Trachea

The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx Larynx The larynx, also commonly called the voice box, is a cylindrical space located in the neck at the level of the C3-C6 vertebrae. The major structures forming the framework of the larynx are the thyroid cartilage, cricoid cartilage, and epiglottis. The larynx serves to produce sound (phonation), conducts air to the trachea, and prevents large molecules from reaching the lungs. Larynx and inferiorly becomes 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 within the lungs 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. Lungs. The trachea consists of a support frame of 16–20 semicircular, or C-shaped, rings made out of hyaline 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 and reinforced by collagenous 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 posterior wall of the trachea is free of 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. In this area, the paries membranaceus forms a plate out of smooth tracheal muscle and 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 and forms the border to the dorsally running 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.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Development

The trachea develops as part of the tracheobronchial tree by budding off of the foregut of the embryonic gut tube.

  • Occurs 4–7 weeks after conception
  • Embryonic gut tube: 
    • Forms from the laterally folded endoderm layer 
    • Surrounded by mesoderm
    • Has 3 sections:
      • Foregut
      • Midgut
      • Hindgut
  • Lung bud (respiratory diverticulum): 
    • Buds off of the ventral side of the foregut around week 4
    • Simultaneously grows out (ventrally) and down (caudally)
    • Includes both endoderm and surrounding splanchnopleuric mesoderm
  • Tracheoesophageal groove (or ridge):
    • As the lung bud grows out and down, the tracheoesophageal groove appears as lateral indentations between the new lung bud and the foregut.
    • The grooves/ridges move in medially, pinching off the lung bud, and forming the tracheoesophageal septum.
    • The tracheoesophageal septum creates 2 separate tubes:
      • Esophagus (posteriorly, from the original foregut)
      • Trachea (anteriorly, from the lung bud) 
  • Primary bronchial buds: The trachea bifurcates into the right and left bronchial buds.
    • The primary bronchial buds later continue developing into 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 lungs 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. Lungs through the pseudoglandular, canalicular, saccular, and alveolar stages.

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Gross Anatomy

General characteristics

  • Location:
    • Located in the lower neck and thorax, within the superior 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
    • Continuous superiorly with the larynx Larynx The larynx, also commonly called the voice box, is a cylindrical space located in the neck at the level of the C3-C6 vertebrae. The major structures forming the framework of the larynx are the thyroid cartilage, cricoid cartilage, and epiglottis. The larynx serves to produce sound (phonation), conducts air to the trachea, and prevents large molecules from reaching the lungs. Larynx at the level of C6
    • Ends inferiorly at the level of T5 and continues as the primary bronchi and the rest 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 in the lungs 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. Lungs
  • Shape and structure:
    • The trachea is a D-shaped tubular structure in the lower respiratory tract.
    • Approximately 10–11 cm (3.9–4.3 in.) long, with an inner diameter of 1.5–2 cm (0.59–0.79 in.)
    • Consists of 16–20 semicircular, or C-shaped, rings made out of hyaline 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 reinforced by collagenous 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
    • Has a flat, 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-free, membranous posterior wall that contains smooth muscle called the trachealis muscle (known as paries membranaceus)
Cross sectional structure trachea

The cross-sectional structure of the trachea:
Note the D-shaped, tubular structure and the close proximity to the 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.

Image by Lecturio. License: CC BY-NC-SA 4.0
  • Functions:
    • Transports air in and out of the lungs 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. Lungs during ventilation
    • Warms and moistens the inhaled air in order to protect the respiratory tract
    • Aids in the act of coughing through the trachealis muscle
  • Portions:
    • Cervical portion: 
      • Direct continuation of the larynx Larynx The larynx, also commonly called the voice box, is a cylindrical space located in the neck at the level of the C3-C6 vertebrae. The major structures forming the framework of the larynx are the thyroid cartilage, cricoid cartilage, and epiglottis. The larynx serves to produce sound (phonation), conducts air to the trachea, and prevents large molecules from reaching the lungs. Larynx below the cricoid 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
      • Begins at approximately the level of C6
      • Ends at the level of the jugular notch of the sternum
    • Thoracic portion: 
      • Direct continuation of the cervical trachea
      • Begins at the upper border of the superior 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 (jugular notch of the sternum)
      • Ends at the bifurcation of the trachea into the main bronchi, at the level of T4–T7 (most commonly T5)
Main structure and portions of trachea

The main structure and portions of the trachea

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

Spatial relations

  • Posterior:
    • Esophagus
    • Descending aorta
    • Vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column
  • Anterior:
    • 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 and cervical fascia
    • Jugular venous arch
    • Ascending aorta
    • Brachiocephalic trunk
    • Superior vena cava
    • Thyroid gland Thyroid gland The thyroid gland is one of the largest endocrine glands in the human body. The thyroid gland is a highly vascular, brownish-red gland located in the visceral compartment of the anterior region of the neck. Thyroid Gland and vessels
    • Thymus gland or remnants
  • Lateral:
    • Carotid 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
    • Inferior thyroid 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
    • Recurrent laryngeal nerves
    • Vagus nerves
    • Lungs and 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

Microscopic Anatomy

The tracheal wall consists of 4 layers:

