sympathetic fibers:
    • Postsynaptic fibers with cell bodies in the paravertebral sympathetic ganglia (sympathetic trunks)
    • Innervation to the smooth muscle of the bronchial tree Bronchial tree The collective term "bronchial tree" refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further. Bronchial Tree (dilator), pulmonary vessels (constrictor), and type II secretory cells of alveoli (inhibitor)
  • Visceral fibers:
    • Reflexive: conduct subconscious sensations connected to the regulation and control of reflexes (accompany parasympathetic fibers)
    • Nociceptive: pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain as a response to injury (accompany sympathetic fibers)

Functions

The main role of the lungs is to oxygenate the body and rid it of CO₂. 

Ventilation:

  • Movement of air between the environment and the lungs via inhalation and exhalation
  • Requires assistance of the intercostal muscles, diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm, and accessory muscles of respiration

Respiration or gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange:

  • Removal of CO₂ from the blood into the air, and absorption of oxygen from environmental air into the blood
  • Relies on the concentration and structural integrity of hemoglobin

Respiratory regulation Respiratory regulation Human cells are reliant on aerobic metabolism. Chemoreceptors in the lungs and tissues sense changes in the concentration of respiratory gasses and send messages to the CNS, which, in turn, modifies breathing parameters such as the respiratory rate or tidal volume to compensate for any imbalance. Disruption of this control mechanism can be caused by severe disease and also result in severe disease. Respiratory Regulation:

  • By the chemoreceptors in the lungs and tissues that sense changes in the concentration of oxygen and CO₂
  • By the respiratory center of the brain, with constant feedback from the central and peripheral chemoreceptors

Clinical Relevance

The following conditions of various origins can affect the lungs:

