The trachea, also known as the windpipe, is a

cartilaginous Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck a ...

tube that connects the larynx to the bronchi of the lungs, allowing the passage of

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

, and so is present in almost all air- breathing

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s with lungs. The trachea extends from the larynx and branches into the two primary

bronchi A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus. ...

. At the top of the trachea the cricoid cartilage attaches it to the larynx. The trachea is formed by a number of horseshoe-shaped rings, joined together vertically by overlying ligaments, and by the trachealis muscle at their ends. The epiglottis closes the opening to the larynx during swallowing. The trachea begins to form in the second month of embryo development, becoming longer and more fixed in its position over time. It is epithelium lined with column-shaped cells that have hair-like extensions called cilia, with scattered goblet cells that produce protective

mucin Mucins () are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most ...

s. The trachea can be affected by inflammation or infection, usually as a result of a viral illness affecting other parts of the respiratory tract, such as the larynx and bronchi, called croup, that can result in a barking cough. Infection with bacteria usually affects the trachea only and can cause narrowing or even obstruction. As a major part of the respiratory tract, when obstructed the trachea prevents air entering the lungs and so a tracheostomy may be required if the trachea is obstructed. Additionally, during surgery if mechanical ventilation is required when a person is sedated, a tube is inserted into the trachea, called

intubation Intubation (sometimes entubation) is a medical procedure involving the insertion of a tube into the body. Patients are generally anesthetized beforehand. Examples include tracheal intubation, and the balloon tamponade with a Sengstaken-Blake ...

. The word ''trachea'' is used to define a very different organ in invertebrates than in vertebrates. Insects have an open

respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies ...

made up of spiracles, tracheae, and

tracheole Tracheole (trā'kē-ōl') is a fine respiratory tube of the trachea of an insect or a spider, part of the respiratory system. Tracheoles are about 1 µm in diameter, and they convey oxygen to cells while providing a means for carbon dioxide ...

s to transport metabolic gases to and from tissues.

Structure

An adult's trachea has an inner diameter of about and a length of about ; wider in males than females. The trachea begins at the lower edge of the cricoid cartilage of the larynx at the level of sixth

cervical vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sa ...

(C6) and ends at the carina, the point where the trachea branches into left and right main bronchi., at the level of the fourth

thoracic vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical ...

(T4), although its position may change with breathing. The trachea is surrounded by 16–20 rings of hyaline cartilage; these 'rings' are 4 millimetres high in the adult, incomplete and C-shaped. Ligaments connect the rings. The trachealis muscle connects the ends of the incomplete rings and runs along the back wall of the trachea. Also adventitia, which is the outermost layer of connective tissue that surrounds the hyaline cartilage, contributes to the trachea's ability to bend and stretch with movement. Although trachea is a midline structure, it can be displaced normally to the right by the aortic arch.

Nearby structures

The trachea passes by many structures of the neck and chest ( thorax) along its course. In front of the upper trachea lies connective tissue and skin. Several other structures pass over or sit on the trachea; the jugular arch, which joins the two

anterior jugular vein The anterior jugular vein is a vein in the neck. Structure The anterior jugular vein lies lateral to the cricothyroid ligament. It begins near the hyoid bone by the confluence of several superficial veins from the submandibular region. Its trib ...

s, sits in front of the upper part of the trachea. The

sternohyoid The sternohyoid muscle is a thin, narrow muscle attaching the hyoid bone to the sternum. It is one of the paired strap muscles of the infrahyoid muscles. It is supplied by the ansa cervicalis. It depresses the hyoid bone. Structure The sternoh ...

and

sternothyroid muscle The sternothyroid muscle, or sternothyroideus, is an infrahyoid muscle in the neck. It acts to depress the hyoid bone. It is below the sternohyoid muscle. It is shorter and wider than the sternohyoid. Structure The sternothyroid arises from the ...

s stretch along its length. The thyroid gland also stretches across the upper trachea, with the isthmus overlying the second to fourth rings, and the lobes stretching to the level of the fifth or sixth cartilage. The blood vessels of the thyroid rest on the trachea next to the isthmus; superior thyroid arteries join just above it, and the inferior thyroid veins below it. In front of the lower trachea lies the manubrium of the

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...

