Trachea: Anatomy

Development

The trachea develops as part of the tracheobronchial tree by budding Budding Mycology off of the foregut Foregut Development of the Abdominal Organs of the embryonic gut tube.

• Occurs 4–7 weeks after conception
• Embryonic gut tube:
  • Forms from the laterally folded endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation layer 
  • Surrounded by mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation
  • Has 3 sections:
    • Foregut Foregut Development of the Abdominal Organs
    • Midgut Midgut Development of the Abdominal Organs
    • Hindgut Hindgut Development of the Abdominal Organs
• Lung bud Lung bud Development of the Respiratory System ( respiratory diverticulum Respiratory diverticulum Development of the Respiratory System):
  • Buds off of the ventral side of the foregut Foregut Development of the Abdominal Organs around week 4
  • Simultaneously grows out (ventrally) and down (caudally)
  • Includes both endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation and surrounding splanchnopleuric mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation
• Tracheoesophageal groove Tracheoesophageal groove Development of the Respiratory System (or ridge):
  • As the lung bud Lung bud Development of the Respiratory System grows out and down, the tracheoesophageal groove Tracheoesophageal groove Development of the Respiratory System appears as lateral indentations between the new lung bud Lung bud Development of the Respiratory System and the foregut Foregut Development of the Abdominal Organs.
  • The grooves/ridges move in medially, pinching off the lung bud Lung bud Development of the Respiratory System, and forming the tracheoesophageal septum.
  • The tracheoesophageal septum creates 2 separate tubes:
    • 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 (posteriorly, from the original foregut Foregut Development of the Abdominal Organs)
    • Trachea (anteriorly, from the lung bud Lung bud Development of the Respiratory System) 
• Primary bronchial buds Primary bronchial buds Development of the Respiratory System: The trachea bifurcates into the right and left bronchial buds.
  • The primary bronchial buds Primary bronchial buds Development of the Respiratory System 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: Anatomy 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: Anatomy through the pseudoglandular, canalicular, saccular, and alveolar stages.

Gross Anatomy

General characteristics

• Location:
  • Located in the lower neck Neck The part of a human or animal body connecting the head to the rest of the body. Peritonsillar Abscess and thorax, within the superior mediastinum Superior mediastinum Mediastinum and Great Vessels: Anatomy
  • 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: Anatomy at the level of C6
  • Ends inferiorly at the level of T5 and continues as the primary 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 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: Anatomy 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: Anatomy
• 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 Hyaline cartilage A type of cartilage characterized by a homogeneous amorphous matrix containing predominantly type II collagen and ground substance. Hyaline cartilage is found in articular cartilage; costal cartilage; laryngeal cartilages; and the nasal septum. Cartilage: Histology 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: Histology
  • Has a flat, cartilage-free, membranous posterior wall that contains smooth muscle called the trachealis muscle (known as paries membranaceus)

• 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: Anatomy during ventilation Ventilation The total volume of gas inspired or expired per unit of time, usually measured in liters per minute. Ventilation: Mechanics of Breathing
  • 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: Anatomy 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: Histology
    • Begins at approximately the level of C6
    • Ends at the level of the jugular notch of the sternum Sternum A long, narrow, and flat bone commonly known as breastbone occurring in the midsection of the anterior thoracic segment or chest region, which stabilizes the rib cage and serves as the point of origin for several muscles that move the arms, head, and neck. Chest Wall: Anatomy
  • Thoracic portion:
    • Direct continuation of the cervical trachea
    • Begins at the upper border of the superior mediastinum Superior mediastinum Mediastinum and Great Vessels: Anatomy (jugular notch of the sternum Sternum A long, narrow, and flat bone commonly known as breastbone occurring in the midsection of the anterior thoracic segment or chest region, which stabilizes the rib cage and serves as the point of origin for several muscles that move the arms, head, and neck. Chest Wall: Anatomy)
    • Ends at the bifurcation of the trachea into 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, at the level of T4 T4 The major hormone derived from the thyroid gland. Thyroxine is synthesized via the iodination of tyrosines (monoiodotyrosine) and the coupling of iodotyrosines (diiodotyrosine) in the thyroglobulin. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form triiodothyronine which exerts a broad spectrum of stimulatory effects on cell metabolism. Thyroid Hormones–T7 (most commonly T5)

Spatial relations

• Posterior:
  • 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
  • Descending aorta Descending aorta Mediastinum and Great Vessels: Anatomy
  • 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: Anatomy
• 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. Skin: Structure and Functions and cervical fascia Fascia Layers of connective tissue of variable thickness. The superficial fascia is found immediately below the skin; the deep fascia invests muscles, nerves, and other organs. Cellulitis
  • Jugular venous arch
  • Ascending aorta Ascending aorta Mediastinum and Great Vessels: Anatomy
  • Brachiocephalic trunk
  • 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
  • Thyroid Thyroid 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: Anatomy gland and vessels
  • Thymus Thymus A single, unpaired primary lymphoid organ situated in the mediastinum, extending superiorly into the neck to the lower edge of the thyroid gland and inferiorly to the fourth costal cartilage. It is necessary for normal development of immunologic function early in life. By puberty, it begins to involute and much of the tissue is replaced by fat. Lymphatic Drainage System: Anatomy gland or remnants
• Lateral:
  • Carotid arteries Carotid Arteries Either of the two principal arteries on both sides of the neck that supply blood to the head and neck; each divides into two branches, the internal carotid artery and the external carotid artery. Carotid Arterial System: Anatomy
  • Inferior thyroid Thyroid 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: Anatomy 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
  • Recurrent laryngeal nerves Laryngeal Nerves Branches of the vagus nerve. The superior laryngeal nerves originate near the nodose ganglion and separate into external branches, which supply motor fibers to the cricothyroid muscles, and internal branches, which carry sensory fibers. The recurrent laryngeal nerve originates more caudally and carries efferents to all muscles of the larynx except the cricothyroid. The laryngeal nerves and their various branches also carry sensory and autonomic fibers to the laryngeal, pharyngeal, tracheal, and cardiac regions. Larynx: Anatomy
  • Vagus nerves
  • 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: Anatomy 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: Anatomy

