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Lungs: Anatomy

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 Elastic Connective Tissue: Histology morphology. Each lung is enclosed within the visceral pleura Visceral pleura Pleura: Anatomy and completely fills 1 of the non- symmetrical Symmetrical Dermatologic Examination 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: Anatomy. 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: Anatomy and are divided functionally and anatomically into lobes.

Last updated: 9 Mar, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Development

Development of the tracheobronchial tree and lungs occurs in 5 stages. The tracheobronchial tree originates from the foregut Foregut Development of the Abdominal Organs 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 Diverticulum A pouch or sac opening from the colon. Diverticular Disease buds off the foregut Foregut Development of the Abdominal Organs.
  • Tracheoesophageal groove Tracheoesophageal groove Development of the Respiratory System “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: Anatomy 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: Anatomy
  • 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: Anatomy 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 Bronchioles The small airways branching off the tertiary bronchi. Terminal bronchioles lead into several orders of respiratory bronchioles which in turn lead into alveolar ducts and then into pulmonary alveoli. Bronchial Tree: Anatomy
  • Mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation → 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: Histology
  • Development of type II pneumocyte precursors → produce amniotic fluid Amniotic fluid A clear, yellowish liquid that envelopes the fetus inside the sac of amnion. In the first trimester, it is likely a transudate of maternal or fetal plasma. In the second trimester, amniotic fluid derives primarily from fetal lung and kidney. Cells or substances in this fluid can be removed for prenatal diagnostic tests (amniocentesis). Placenta, Umbilical Cord, and Amniotic Cavity
  • Defects:
  • 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 Bronchioles The small airways branching off the tertiary bronchi. Terminal bronchioles lead into several orders of respiratory bronchioles which in turn lead into alveolar ducts and then into pulmonary alveoli. Bronchial Tree: Anatomy → respiratory bronchioles Bronchioles The small airways branching off the tertiary bronchi. Terminal bronchioles lead into several orders of respiratory bronchioles which in turn lead into alveolar ducts and then into pulmonary alveoli. Bronchial Tree: Anatomy → alveolar ducts → primitive alveoli Alveoli Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. Acute Respiratory Distress Syndrome (ARDS)
  • 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: Histology
  • Surfactant Surfactant Substances and drugs that lower the surface tension of the mucoid layer lining the pulmonary alveoli. Acute Respiratory Distress Syndrome (ARDS) production
  • Airway Airway ABCDE Assessment diameter ↑
  • Defects:
  • Respiration Respiration The act of breathing with the lungs, consisting of inhalation, or the taking into the lungs of the ambient air, and of exhalation, or the expelling of the modified air which contains more carbon dioxide than the air taken in. Nose and Nasal Cavity: Anatomy 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 Surfactant Substances and drugs that lower the surface tension of the mucoid layer lining the pulmonary alveoli. Acute Respiratory Distress Syndrome (ARDS) 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 Alveoli Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. Acute Respiratory Distress Syndrome (ARDS)
  • Following birth, alveoli Alveoli Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. Acute Respiratory Distress Syndrome (ARDS) ↑ in number:
    • At birth: 50 million
    • By 8 years of age: 300 million
  • In utero: ↑ vascular resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing due to aspiration of amniotic fluid Amniotic fluid A clear, yellowish liquid that envelopes the fetus inside the sac of amnion. In the first trimester, it is likely a transudate of maternal or fetal plasma. In the second trimester, amniotic fluid derives primarily from fetal lung and kidney. Cells or substances in this fluid can be removed for prenatal diagnostic tests (amniocentesis). Placenta, Umbilical Cord, and Amniotic Cavity
  • Postpartum: Inspiration Inspiration Ventilation: Mechanics of Breathing of air leads to a drop in pulmonary vascular resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing.
Bronchial buds development