, the remnants of the thymus in adults. To the front left lie the large blood vessels the aortic arch and its branches the left

common carotid artery In anatomy, the left and right common carotid arteries (carotids) (Entry "carotid"
in
brachiocephalic trunk; and the left brachiocephalic vein. The deep

cardiac plexus The cardiac plexus is a plexus of nerves situated at the base of the heart that innervates the heart. Structure The cardiac plexus is divided into a superficial part, which lies in the concavity of the aortic arch, and a deep part, between the ao ...

and lymph nodes are also positioned in front of the lower trachea. Behind the trachea, along its length, sits the oesophagus, followed by connective tissue and the

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

. To its sides run the

carotid arteries In anatomy, the left and right common carotid arteries (carotids) (Entry "carotid"
in
recurrent laryngeal nerves in the upper trachea, and the vagus nerves in the lower trachea. The trachealis muscle contracts during

cough A cough is a sudden expulsion of air through the large breathing passages that can help clear them of fluids, irritants, foreign particles and microbes. As a protective reflex, coughing can be repetitive with the cough reflex following three ph ...

ing, reducing the size of the lumen of the trachea. File:CT-Thorax-5.0-B70f.ogg, CT scan of the thorax (axial lung window) File:COR-2-STND-CHEST.ogg, CT scan of the thorax (coronal lung window) File:COR-2-STND-CHEST-MEDIASTINUM.ogg, CT scan of the thorax (coronal mediastinal window) File:Cross section of a trachea and esophagus.svg, Cross section of a trachea and esophagus File:Sternohyoid muscle.PNG, The

and

s lie on top of the upper part of the trachea File:Blausen 0534 Goiter.png, The thyroid gland also lies on top of the trachea, and lies below the cricoid cartilage.

Blood and lymphatic supply

The upper part of trachea receives and drains blood through the inferior thyroid arteries and veins; the lower trachea receives blood from bronchial arteries. Arteries that supply the trachea do so via small branches that supply the trachea from the sides. As the branches approach the wall of the trachea, they split into inferior and superior branches, which join with the branches of the arteries above and below; these then split into branches that supply the anterior and posterior parts of the trachea. The inferior thyroid arteries arise just below the isthmus of the thyroid, which sits atop the trachea. These arteries join () with ascending branches of the bronchial arteries, which are direct branches from the

aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...

, to supply blood to the trachea. The

lymphatic vessel The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph ve ...

s of the trachea drain into the pretracheal nodes that lie in front of the trachea, and paratracheal lymph nodes that lie beside it.

Development

In the fourth week of development of the human embryo as the

respiratory bud The lung bud sometimes referred to as the respiratory bud forms from the respiratory diverticulum, an embryological endodermal structure that develops into the respiratory tract organs such as the larynx, trachea, bronchi and lungs. It arises from ...

grows, the trachea separates from the foregut through the formation of ridges which eventually separate the trachea from the oesophagus, the tracheoesophageal septum. This separates the future trachea from the oesophagus and divides the foregut tube into the laryngotracheal tube. By the start of the fifth week, the left and right main bronchi have begin to form, initially as buds at the terminal end of the trachea. The trachea is no more than 4mm diameter during the first year of life, expanding to its adult diameter of approximately 2cm by late childhood. The trachea is more circular and more vertical in children compared to adults, varies more in size, and also varies more in its position in relation to its surrounding structures.

Microanatomy

The trachea is lined with a layer of interspersed layers of column-shaped cells with cilia. The epithelium contains goblet cells, which are glandular, column-shaped cells that produce mucins, the main component of

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

. Mucus helps to moisten and protect the airways. Mucus lines the ciliated cells of the trachea to trap inhaled foreign particles that the cilia then waft upward toward the larynx and then the pharynx where it can be either swallowed into the

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

or expelled as phlegm. This self-clearing mechanism is termed

mucociliary clearance Mucociliary clearance (MCC), mucociliary transport, or the mucociliary escalator, describes the self-clearing mechanism of the airways in the respiratory system. It is one of the two protective processes for the lungs in removing inhaled particl ...

. The trachea is surrounded by 16 to 20 rings of hyaline cartilage; these 'rings' are incomplete and C-shaped. Two or more of the cartilages often unite, partially or completely, and they are sometimes bifurcated at their extremities. The rings are generally highly elastic but they may calcify with age. File:Gray964.png, Cross-section File:2308 The Trachea-b.jpg, Cross-section of the trachea, with pseudostratified ciliated columnar epithelium and goblet cells labelled File:Cartilage01.JPG, Magnified cross-section of the cartilage of the trachea.

