Incertae sedis or problematica are terms used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty at specific taxonomic levels is indicated by incertae familiae, incerti subordinis, incerti ordinis and similar terms; the fossil plant Paradinandra suecica could not be assigned to any family, but was placed incertae sedis within the order Ericales when described in 2001. The fossil Gluteus minimus, described in 1975, could not be assigned to any known animal phylum; the genus is therefore incertae sedis within the kingdom Animalia. While it was unclear to which order the New World vultures should be assigned, they were placed in Aves incertae sedis, it was agreed to place them in a separate order, Cathartiformes. Bocage's longbill, Amaurocichla bocagei, a species of passerine bird, belongs to the superfamily Passeroidea. Since it is unclear to which family it belongs, it is classified as Passeroidea incertae sedis.
HeLa cells, descended from human cervical cancer cells, may diverge genetically from normal human cells sufficiently to be categorized as a new species with incertae sedis taxonomy. When formally naming a taxon, uncertainty about its taxonomic classification can be problematic; the International Code of Nomenclature for algae and plants, stipulates that "species and subdivisions of genera must be assigned to genera, infraspecific taxa must be assigned to species, because their names are combinations", but ranks higher than the genus may be assigned incertae sedis. This excerpt from a 2007 scientific paper about crustaceans of the Kuril-Kamchatka Trench and the Japan Trench describes typical circumstances through which this category is applied in discussing:...the removal of many genera from new and existing families into a state of incertae sedis. Their reduced status was attributed to poor or inadequate descriptions but it was accepted that some of the vagueness in the analysis was due to insufficient character states.
It is evident that a proportion of the characters used in the analysis, or their given states for particular taxa, were inappropriate or invalid. Additional complexity, factors that have misled earlier authorities, are intrusion by extensive homoplasies, apparent character state reversals and convergent evolution. If a formal phylogenetic analysis is conducted that does not include a certain taxon, the authors might choose to label the taxon incertae sedis instead of guessing its placement; this is common when molecular phylogenies are generated, since tissue for many rare organisms is hard to obtain. It is a common scenario when fossil taxa are included, since many fossils are defined based on partial information. For example, if the phylogeny was constructed using soft tissue and vertebrae as principal characters and the taxon in question is only known from a single tooth, it would be necessary to label it incertae sedis. If conflicting results exist or if there is not a consensus among researchers as to how a taxon relates to other organisms, it may be listed as incertae sedis until the conflict is resolved.
In botany, a name is not validly published if it is not accepted by the author in the same publication. Article 36.1 In zoology, a name proposed conditionally may be available under certain conditions. Articles 11 and 15 For uncertainties at lower levels, the system of open nomenclature suggests that question marks be used to denote a questionable assignment. For example, if a new species was given the specific epithet album by Anton and attributed with uncertainty to Agenus, it could be denoted "Agenus? Album Anton". So if Anton described Agenus album, Bruno called the assignment into doubt, this could be denoted "Agenus? Album ", with the parentheses around Anton because the original assignment was modified by Bruno. Glossary of scientific naming Nomen dubium, a name of unknown or doubtful application Species inquirenda, a species that in the opinion of the taxonomist requires further investigation Wastebasket taxon Sui generis Unclassified language The dictionary definition of incertae sedis at Wiktionary Media related to Incertae sedis at Wikimedia Commons
Chemokines are a family of small cytokines, or signaling proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells. Cytokine proteins are classified as chemokines according to structural characteristics. In addition to being known for mediating chemotaxis, chemokines are all 8-10 kilodaltons in mass and have four cysteine residues in conserved locations that are key to forming their 3-dimensional shape; these proteins have been known under several other names including the SIS family of cytokines, SIG family of cytokines, SCY family of cytokines, Platelet factor-4 superfamily or intercrines. Some chemokines are considered pro-inflammatory and can be induced during an immune response to recruit cells of the immune system to a site of infection, while others are considered homeostatic and are involved in controlling the migration of cells during normal processes of tissue maintenance or development. Chemokines are found in all vertebrates, some viruses and some bacteria, but none have been described for other invertebrates.