Microscopic Anatomy

The tracheal wall consists of 4 layers:

1. Mucosa: tall, columnar, pseudostratified with cilia and mucin-producing 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
2. Submucosa:
   • 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 provides support and elastic Elastic Connective Tissue: Histology recoil Recoil Vessels can stretch and return to their original shape after receiving the stroke volume of blood ejected by the left ventricle during systole. Arteries: Histology
   • Contains elastin fibers, mucus glands, smooth muscle, vessels, nerves, and lymphatics
3. Musculo-cartilaginous layer: contains the hyaline cartilage Hyaline cartilage A type of cartilage characterized by a homogeneous amorphous matrix containing predominantly type II collagen and ground substance. Hyaline cartilage is found in articular cartilage; costal cartilage; laryngeal cartilages; and the nasal septum. Cartilage: Histology 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: Anatomy
4. Adventitia: composed of fibroelastic, loose connective tissue Loose connective tissue Connective Tissue: Histology

Neurovasculature

Blood supply

• Arterial supply:
  • The trachea is mainly supplied by branches of the inferior thyroid Thyroid 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: Anatomy 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, 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: Histology. 
  • 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: Histology 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: Histology drain into the inferior thyroid Thyroid 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: Anatomy venous plexus.
  • The inferior thyroid Thyroid 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: Anatomy 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 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: Histology.

Innervation

• The trachea receives innervation from the pulmonary plexus.
  • Parasympathetic supply: recurrent laryngeal nerves Laryngeal Nerves Branches of the vagus nerve. The superior laryngeal nerves originate near the nodose ganglion and separate into external branches, which supply motor fibers to the cricothyroid muscles, and internal branches, which carry sensory fibers. The recurrent laryngeal nerve originates more caudally and carries efferents to all muscles of the larynx except the cricothyroid. The laryngeal nerves and their various branches also carry sensory and autonomic fibers to the laryngeal, pharyngeal, tracheal, and cardiac regions. Larynx: Anatomy, branches of 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
  • Sympathetic supply: sympathetic trunks
• Within the walls of the tracheobronchial tree, efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology preganglionic axons Axons Nerve fibers that are capable of rapidly conducting impulses away from the neuron cell body. Nervous System: Histology 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 Local reflexes Gastrointestinal Secretions and airway Airway ABCDE Assessment 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: Anatomy that can occur after an infection of the nasal cavity Nasal cavity The proximal portion of the respiratory passages on either side of the nasal septum. Nasal cavities, extending from the nares to the nasopharynx, are lined with ciliated nasal mucosa. Nose Anatomy (External & Internal) and pharyngeal region due to viruses Viruses Minute infectious agents whose genomes are composed of DNA or RNA, but not both. They are characterized by a lack of independent metabolism and the inability to replicate outside living host cells. Virology 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, which can get into the trachea. Also, overuse of the voice in a dry air environment can lead to symptoms of 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: reddened pharyngeal mucosa, burning sore throat Sore throat Pharyngitis is an inflammation of the back of the throat (pharynx). Pharyngitis is usually caused by an upper respiratory tract infection, which is viral in most cases. It typically results in a sore throat and fever. Other symptoms may include a runny nose, cough, headache, and hoarseness. Pharyngitis, and hoarseness Hoarseness An unnaturally deep or rough quality of voice. Parapharyngeal Abscess or loss of voice. 
• Congenital:
  • Tracheoesophageal fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal 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: Anatomy and the trachea: Tracheoesophageal fistula Fistula Abnormal communication most commonly seen between two internal organs, or between an internal organ and the surface of the body. Anal Fistula is a common congenital abnormality, but when it occurs late in life, it is usually a sequela of a surgical procedure.
  • Tracheomalacia Tracheomalacia A congenital or acquired condition of underdeveloped or degeneration of cartilage in the trachea. This results in a floppy tracheal wall making patency difficult to maintain. It is characterized by wheezing and difficult breathing. Laryngomalacia and Tracheomalacia: an abnormal collapsing of the walls of the trachea: Tracheomalacia Tracheomalacia A congenital or acquired condition of underdeveloped or degeneration of cartilage in the trachea. This results in a floppy tracheal wall making patency difficult to maintain. It is characterized by wheezing and difficult breathing. 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: Histology disorder or can be caused by an external source ( vascular rings Vascular Rings Pediatric Vomiting, masses, etc ETC The electron transport chain (ETC) sends electrons through a series of proteins, which generate an electrochemical proton gradient that produces energy in the form of adenosine triphosphate (ATP). Electron Transport Chain (ETC)). Symptoms are similar to those of airway Airway ABCDE Assessment obstruction and include stridor Stridor Laryngomalacia and Tracheomalacia 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.