Function

The trachea is one part of the respiratory tree that is a conduit for

to pass through on its way to or from the alveoli of the lungs. This transmits

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

to the body and removes

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

Clinical significance

Inflammation and infection

Inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...

of the trachea is known as tracheitis, usually due to an infection. It is usually caused by

viral infection A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells. Structural Characteristics Basic structural characteristics, ...

s, with bacterial infections occurring almost entirely in children. Most commonly, infections occur with inflammation of other parts of the respiratory tract, such as the larynx and bronchi, known as croup, however bacterial infections may also affect the trachea alone, although they are often associated with a recent viral infection. Viruses that cause croup are generally the parainfluenza viruses 1–3, with

influenza virus ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ' ...

es A and B also causing croup, but usually causing more serious infections; bacteria may also cause croup and include '' Staphylococcus aureus'', '' Haemophilus influenzae'', ''

Streptococcus pneumoniae ''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, alpha-hemolytic (under aerobic conditions) or beta-hemolytic (under anaerobic conditions), aerotolerant anaerobic member of the genus Streptococcus. They ar ...

'' and ''

Moraxella catarrhalis ''Moraxella catarrhalis'' is a fastidious, nonmotile, Gram-negative, aerobic, oxidase-positive diplococcus that can cause infections of the respiratory system, middle ear, eye, central nervous system, and joints of humans. It causes the inf ...

''. Causes of bacterial infection of the trachea are most commonly '' Staphylococcus aureus'' and ''

''. In patients who are in hospital, additional bacteria that may cause tracheitis include '' Escherichia coli'', ''

Klebsiella pneumoniae ''Klebsiella pneumoniae'' is a Gram-negative, non-motile, encapsulated, lactose- fermenting, facultative anaerobic, rod-shaped bacterium. It appears as a mucoid lactose fermenter on MacConkey agar. Although found in the normal flora of the mo ...

'', and '' Pseudomonas aeruginosa''. A person affected with tracheitis may start with symptoms that suggest an

upper respiratory tract infection An upper respiratory tract infection (URTI) is an illness caused by an acute infection, which involves the upper respiratory tract, including the nose, sinuses, pharynx, larynx or trachea. This commonly includes nasal obstruction, sore thro ...

such as a cough, sore throat, or coryzal symptoms such as a runny nose. Fevers may develop and an affected child may develop difficulty breathing and

sepsis Sepsis, formerly known as septicemia (septicaemia in British English) or blood poisoning, is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage is follo ...

. Swelling of the airway can cause narrowing of the airway, causing a hoarse breathing sound called stridor, or even cause complete blockage. Unfortunately, up to 80% of people affected by bacterial tracheitis require the use of mechanical ventilation, and treatment may include

endoscopy An endoscopy is a procedure used in medicine to look inside the body. The endoscopy procedure uses an endoscope to examine the interior of a hollow organ or cavity of the body. Unlike many other medical imaging techniques, endoscopes are inse ...

for the purposes of acquiring microbiological specimens for culture and sensitivity, as well as removal of any dead tissue associated with the infection. Treatment in such situations usually includes

antibiotics An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention o ...

Narrowing

A trachea may be narrowed or compressed, usually a result of enlarged nearby lymph nodes; cancers of the trachea or nearby structures; large thyroid

goitre A goitre, or goiter, is a swelling in the neck resulting from an enlarged thyroid gland. A goitre can be associated with a thyroid that is not functioning properly. Worldwide, over 90% of goitre cases are caused by iodine deficiency. The term is ...

s; or rarely as a result of other processes such as unusually swollen blood vessels. Scarring from tracheobronchial injury or intubation; or inflammation associated with

granulomatosis with polyangiitis Granulomatosis with polyangiitis (GPA), previously known as Wegener's granulomatosis (WG), is a rare long-term systemic disorder that involves the formation of granulomas and inflammation of blood vessels (vasculitis). It is a form of vasculitis ...

may also cause a narrowing of the trachea ( tracheal stenosis). Obstruction invariably causes a harsh breathing sound known as stridor. A camera inserted via the mouth down into the trachea, called

bronchoscopy Bronchoscopy is an endoscopic technique of visualizing the inside of the airways for diagnostic and therapeutic purposes. An instrument (bronchoscope) is inserted into the airways, usually through the nose or mouth, or occasionally through a trac ...