Chemokines have been classified into four main subfamilies: CXC, CC, CX3C and XC. All of these proteins exert their biological effects by interacting with G protein-linked transmembrane receptors called chemokine receptors, that are selectively found on the surfaces of their target cells; the major role of chemokines is to act as a chemoattractant to guide the migration of cells. Cells that are attracted by chemokines follow a signal of increasing chemokine concentration towards the source of the chemokine; some chemokines control cells of the immune system during processes of immune surveillance, such as directing lymphocytes to the lymph nodes so they can screen for invasion of pathogens by interacting with antigen-presenting cells residing in these tissues. These are known as homeostatic chemokines and are produced and secreted without any need to stimulate their source cell; some chemokines have roles in development. Other chemokines are inflammatory and are released from a wide variety of cells in response to bacterial infection and agents that cause physical damage such as silica or the urate crystals that occur in gout.
Their release is stimulated by pro-inflammatory cytokines such as interleukin 1. Inflammatory chemokines function as chemoattractants for leukocytes, recruiting monocytes and other effector cells from the blood to sites of infection or tissue damage. Certain inflammatory chemokines activate cells to initiate an immune response or promote wound healing, they are released by many different cell types and serve to guide cells of both innate immune system and adaptive immune system. Chemokines are functionally divided into two groups: Homeostatic: are constitutively produced in certain tissues and are responsible for basal leukocyte migration; these include: CCL14, CCL19, CCL20, CCL21, CCL25, CCL27, CXCL12 and CXCL13. This classification is not strict. Inflammatory: these are formed under pathological conditions and participate in the inflammatory response attracting immune cells to the site of inflammation. Examples are: CXCL-8, CCL2, CCL3, CCL4, CCL5, CCL11, CXCL10; the main function of chemokines is to manage the migration of leukocytes in the respective anatomical locations in inflammatory and homeostatic processes.
Basal: homeostatic chemokines are basal produced in the thymus and lymphoid tissues. Their homeostatic function in homing is best exemplified by the chemokines CCL19 and CCL21 and their receptor CCR7. Using these ligands is possible routing antigen-presenting cells to lymph nodes during the adaptive immune response. Among other homeostatic chemokine receptors include: CCR9, CCR10, CXCR5, which are important as part of the cell addresses for tissue-specific homing of leukocytes. CCR9 supports the migration of leukocytes into the intestine, CCR10 to the skin and CXCR5 supports the migration of B-cell to follicles of lymph nodes; as well CXCL12 constitutively produced in the bone marrow promotes proliferation of progenitor B cells in the bone marrow microenvironment. Inflammatory: inflammatory chemokines are produced in high concentrations during infection or injury and determine the migration of inflammatory leukocytes into the damaged area. Typical inflammatory chemokines include: CCL2, CCL3 and CCL5, CXCL1, CXCL2 and CXCL8.
A typical example is CXCL-8. In contrast to the homeostatic chemokine receptors, there is significant promiscuity associated with binding receptor and inflammatory chemokines; this complicates research on receptor-specific therapeutics in this area. Monocytes / macrophages: the key chemokines that attract these cells to the site of inflammation include: CCL2, CCL3, CCL5, CCL7, CCL8, CCL13, CCL17 and CCL22. T-lymphocytes: the four key chemokines that are involved in the recruitment of T lymphocytes to the site of inflammation are: CCL2, CCL1, CCL22 and CCL17. Furthermore, CXCR3 expression by T-cells is induced following T-cell activation and activated T-cells are attracted to sites of inflammation where the IFN-y inducible chemokines CXCL9, CXCL10 and CXCL11 are secreted. Mast cells: on their surface express several receptors for chemokines: CCR1, CCR2, CCR3, CCR4, CCR5, CXCR2, CXCR4. Ligands of
Cladistics is an approach to biological classification in which organisms are categorized in groups based on the most recent common ancestor. Hypothesized relationships are based on shared derived characteristics that can be traced to the most recent common ancestor and are not present in more distant groups and ancestors. A key feature of a clade is that all its descendants are part of the clade. All descendants stay in their overarching ancestral clade. For example, if within a strict cladistic framework the terms animals, bilateria/worms, fishes/vertebrata, or monkeys/anthropoidea were used, these terms would include humans. Many of these terms are used paraphyletically, outside of cladistics, e.g. as a'grade'. Radiation results in the generation of new subclades by bifurcation; the techniques and nomenclature of cladistics have been applied to other disciplines. Cladistics is now the most used method to classify organisms; the original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics.