Early stage of lung development:
During the early (embryonic and pseudoglandular) stages of lung development, the lung bud Lung bud Development of the Respiratory System 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: Anatomy and then bifurcates, forming the 2 mainstem bronchi Bronchi The larger air passages of the lungs arising from the terminal bifurcation of the trachea. They include the largest two primary bronchi which branch out into secondary bronchi, and tertiary bronchi which extend into bronchioles and pulmonary alveoli. Bronchial Tree: Anatomy. These bronchi Bronchi The larger air passages of the lungs arising from the terminal bifurcation of the trachea. They include the largest two primary bronchi which branch out into secondary bronchi, and tertiary bronchi which extend into bronchioles and pulmonary alveoli. Bronchial Tree: Anatomy further divide, giving rise to lobar and segmental bronchi Bronchi The larger air passages of the lungs arising from the terminal bifurcation of the trachea. They include the largest two primary bronchi which branch out into secondary bronchi, and tertiary bronchi which extend into bronchioles and pulmonary alveoli. Bronchial Tree: Anatomy.

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: Anatomy
    • 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: Anatomy
    • Enclosed within the visceral pleura Visceral pleura Pleura: Anatomy
    • 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: Anatomy via the main bronchi Bronchi The larger air passages of the lungs arising from the terminal bifurcation of the trachea. They include the largest two primary bronchi which branch out into secondary bronchi, and tertiary bronchi which extend into bronchioles and pulmonary alveoli. Bronchial Tree: Anatomy 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: Anatomy.
      • The apex projects above the 1st rib.
      • 1 surface faces the ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy (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: Anatomy-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: Histology
      • Bronchial vessels
      • Pulmonary autonomic plexus
      • Lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy 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: Histology
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).

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Right versus left lung

The 2 lungs are not symmetrical Symmetrical Dermatologic Examination 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 Inferior vena cava The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy
    • Superior vena cava Superior vena cava The venous trunk which returns blood from the head, neck, upper extremities and chest. Mediastinum and Great Vessels: Anatomy
    • Azygos vein 
    • 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: Anatomy

Left lung:

Lobes and fissures of the lungs

Lobes and fissures of the lungs

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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: Anatomy, 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 Bronchi The larger air passages of the lungs arising from the terminal bifurcation of the trachea. They include the largest two primary bronchi which branch out into secondary bronchi, and tertiary bronchi which extend into bronchioles and pulmonary alveoli. Bronchial Tree: Anatomy
    • 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: Histology
  • 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: Anatomy 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 Bronchi The larger air passages of the lungs arising from the terminal bifurcation of the trachea. They include the largest two primary bronchi which branch out into secondary bronchi, and tertiary bronchi which extend into bronchioles and pulmonary alveoli. Bronchial Tree: Anatomy Bronchioles Bronchioles The small airways branching off the tertiary bronchi. Terminal bronchioles lead into several orders of respiratory bronchioles which in turn lead into alveolar ducts and then into pulmonary alveoli. Bronchial Tree: Anatomy Respiratory bronchioles Bronchioles The small airways branching off the tertiary bronchi. Terminal bronchioles lead into several orders of respiratory bronchioles which in turn lead into alveolar ducts and then into pulmonary alveoli. Bronchial Tree: Anatomy Alveoli Alveoli Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. Acute Respiratory Distress Syndrome (ARDS)
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: Histology Pseudostratified ciliated columnar 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: Histology Ciliated columnar to cuboidal 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: Histology Non-ciliated simple cuboidal epithelium Simple cuboidal epithelium Surface Epithelium: Histology Simple squamous epithelium Simple squamous epithelium Surface Epithelium: Histology
Special cells Goblet cells Goblet cells A glandular epithelial cell or a unicellular gland. Goblet cells secrete mucus. They are scattered in the epithelial linings of many organs, especially the small intestine and the respiratory tract. Glandular Epithelium: Histology (mucus) Club (a component of surfactant Surfactant Substances and drugs that lower the surface tension of the mucoid layer lining the pulmonary alveoli. Acute Respiratory Distress Syndrome (ARDS)) Club (a component of surfactant Surfactant Substances and drugs that lower the surface tension of the mucoid layer lining the pulmonary alveoli. Acute Respiratory Distress Syndrome (ARDS)) 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: Histology X X X