, may be performed to investigate the cause of an obstruction. Management of obstructions depends on the cause. Obstructions as a result of malignancy may be managed with surgery,

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs ( chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemothe ...

or radiotherapy. A stent may be inserted over the obstruction. Benign lesions, such as narrowing resulting from scarring, are likely to be surgically excised. One cause of narrowing is

tracheomalacia Tracheomalacia is a condition or incident where the cartilage that keeps the airway (trachea) open is soft such that the trachea partly collapses especially during increased airflow. This condition is most commonly seen in infants and young child ...

, which is the tendency for the trachea to collapse when there is increased external pressure, such as when airflow is increased during breathing in or out, due to decreased compliance. It can be due to congenital causes, or due to things that develop after birth, such as compression from nearby masses or swelling, or trauma. Congenital tracheomalacia can occur by itself or in association with other abnormalities such as bronchomalacia or laryngomalacia, and abnormal connections between the trachea and the oesophagus, amongst others. Congenital tracheomalacia often improves without specific intervention; when required, interventions may include beta agonists and muscarinic agonists, which enhance the tone of the smooth muscle surrounding the trachea; positive pressure ventilation, or surgery, which may include the placement of a

stent In medicine, a stent is a metal or plastic tube inserted into the lumen of an anatomic vessel or duct to keep the passageway open, and stenting is the placement of a stent. A wide variety of stents are used for different purposes, from expandab ...

, or the removal of the affected part of the trachea. In dogs, particularly miniature dogs and

toy dogs Toy dog traditionally refers to a very small dog or a grouping of small and very small breeds of dog. A toy dog may be of any of various dog types. Types of dogs referred to as toy dogs may include spaniels, pinschers and terriers that have been ...

, tracheomalacia, as well as bronchomalacia, can lead to

tracheal collapse Tracheal collapse in dogs is a condition characterized by incomplete formation or weakening of the cartilaginous rings of the trachea resulting in flattening of the trachea. It can be congenital or acquired, and extrathoracic or intrathoracic ...

, which often presents with a honking goose-like cough.

Injury

The trachea may be injured by trauma such as in a vehicle accident, or intentionally by another wilfully inflicting damage for example as practiced in some martial arts.

Intubation

Tracheal intubation refers to the insertion of a tube down the trachea. This procedure is commonly performed during surgery, in order to ensure a person receives enough oxygen when sedated. The catheter is connected to a machine that monitors the airflow, oxygenation and several other metrics. This is often one of the responsibilities of an

anaesthetist Anesthesiology, anaesthesiology, or anaesthesia is the medical specialty concerned with the total perioperative care of patients before, during and after surgery. It encompasses anesthesia, intensive care medicine, critical emergency medicine ...

during surgery. In an emergency, or when tracheal intubation is deemed impossible, a

tracheotomy Tracheotomy (, ), or tracheostomy, is a surgical airway management procedure which consists of making an incision (cut) on the anterior aspect (front) of the neck and opening a direct airway through an incision in the trachea (windpipe). The r ...

is often performed to insert a tube for ventilation, usually when needed for particular types of surgery to be carried out so that the airway can be kept open. The provision of the opening via a tracheotomy is called a tracheostomy. Another method procedure can be carried, in an emergency situation, and this is a cricothyrotomy.

Congenital disorders

Tracheal agenesis is a rare birth defect in which the trachea fails to develop. The defect is usually fatal though sometimes surgical intervention has been successful. A tracheoesophageal fistula is a congenital defect in which the trachea and esophagus are abnormally connected (a ). This is because of abnormalities in the separation between the trachea and oesophagus during development. This occurs in approximately 1 in 3000 births, and the most common abnormalities is a separation of the upper and lower ends of the oesophagus, with the upper end finishing in a closed pouch. Other abnormalities may be associated with this, including cardiac abnormalities, or VACTERL syndrome. Such fistulas may be detected before a baby is born because of excess amniotic fluid; after birth, they are often associated with pneumonitis and pneumonia because of of food contents. Congenital fistulas are often treated by surgical repair. In adults, fistulas may occur because of erosion into the trachea from nearby malignant tumours, which erode into both the trachea and the oesophagus. Initially, these often result in coughing from swallowed contents of the oesophagus that are aspirated through the trachea, often progressing to fatal pneumonia; unfortunately, there is rarely a curative treatment. A tracheo-oesophageal puncture is a surgically created hole between the trachea and the esophagus in a person who has had their larynx removed. Air travels upwards from the surgical connection to the upper oesophagus and the pharynx, creating vibrations that create sound that can be used for speech. The purpose of the puncture is to restore a person's ability to speak after the vocal cords have been removed. Sometimes as an anatomical variation one or more of the tracheal rings are formed as complete rings, rather than horseshoe shaped rings. These ''O'' rings are smaller than the normal C-shaped rings and can cause narrowing () of the trachea, resulting in breathing difficulties. An operation called a ''slide tracheoplasty'' can open up the rings and rejoin them as wider rings, shortening the length of the trachea. Slide tracheoplasty is said to be the best option in treating tracheal stenosis. Mounier-Kuhn syndrome is a rare congenital disorder of an abnormally enlarged trachea, characterised by absent elastic fibres, smooth muscle thinning, and a tendency to get recurrent respiratory tract infections.