Cladistics in the original sense refers to a particular set of methods used in phylogenetic analysis, although it is now sometimes used to refer to the whole field. What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birds and subsequently by Robert John Tillyard in 1921, W. Zimmermann in 1943; the term "clade" was introduced in 1958 by Julian Huxley after having been coined by Lucien Cuénot in 1940, "cladogenesis" in 1958, "cladistic" by Cain and Harrison in 1960, "cladist" by Mayr in 1965, "cladistics" in 1966. Hennig referred to his own approach as "phylogenetic systematics". From the time of his original formulation until the end of the 1970s, cladistics competed as an analytical and philosophical approach to systematics with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal, evolutionary taxonomy by Ernst Mayr. Conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966.
Today, cladistics is the most popular method for constructing phylogenies from morphological data. In the 1990s, the development of effective polymerase chain reaction techniques allowed the application of cladistic methods to biochemical and molecular genetic traits of organisms, vastly expanding the amount of data available for phylogenetics. At the same time, cladistics became popular in evolutionary biology, because computers made it possible to process large quantities of data about organisms and their characteristics; the cladistic method interprets each character state transformation implied by the distribution of shared character states among taxa as a potential piece of evidence for grouping. The outcome of a cladistic analysis is a cladogram – a tree-shaped diagram, interpreted to represent the best hypothesis of phylogenetic relationships. Although traditionally such cladograms were generated on the basis of morphological characters and calculated by hand, genetic sequencing data and computational phylogenetics are now used in phylogenetic analyses, the parsimony criterion has been abandoned by many phylogeneticists in favor of more "sophisticated" but less parsimonious evolutionary models of character state transformation.
Cladists contend. Every cladogram is based on a particular dataset analyzed with a particular method. Datasets are tables consisting of molecular, ethological and/or other characters and a list of operational taxonomic units, which may be genes, populations, species, or larger taxa that are presumed to be monophyletic and therefore to form, all together, one large clade. Different datasets and different methods, not to mention violations of the mentioned assumptions result in different cladograms. Only scientific investigation can show, more to be correct; until for example, cladograms like the following have been accepted as accurate representations of the ancestral relations among turtles, lizards and birds: If this phylogenetic hypothesis is correct the last common ancestor of turtles and birds, at the branch near the ▼ lived earlier than the last common ancestor of lizards and birds, near the ♦. Most molecular evidence, produces cladograms more like this: If this is accurate the last common ancestor of turtles and birds lived than the last common ancestor of lizards and birds.
Since the cladograms provide competing accounts of real events, at most one of them is correct. The cladogram to the right represents the current universally accepted hypothesis that all primates, including strepsirrhines like the lemurs and lorises, had a common ancestor all of whose descendants were primates, so form a clade. Within the primates, all anthropoids are hypothesized to have had a common ancestor all of whose descendants were anthropoids, so they form the clade called Anthropoidea; the "prosimians", on the other hand, form a paraphyletic taxon. The name Prosimii is not used in phylogenetic nomenclature, whic
A sneeze, or sternutation, is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth caused by foreign particles irritating the nasal mucosa. A sneeze expels air forcibly from the mouth and nose in an explosive, spasmodic involuntary action resulting chiefly from irritation of the nasal mucous membrane; this action allows for mucus to escape through the nasal cavity. Sneezing is linked to sudden exposure to bright light, sudden change in temperature, breeze of cold air, a full stomach, or viral infection, can lead to the spread of disease; the function of sneezing is to expel mucus containing foreign particles or irritants and cleanse the nasal cavity. During a sneeze, the soft palate and palatine uvula depress while the back of the tongue elevates to close the passage to the mouth so that air ejected from the lungs may be expelled through the nose; because the closing of the mouth is partial, a considerable amount of this air is also expelled from the mouth.