Alveoli Alveoli Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. Acute Respiratory Distress Syndrome (ARDS)

  • 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 Bronchioles The small airways branching off the tertiary bronchi. Terminal bronchioles lead into several orders of respiratory bronchioles which in turn lead into alveolar ducts and then into pulmonary alveoli. Bronchial Tree: Anatomy along the walls ducts, and in the alveolar sacs
  • Separated by interalveolar septa, which is made of elastic Elastic Connective Tissue: Histology 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: Histology
  • 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 Surfactant Substances and drugs that lower the surface tension of the mucoid layer lining the pulmonary alveoli. Acute Respiratory Distress Syndrome (ARDS)
Alveolar stage

Representation of pulmonary histology in the alveolar stage Alveolar Stage During the alveolar stage, the respiratory units continue to grow in number and maturity. Development of the Respiratory System:
1: Respiratory bronchiole
2: Primary septum
3: Alveolar sac
4: 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: Histology
5: Type II pneumocyte
6: Type I pneumocyte
7: Alveolar duct

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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: Histology:
      • Originate as the pulmonary trunk Pulmonary Trunk Truncus Arteriosus
      • 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: Histology 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: Anatomy
      • 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: Histology:
      • Originate from the millions of alveoli Alveoli Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. Acute Respiratory Distress Syndrome (ARDS) 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: Anatomy.
      • 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: Histology
      • 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: Histology 
      • 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 Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology fibers.

  • Parasympathetic fibers: 
    • Presynaptic fibers from the vagus nerve Vagus nerve The 10th cranial nerve. The vagus is a mixed nerve which contains somatic afferents (from skin in back of the ear and the external auditory meatus), visceral afferents (from the pharynx, larynx, thorax, and abdomen), parasympathetic efferents (to the thorax and abdomen), and efferents to striated muscle (of the larynx and pharynx). Pharynx: Anatomy
    • 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 Ganglion Cells The Visual Pathway and Related Disorders.
    • Motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology 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: Anatomy (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: Anatomy (secretory- motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology)
  • 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: Anatomy (dilator), pulmonary vessels (constrictor), and type II secretory cells of alveoli Alveoli Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. Acute Respiratory Distress Syndrome (ARDS) (inhibitor)
  • Visceral fibers:
    • Reflexive: conduct subconscious sensations connected to the regulation and control of reflexes (accompany parasympathetic fibers)
    • Nociceptive: pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways 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 Ventilation The total volume of gas inspired or expired per unit of time, usually measured in liters per minute. Ventilation: Mechanics of Breathing:

  • Movement of air between the environment and the lungs via inhalation and exhalation
  • Requires assistance of the intercostal muscles Intercostal Muscles Respiratory muscles that arise from the lower border of one rib and insert into the upper border of the adjoining rib, and contract during inspiration or respiration. Chest Wall: Anatomy, 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: Anatomy, and accessory muscles of respiration Respiration The act of breathing with the lungs, consisting of inhalation, or the taking into the lungs of the ambient air, and of exhalation, or the expelling of the modified air which contains more carbon dioxide than the air taken in. Nose and Nasal Cavity: Anatomy

Respiration Respiration The act of breathing with the lungs, consisting of inhalation, or the taking into the lungs of the ambient air, and of exhalation, or the expelling of the modified air which contains more carbon dioxide than the air taken in. Nose and Nasal Cavity: Anatomy 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 Absorption Absorption involves the uptake of nutrient molecules and their transfer from the lumen of the GI tract across the enterocytes and into the interstitial space, where they can be taken up in the venous or lymphatic circulation. Digestion 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 Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification, with constant feedback from the central and peripheral chemoreceptors Peripheral chemoreceptors Respiratory Regulation