  • Infectious:
    • Pneumonia Pneumonia Pneumonia or pulmonary inflammation is an acute or chronic inflammation of lung tissue. Causes include infection with bacteria, viruses, or fungi. In more rare cases, pneumonia can also be caused through toxic triggers through inhalation of toxic substances, immunological processes, or in the course of radiotherapy. Pneumonia: acute or chronic inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the lung tissue commonly known as pulmonary inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation. Causes of pneumonia include bacterial, viral, or fungal infections. In rare cases, pneumonia can also be caused by toxic triggers via inhalation, in immunological processes, or during the course of radiotherapy.
    • Tuberculosis Tuberculosis Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis complex bacteria. The bacteria usually attack the lungs but can also damage other parts of the body. Approximately 30% of people around the world are infected with this pathogen, with the majority harboring a latent infection. Tuberculosis spreads through the air when a person with active pulmonary infection coughs or sneezes. Tuberculosis: an infectious disease caused by bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology: Overview of the Mycobacterium Mycobacterium Mycobacterium is a genus of the family Mycobacteriaceae in the phylum Actinobacteria. Mycobacteria comprise more than 150 species of facultative intracellular bacilli that are mostly obligate aerobes. Mycobacteria are responsible for multiple human infections including serious diseases, such as tuberculosis (M. tuberculosis), leprosy (M. leprae), and M. avium complex infections. Mycobacterium tuberculosis complex, which usually attack the lungs, but can also affect other parts of the body. Tuberculosis Tuberculosis Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis complex bacteria. The bacteria usually attack the lungs but can also damage other parts of the body. Approximately 30% of people around the world are infected with this pathogen, with the majority harboring a latent infection. Tuberculosis spreads through the air when a person with active pulmonary infection coughs or sneezes. Tuberculosis spreads through air when an individual with an active pulmonary infection coughs or sneezes. Diagnosis is established using the tuberculin skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin test, sputum culture, and lung imaging. Management is with antimycobacterial drugs.
    • COVID-19: an infectious disease caused by SARS-CoV-2 that mainly affects the respiratory system, but can also damage other organs. Coronavirus Coronavirus Coronaviruses are a group of related viruses that contain positive-sense, single-stranded RNA. Coronavirus derives its name from "κορώνη korṓnē" in Greek, which translates as "crown," after the small club-shaped proteins visible as a ring around the viral envelope in electron micrographs. Coronavirus disease 2019 may present asymptomatically as a mild “flu-like” illness, or severely, with shortness of breath and life-threatening complications. Management is based on supportive care.
  • Malignant:
    • Lung cancer Lung cancer Lung cancer is the malignant transformation of lung tissue and the leading cause of cancer-related deaths. The majority of cases are associated with long-term smoking. The disease is generally classified histologically as either small cell lung cancer or non-small cell lung cancer. Symptoms include cough, dyspnea, weight loss, and chest discomfort. Lung Cancer: a condition associated with a relatively high risk of mortality. Lung cancer Lung cancer Lung cancer is the malignant transformation of lung tissue and the leading cause of cancer-related deaths. The majority of cases are associated with long-term smoking. The disease is generally classified histologically as either small cell lung cancer or non-small cell lung cancer. Symptoms include cough, dyspnea, weight loss, and chest discomfort. Lung Cancer has become the leading cause of death among men worldwide. This malignant tumor originates from the respiratory epithelium of the bronchi, bronchioles, and alveoli. In 90% of men and 80% of women, bronchial carcinoma appears to be associated with the inhalation of tobacco smoke.
  • Obstructive:
    • Asthma Asthma Asthma is a chronic inflammatory respiratory condition characterized by bronchial hyperresponsiveness and airflow obstruction. The disease is believed to result from the complex interaction of host and environmental factors that increase disease predisposition, with inflammation causing symptoms and structural changes. Patients typically present with wheezing, cough, and dyspnea. Asthma: a chronic inflammatory respiratory condition characterized by bronchial hyperresponsiveness and airflow obstruction. Asthma Asthma Asthma is a chronic inflammatory respiratory condition characterized by bronchial hyperresponsiveness and airflow obstruction. The disease is believed to result from the complex interaction of host and environmental factors that increase disease predisposition, with inflammation causing symptoms and structural changes. Patients typically present with wheezing, cough, and dyspnea. Asthma presents with wheezing Wheezing Wheezing is an abnormal breath sound characterized by a whistling noise that can be relatively high-pitched and shrill (more common) or coarse. Wheezing is produced by the movement of air through narrowed or compressed small (intrathoracic) airways. Wheezing, cough, and dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea. The diagnosis is confirmed with pulmonary function tests, which show a reversible obstructive pattern. Treatment of asthma is with bronchodilators and inhaled corticosteroids. 
    • Chronic obstructive pulmonary disease Chronic obstructive pulmonary disease Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD) ( COPD COPD Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD)): a lung disease characterized by progressive, irreversible airflow obstruction. Chronic obstructive pulmonary disease Chronic obstructive pulmonary disease Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD) presents with progressive dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea, chronic cough, prolonged expiration, and wheezing Wheezing Wheezing is an abnormal breath sound characterized by a whistling noise that can be relatively high-pitched and shrill (more common) or coarse. Wheezing is produced by the movement of air through narrowed or compressed small (intrathoracic) airways. Wheezing. The diagnosis is based on pulmonary function tests. Management includes smoking cessation, pulmonary rehabilitation, and pharmacotherapy.
    • Emphysema: a condition characterized by dilation of the airways, with decreased elasticity and increased compliance, due to destruction of the alveolar walls. The most common cause of emphysema is smoking. Alpha-1-antitrypsin deficiency may also result in the development of emphysema.