The force and extent of the expulsion of the air through the nose varies. Sneezing cannot occur during sleep due to REM atonia – a bodily state where motor neurons are not stimulated and reflex signals are not relayed to the brain. Sufficient external stimulants, may cause a person to wake from sleep to sneeze, but any sneezing occurring afterwards would take place with a awake status at minimum. Sneezing occurs when foreign particles or sufficient external stimulants pass through the nasal hairs to reach the nasal mucosa; this triggers the release of histamines, which irritate the nerve cells in the nose, resulting in signals being sent to the brain to initiate the sneeze through the trigeminal nerve network. The brain relates this initial signal, activates the pharyngeal and tracheal muscles and creates a large opening of the nasal and oral cavities, resulting in a powerful release of air and bioparticles; the powerful nature of a sneeze is attributed to its involvement of numerous organs of the upper body – it is a reflexive response involving the face and chest muscles.
Sneezing is triggered by sinus nerve stimulation caused by nasal congestion and allergies. The neural regions involved in the sneeze reflex are located in the brainstem along the ventromedial part of the spinal trigeminal nucleus and the adjacent pontine-medullary lateral reticular formation; this region appears to control the epipharyngeal, intrinsic laryngeal and respiratory muscles, the combined activity of these muscles serve as the basis for the generation of a sneeze. The sneeze reflex involves contraction of a number of different muscles and muscle groups throughout the body including the eyelids; the common suggestion that it is impossible to sneeze with one's eyes open is, inaccurate. Other than irritating foreign particles, allergies or possible illness, another stimulus is sudden exposure to bright light – a condition known as photic sneeze reflex. Walking out of a dark building into sunshine may trigger PSR, or the ACHOO syndrome as it's called; the tendency to sneeze upon exposure to bright light is an autosomal dominant trait and affects 18-35% of the human population.
A rarer trigger, observed in some individuals, is the fullness of the stomach after a large meal. This is known as snatiation and is regarded as a medical disorder passed along genetically as an autosomal dominant trait. While harmless in healthy individuals, sneezes spread disease through the infectious aerosol droplets ranging from 0.5 to 5 µm. A sneeze can produce 40,000 droplets. To reduce the possibility of thus spreading disease, one holds the forearm or the inside of the elbow in front of one's mouth and nose when sneezing. Using one's hand for that purpose has fallen into disuse as it is considered inappropriate, since it promotes spreading germs through human contact or by touched objects. Examples of preventive techniques are: the deep exhalation of the air in the lungs that would otherwise be used in the act of sneezing, holding the breath in while counting to ten or pinching the bridge of the nose for several seconds. Proven methods to reduce sneezing advocate reducing interaction with irritants, such as keeping pets out of the house to avoid animal dander.