  1. Mucosa: tall, columnar, pseudostratified with cilia and mucin-producing goblet cells
  2. Submucosa: 
    • Connective tissue provides support and elastic recoil
    • Contains elastin fibers, mucus glands, smooth muscle, vessels, nerves, and lymphatics
  3. Musculo-cartilaginous layer: contains the hyaline 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 of the C-shaped rings and the intervening smooth muscle
    • Trachealis muscle:
      • Can alter the dimensions of the trachea
      • Regulates the amount of air coming into or out of the lungs 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. Lungs
  4. Adventitia: composed of fibroelastic, loose 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
Various layers of tissue trachea and larynx

The various layers of tissue that comprise the trachea

Image: “Layers of tissue: trachea and larynx Larynx The larynx, also commonly called the voice box, is a cylindrical space located in the neck at the level of the C3-C6 vertebrae. The major structures forming the framework of the larynx are the thyroid cartilage, cricoid cartilage, and epiglottis. The larynx serves to produce sound (phonation), conducts air to the trachea, and prevents large molecules from reaching the lungs. Larynx” by Phil Schatz. License: CC BY 4.0, edited by Lecturio.

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Neurovaculature

Blood supply

  • Arterial supply:
    • The trachea is mainly supplied by branches of the inferior thyroid 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, branches of the thyrocervical trunk.
    • Irrigation is supported by ascending branches 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
    • These 2 sets of 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 anastomose along the structure of the trachea.
  • Venous drainage: 
    • Analogous 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 inferior thyroid venous plexus.
    • The inferior thyroid 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 brachiocephalic 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.

Innervation

  • The trachea receives innervation from the pulmonary plexus. 
    • Parasympathetic supply: recurrent laryngeal nerves, branches of the vagus nerve
    • Sympathetic supply: sympathetic trunks
  • Within the walls of the tracheobronchial tree, efferent preganglionic axons from the vagus nerves 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 on small ganglia → regulation of local reflexes and airway control

Clinical Relevance

The following conditions of various origins can affect the trachea:

  • Infectious:
    • Laryngitis Laryngitis Laryngitis is an inflammation of the larynx most commonly due to infection or trauma that can be either acute or chronic. In this condition, the 2 folds of mucous membranes that make up the vocal cords become inflamed and irritated. The inflammation results in a distortion of the voice produced, resulting in a hoarse sound or aphonia. Laryngitis is an 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 larynx Larynx The larynx, also commonly called the voice box, is a cylindrical space located in the neck at the level of the C3-C6 vertebrae. The major structures forming the framework of the larynx are the thyroid cartilage, cricoid cartilage, and epiglottis. The larynx serves to produce sound (phonation), conducts air to the trachea, and prevents large molecules from reaching the lungs. Larynx that can occur after an infection of the nasal cavity and pharyngeal region due to viruses or 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, which can get into the trachea. Also, overuse of the voice in a dry air environment can lead to symptoms of laryngitis: reddened pharyngeal mucosa, burning sore throat, and hoarseness or loss of voice. 
  • Congenital:
    • Tracheoesophageal fistula Tracheoesophageal fistula Tracheoesophageal fistula is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula: an abnormal connection between the 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 and the trachea: Tracheoesophageal fistula Tracheoesophageal fistula Tracheoesophageal fistula is an abnormal connection between the trachea and esophagus. Esophageal Atresia and Tracheoesophageal Fistula is a common congenital abnormality, but when it occurs late in life, it is usually a sequela of a surgical procedure.
    • Tracheomalacia Tracheomalacia Laryngomalacia and tracheomalacia are the most common upper airway conditions that produce stridor in newborns. Laryngomalacia and tracheomalacia tend to present in the 1st 2 weeks of life, with symptoms ranging from stridor to respiratory distress. The symptoms are caused by narrowing of the airway, which may be due to weakened cartilage, redundant tissue, external compression, or hypotonia of the affected area. Laryngomalacia and Tracheomalacia: an abnormal collapsing of the walls of the trachea: Tracheomalacia Tracheomalacia Laryngomalacia and tracheomalacia are the most common upper airway conditions that produce stridor in newborns. Laryngomalacia and tracheomalacia tend to present in the 1st 2 weeks of life, with symptoms ranging from stridor to respiratory distress. The symptoms are caused by narrowing of the airway, which may be due to weakened cartilage, redundant tissue, external compression, or hypotonia of the affected area. Laryngomalacia and Tracheomalacia is due to a 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 disorder or can be caused by an external source ( vascular rings Vascular rings Vascular rings are a group of rare malformations featuring congenital abnormalities of the aortic arch. The aberrant arteries often form a ring around the esophagus and trachea, putting pressure on these structures. Vascular Rings, masses, etc). Symptoms are similar to those of airway obstruction Airway obstruction Airway obstruction is a partial or complete blockage of the airways that impedes airflow. An airway obstruction can be classified as upper, central, or lower depending on location. Lower airway obstruction (LAO) is usually a manifestation of chronic disease, such as asthma or chronic obstructive pulmonary disease (COPD). Airway Obstruction and include stridor 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.

References

  1. Moore, K. L., et al. Clinically oriented anatomy. Lippincott Williams & Wilkins, 2017.
  2. Drake, R., 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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