Replacement

From 2008, operations have experimentally replaced tracheas, with those grown from stem cells, or with synthetic substitutes, however this is regarded as experimental and there is no standardised method. Difficulties with ensuring adequate blood supply to the replaced trachea is considered a major challenge to any replacement. Additionally, no evidence has been found to support the placement of stem cells taken from bone marrow on the trachea as a way of stimulating tissue regeneration, and such a method remains hypothetical. In January 2021, surgeons at Mount Sinai Hospital in New York performed the first complete trachea transplantation. The 18-hour procedure included harvesting a trachea from a donor and implanting it in the patient, connecting numerous veins and arteries to provide sufficient blood flow to the organ.

Other animals

Allowing for variations in the length of the neck, the trachea in other mammals is, in general, similar to that in humans. Generally, it is also similar to the reptilian trachea.

Vertebrates

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s, the trachea runs from the pharynx to the syrinx, from which the primary bronchi diverge. Swans have an unusually elongated trachea, part of which is coiled beneath the sternum; this may act as a resonator to amplify sound. In some birds, the tracheal rings are complete, and may even be

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

. In amphibians, the trachea is normally extremely short, and leads directly into the lungs, without clear primary bronchi. A longer trachea is, however, found in some long-necked

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...

s, and in

caecilian Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics o ...

s. While there are irregular cartilagenous nodules on the amphibian trachea, these do not form the rings found in amniotes. The only vertebrates to have lungs, but no trachea, are the lungfish and the ''

Polypterus ''Polypterus'' is a genus of freshwater fish in the bichir family (Polypteridae) of order Polypteriformes. The type species is the Nile bichir (''P. bichir''). Fish in this genus live in various areas in Africa. ''Polypterus'' is the only kno ...

'', in which the lungs arise directly from the pharynx.

Invertebrates

The word ''trachea'' is used to define a very different organ in invertebrates than in vertebrates. Insects have an open

made up of spiracles, tracheae, and

s to transport metabolic gases to and from tissues. The distribution of spiracles can vary greatly among the many orders of

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...

s, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticular exoskeleton that branch (

anastomose An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be normal ...

) throughout the body with diameters from only a few micrometres up to 0.8 mm.

Diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...

of oxygen and

takes place across the walls of the smallest tubes, called tracheoles, which penetrate tissues and even indent individual cells. Gas may be conducted through the respiratory system by means of active

ventilation Ventilation may refer to: * Ventilation (physiology), the movement of air between the environment and the lungs via inhalation and exhalation ** Mechanical ventilation, in medicine, using artificial methods to assist breathing *** Ventilator, a m ...

or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their

haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

. This is one of the factors that may limit their size. A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices. Taenidia provide strength and flexibility to the trachea. In the head, thorax, or

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...

, tracheae may also be connected to air sacs. Many insects, such as grasshoppers and bees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a gill, or function essentially as normal, via a plastron. Note that despite being internal, the tracheae of arthropods are lined with cuticular tissue and are shed during moulting ( ecdysis).

Additional images

File:Trachea (mammal) histology cross-section.png, alt=Trachea (mammal) cross-section high resolution, Trachea (mammal) cross-section high resolution File:Trachea (mammal) histology cross-section low mag.png, alt=Trachea (mammal) cross-section low resolution, Trachea (mammal) cross-section low resolution File:Larynx detailed.jpg, Trachea File:Gray954.png, Coronal section of larynx and upper part of trachea

References

{{good article Arthropod anatomy Invertebrate respiratory system Human head and neck Thorax (human anatomy) Respiratory system