Some people, find sneezes to be pleasurable and would not want to prevent them. In Ancient Greece, sneezes were believed to be prophetic signs from the gods. In 401 BC, for instance, the Athenian general Xenophon gave a speech exhorting his fellow soldiers to fight against the Persians. A soldier underscored his conclusion with a sneeze. Thinking that this sneeze was a favorable sign from the gods, the soldiers were impressed. Another divine moment of sneezing for the Greeks occurs in the story of Odysseus, his waiting wife Penelope, hearing Odysseus may be alive, says that he and his son would take revenge on the suitors if he were to return. At that moment, their son sneezes loudly and Penelope laughs with joy, reassured that it is a sign from the gods, it may be because this belief survived through the centuries, that in certain parts of Greece today, when someone is asserting something and the listener sneezes promptly at the end of the assertion, the former responds "bless you and I am speaking the truth", or "bless you and here is the truth", "γεια σου κι αλήθεια λ
Ribonucleic acid is a polymeric molecule essential in various biological roles in coding, decoding and expression of genes. RNA and DNA are nucleic acids, along with lipids and carbohydrates, constitute the four major macromolecules essential for all known forms of life. Like DNA, RNA is assembled as a chain of nucleotides, but unlike DNA it is more found in nature as a single-strand folded onto itself, rather than a paired double-strand. Cellular organisms use messenger RNA to convey genetic information that directs synthesis of specific proteins. Many viruses encode their genetic information using an RNA genome; some RNA molecules play an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals. One of these active processes is protein synthesis, a universal function in which RNA molecules direct the assembly of proteins on ribosomes; this process uses transfer RNA molecules to deliver amino acids to the ribosome, where ribosomal RNA links amino acids together to form proteins.
Like DNA, most biologically active RNAs, including mRNA, tRNA, rRNA, snRNAs, other non-coding RNAs, contain self-complementary sequences that allow parts of the RNA to fold and pair with itself to form double helices. Analysis of these RNAs has revealed that they are structured. Unlike DNA, their structures do not consist of long double helices, but rather collections of short helices packed together into structures akin to proteins. In this fashion, RNAs can achieve chemical catalysis. For instance, determination of the structure of the ribosome—an RNA-protein complex that catalyzes peptide bond formation—revealed that its active site is composed of RNA; each nucleotide in RNA contains a ribose sugar, with carbons numbered 1' through 5'. A base is attached to the 1' position, in general, cytosine, guanine, or uracil. Adenine and guanine are purines and uracil are pyrimidines. A phosphate group is attached to the 5' position of the next; the phosphate groups have a negative charge each. The bases form hydrogen bonds between cytosine and guanine, between adenine and uracil and between guanine and uracil.
However, other interactions are possible, such as a group of adenine bases binding to each other in a bulge, or the GNRA tetraloop that has a guanine–adenine base-pair. An important structural component of RNA that distinguishes it from DNA is the presence of a hydroxyl group at the 2' position of the ribose sugar; the presence of this functional group causes the helix to take the A-form geometry, although in single strand dinucleotide contexts, RNA can also adopt the B-form most observed in DNA. The A-form geometry results in a deep and narrow major groove and a shallow and wide minor groove. A second consequence of the presence of the 2'-hydroxyl group is that in conformationally flexible regions of an RNA molecule, it can chemically attack the adjacent phosphodiester bond to cleave the backbone. RNA is transcribed with only four bases, but these bases and attached sugars can be modified in numerous ways as the RNAs mature. Pseudouridine, in which the linkage between uracil and ribose is changed from a C–N bond to a C–C bond, ribothymidine are found in various places.
Another notable modified base is hypoxanthine, a deaminated adenine base whose nucleoside is called inosine. Inosine plays a key role in the wobble hypothesis of the genetic code. There are more than 100 other occurring modified nucleosides; the greatest structural diversity of modifications can be found in tRNA, while pseudouridine and nucleosides with 2'-O-methylribose present in rRNA are the most common. The specific roles of many of these modifications in RNA are not understood. However, it is notable that, in ribosomal RNA, many of the post-transcriptional modifications occur in functional regions, such as the peptidyl transferase center and the subunit interface, implying that they are important for normal function; the functional form of single-stranded RNA molecules, just like proteins requires a specific tertiary structure. The scaffold for this structure is provided by secondary structural elements that are hydrogen bonds within the molecule; this leads to several recognizable "domains" of secondary structure like hairpin loops and internal loops.