    • Atelectasis Atelectasis Atelectasis is the partial or complete collapse of a part of the lung. Atelectasis is almost always a secondary phenomenon from conditions causing bronchial obstruction, external compression, surfactant deficiency, or scarring. Atelectasis: a partial or complete collapse of a part of the lung, usually as a secondary phenomenon from conditions causing bronchial obstruction, external compression, surfactant deficiency, or scarring. Patients are often asymptomatic and the diagnosis is made by imaging. The management of atelectasis includes treatment of the underlying etiology, lung-expansion exercises, and chest physiotherapy.
  • Vascular:
    • Pulmonary embolism Pulmonary Embolism Pulmonary embolism (PE) is a potentially fatal condition that occurs as a result of intraluminal obstruction of the main pulmonary artery or its branches. The causative factors include thrombi, air, amniotic fluid, and fat. In PE, gas exchange is impaired due to the decreased return of deoxygenated blood to the lungs. Pulmonary Embolism: a potentially fatal intraluminal obstruction of the main pulmonary artery or its branches by an embolus. The most common presenting symptom is dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea. The diagnosis of pulmonary embolism is by radiographic imaging. The initial management is supportive and focuses on restoring oxygenation and hemodynamic stability.
    • Pulmonary hypertension Pulmonary Hypertension Pulmonary hypertension (PH) or pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary arterial pressure, which can lead to chronic progressive right heart failure. Pulmonary hypertension is grouped into 5 categories based on etiology, which include primary PAH, and PH due to cardiac disease, lung or hypoxic disease, chronic thromboembolic disease, and multifactorial or unclear etiologies. Pulmonary Hypertension: a condition associated with elevated pulmonary arterial pressure, which can lead to chronic progressive right heart failure. Pulmonary hypertension Pulmonary Hypertension Pulmonary hypertension (PH) or pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary arterial pressure, which can lead to chronic progressive right heart failure. Pulmonary hypertension is grouped into 5 categories based on etiology, which include primary PAH, and PH due to cardiac disease, lung or hypoxic disease, chronic thromboembolic disease, and multifactorial or unclear etiologies. Pulmonary Hypertension presents as progressive dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea, and is either primary or occurs due to other medical conditions. Diagnosis is based on echocardiography, ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Normal Electrocardiogram (ECG), chest X-ray, and pulmonary function tests. Management is aimed at treating the underlying etiology. 
    • Cor pulmonale Cor Pulmonale Cor pulmonale is right ventricular (RV) dysfunction caused by lung disease that results in pulmonary artery hypertension. The most common cause of cor pulmonale is chronic obstructive pulmonary disease. Dyspnea is the usual presenting symptom. Cor Pulmonale: right ventricular dysfunction caused by lung disease, which results in pulmonary hypertension. Cor pulmonale Cor Pulmonale Cor pulmonale is right ventricular (RV) dysfunction caused by lung disease that results in pulmonary artery hypertension. The most common cause of cor pulmonale is chronic obstructive pulmonary disease. Dyspnea is the usual presenting symptom. Cor Pulmonale is caused by COPD COPD Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD). Dyspnea is the usual presenting symptom, and signs of right-sided heart failure and hypoxemia may be observed. Most patients are diagnosed clinically. Initial treatment is by using oxygen therapy and is targeted at identifying the underlying disease.
  • Interstitial:
    • Idiopathic pulmonary fibrosis Pulmonary Fibrosis Idiopathic pulmonary fibrosis is a specific entity of the major idiopathic interstitial pneumonia classification of interstitial lung diseases. As implied by the name, the exact causes are poorly understood. Patients often present in the moderate to advanced stage with progressive dyspnea and nonproductive cough. Pulmonary Fibrosis: a specific entity of the major idiopathic interstitial pneumonia classification within interstitial lung diseases Interstitial Lung Diseases Interstitial lung diseases are a heterogeneous group of disorders characterized by the inflammation and fibrosis of lung parenchyma, especially the pulmonary connective tissue in the alveolar walls. It may be idiopathic (e.g., idiopathic pulmonary fibrosis) or secondary to connective tissue diseases, medications, malignancies, occupational exposure, or allergens. Interstitial Lung Diseases. Idiopathic pulmonary fibrosis Pulmonary Fibrosis Idiopathic pulmonary fibrosis is a specific entity of the major idiopathic interstitial pneumonia classification of interstitial lung diseases. As implied by the name, the exact causes are poorly understood. Patients often present in the moderate to advanced stage with progressive dyspnea and nonproductive cough. Pulmonary Fibrosis is often characterized by its rapid progression. The average life expectancy is 3–4 years from diagnosis. Lung transplantation is the only curative intervention. 
    • Hypersensitivity pneumonitis Hypersensitivity Pneumonitis Hypersensitivity pneumonitis (HP), previously called extrinsic allergic alveolitis, is an immunologically induced inflammatory disease affecting the alveoli, bronchioles, and lung parenchyma. It is caused by repeated inhalation of an inciting agent in a susceptible host that triggers first a type III (complement-mediated) hypersensitivity reaction in the acute phase and then a type IV (delayed) reaction in the subacute and chronic phases. Hypersensitivity Pneumonitis: an immunologically induced inflammatory disease caused by repeated inhalation of an inciting agent that triggers a type III and IV hypersensitivity reaction. Hypersensitivity pneumonitis Hypersensitivity Pneumonitis Hypersensitivity pneumonitis (HP), previously called extrinsic allergic alveolitis, is an immunologically induced inflammatory disease affecting the alveoli, bronchioles, and lung parenchyma. It is caused by repeated inhalation of an inciting agent in a susceptible host that triggers first a type III (complement-mediated) hypersensitivity reaction in the acute phase and then a type IV (delayed) reaction in the subacute and chronic phases. Hypersensitivity Pneumonitis presents as cough, fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, and malaise. Diagnosis is aided by high-resolution CT scans and analysis of the bronchoalveolar lavage fluid. Management is with steroids and by avoiding exposure to the inciting agent.

References

  1. Moore, Keith, L., et al. Clinically Oriented Anatomy. Lippincott Williams & Wilkins, 2017.
  2. Drake, Richard, et al. Gray’s Anatomy for Students E-Book. Elsevier Health Sciences, 2014.
  3. Standring, S. Gray’s anatomy. The Anatomical Basis of Clinical Practice (41st ed.). Edinburgh: Churchill Livingstone/Elsevier, 2016.

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