Since RNA is charged, metal ions such as Mg2+ are needed to stabilise many secondary and tertiary structures. The occurring enantiomer of RNA is D-RNA composed of D-ribonucleotides. All chirality centers are located in the D-ribose. By the use of L-ribose or rather L-ribonucleotides, L-RNA can be synthesized. L-RNA is much more stable against degradation by RNase. Like other structured biopolymers such as proteins, one can define topology of a folded RNA molecule; this is done based on arrangement of intra-chain contacts within a folded RNA, termed as circuit topology. Synthesis of RNA is catalyzed by an enzyme—RNA polymerase—using DNA as a template, a process known as transcription. Initiation of transcription begins with the binding of the enzyme to a promoter sequence in the DNA; the DNA double helix is unwound by the helicase activity of the enzyme. The enzyme progresses along the template strand in the 3’ to 5’ direction, synthesizing a complementary RNA molecule with elongation occ
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the outermost layer of the skin. There are three principal shapes of epithelial cell: squamous and cuboidal; these can be arranged in a single layer of cells as simple epithelium, either squamous, columnar, or cuboidal, or in layers of two or more cells deep as stratified, either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei; this sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include secretion, selective absorption, transcellular transport, sensing. Epithelial layers contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.
Cell junctions are well employed in epithelial tissues. In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells; the three principal shapes associated with epithelial cells are—squamous and columnar. Squamous epithelium has cells; this is found as the lining of the mouth, the blood vessels and in the alveoli of the lungs. Cuboidal epithelium has cells whose height and width are the same. Columnar epithelium has cells taller. By layer, epithelium is classed as either simple epithelium, only one cell thick or stratified epithelium having two or more cells in thickness or multi-layered – as stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium, both types of layering can be made up of any of the cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia; this kind of epithelium is therefore described as pseudostratified columnar epithelium.
Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium. Simple epithelium is a single layer of cells with every cell in direct contact with the basement membrane that separates it from the underlying connective tissue. In general, it is found where filtration occur; the thinness of the epithelial barrier facilitates these processes. In general, simple epithelial tissues are classified by the shape of their cells; the four major classes of simple epithelium are: simple squamous. Simple squamous. Simple cuboidal: these cells may have secretory, absorptive, or excretory functions. Examples include small collecting ducts of kidney and salivary gland. Simple columnar. Non-ciliated epithelium can possess microvilli; some tissues are referred to as simple glandular columnar epithelium. These secrete mucus and are found in stomach and rectum. Pseudostratified columnar epithelium; the ciliated type is called respiratory epithelium as it is exclusively confined to the larger respiratory airways of the nasal cavity and bronchi.
Stratified epithelium differs from simple epithelium. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers the cells can be squamous, cuboidal or columnar. Stratified epithelia can have the following specializations: The basic cell types are squamous and columnar classed by their shape. Cells of epithelial tissue are scutoid shaped packed and form a continuous sheet, they have no intercellular spaces. All epithelia is separated from underlying tissues by an extracellular fibrous basement membrane; the lining of the mouth, lung alveoli and kidney tubules are all made of epithelial tissue. The lining of the blood and lymphatic vessels are of a specialised form of epithelium called endothelium. Epithelium lines both the outside and the inside cavities and lumina of bodies; the outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells.
Tissues that line the inside of the mouth, the esophagus, the vagina, part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, make up the exocrine and endocrine glands; the outer surface of the cornea is covered with fast-growing regenerated epithelial cells. A specialised form of epithelium – endothelium forms the inner lining of blood vessels and the heart, is known as vascular endotheliu
In humans, the respiratory tract is the part of the anatomy of the respiratory system involved with the process of respiration. Air is breathed in through the mouth. In the nasal cavity, a layer of mucous membrane acts as a filter and traps pollutants and other harmful substances found in the air. Next, air moves into the pharynx, a passage that contains the intersection between the esophagus and the larynx; the opening of the larynx has a special flap of cartilage, the epiglottis, that opens to allow air to pass through but closes to prevent food from moving into the airway. From the larynx, air moves into the trachea and down to the intersection that branches to form the right and left primary bronchi; each of these bronchi branch into secondary bronchi that branch into tertiary bronchi that branch into smaller airways called bronchioles that connect with tiny specialized structures called alveoli that function in gas exchange. The lungs which are located in the thoracic cavity, are protected from physical damage by the rib cage.
At the base of the lungs is a sheet of skeletal muscle called the diaphragm. The diaphragm separates the lungs from intestines; the diaphragm is the main muscle of respiration involved in breathing, is controlled by the sympathetic nervous system. The lungs are encased in a serous membrane that folds in on itself to form the pleurae – a two-layered protective barrier; the inner visceral pleura covers the surface of the lungs, the outer parietal pleura is attached to the inner surface of the thoracic cavity. The pleurae enclose; this fluid is used to decrease the amount of friction. The respiratory tract is divided into lower airways; the upper airways or upper respiratory tract includes the nose and nasal passages, paranasal sinuses, the pharynx, the portion of the larynx above the vocal folds. The lower airways or lower respiratory tract includes the portion of the larynx below the vocal folds, trachea and bronchioles; the lungs can be included in the lower respiratory tract or as separate entity and include the respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli.
The respiratory tract can be divided into a conducting zone and a respiratory zone, based on the distinction of transporting gases or exchanging them. The conducting zone includes structures outside of the lungs – the nose, pharynx and trachea, structures inside the lungs – the bronchi and terminal bronchioles; the conduction zone conducts air breathed in, filtered and moistened, into the lungs. It represents the 1st through the 16th division of the respiratory tract; the conducting zone is most of the respiratory tract that conducts gases into and out of the lungs, but excludes the respiratory zone that exchanges gases. The conducting zone functions to offer a low resistance pathway for airflow, it provides a major defense role in its filtering abilities. The respiratory zone includes the respiratory bronchioles, alveolar ducts and alveoli, is the site of oxygen and carbon dioxide exchange with the blood; the respiratory bronchioles and the alveolar ducts are responsible for 10% of the gas exchange.
The alveoli are responsible for the other 90%. The respiratory zone represents the 16th through the 23rd division of the respiratory tract. From the bronchi, the dividing tubes become progressively smaller with an estimated 20 to 23 divisions before ending at an alveolus; the upper respiratory tract, can refer to the parts of the respiratory system lying above the sternal angle, above the vocal folds, or above the cricoid cartilage. The larynx is sometimes included in both lower airways; the larynx is called the voice box and has the associated cartilage that produces sound. The tract consists of the nasal cavity and paranasal sinuses, the pharynx and sometimes includes the larynx; the lower respiratory tract or lower airway is derived from the developing foregut and consists of the trachea, bronchi and lungs. It sometimes includes the larynx; the lower respiratory tract is called the respiratory tree or tracheobronchial tree, to describe the branching structure of airways supplying air to the lungs, includes the trachea and bronchioles.
Trachea main bronchus lobar bronchus segmental bronchus subsegmental bronchus conducting bronchiole terminal bronchiole respiratory bronchiole alveolar duct alveolar sac alveolusAt each division point or generation, one airway branches into two or more smaller airways. The human respiratory tree may consist on average of 23 generations, while the respiratory tree of the mouse has up to 13 generations. Proximal divisions function to transmit air to the lower airways. Divisions including the respiratory bronchiole, alveolar ducts and alveoli, are specialized for gas exchange; the trachea is the largest tube in the respiratory tract and consists of tracheal rings of hyaline cartilage. It branches off into a left and a right main bronchus; the bronchi branch off into smaller sections inside the lungs, called bronchioles. These bronchioles give rise to the air sacs in the lungs called the alveoli; the lungs are the largest organs in the lower respiratory tract. The lungs are suspended within the pleural cavity of the thorax.
The pleurae are two thin membranes, one