Abdomen
The abdomen constitutes the part of the body between the thorax and pelvis, in humans and in other vertebrates. The abdomen is the frontal part of the abdominal segment of the trunk, the dorsal part of this segment being the back of the abdomen; the region occupied by the abdomen is termed the abdominal cavity. In arthropods it is the posterior tagma of the body; the abdomen stretches from the thorax at the thoracic diaphragm to the pelvis at the pelvic brim. The pelvic brim stretches from the lumbosacral joint to the pubic symphysis and is the edge of the pelvic inlet; the space above this inlet and under the thoracic diaphragm is termed the abdominal cavity. The boundary of the abdominal cavity is the abdominal wall in the front and the peritoneal surface at the rear; the abdomen contains most of the tubelike organs of the digestive tract, as well as several solid organs. Hollow abdominal organs include the stomach, the small intestine, the colon with its attached appendix. Organs such as the liver, its attached gallbladder, the pancreas function in close association with the digestive tract and communicate with it via ducts.
The spleen and adrenal glands lie within the abdomen, along with many blood vessels including the aorta and inferior vena cava. Anatomists may consider the urinary bladder, fallopian tubes, ovaries as either abdominal organs or as pelvic organs; the abdomen contains an extensive membrane called the peritoneum. A fold of peritoneum may cover certain organs, whereas it may cover only one side of organs that lie closer to the abdominal wall. Anatomists call the latter type of organs retroperitoneal. Digestive tract: Stomach, small intestine, large intestine with cecum and appendix Accessory organs of the digestive tract: Liver and pancreas Urinary system: Kidneys and ureters – but technically located in retroperitoneum – outside peritoneal membrane Other organs: SpleenAbdominal organs can be specialized in some animals. For example, the stomach of ruminants is divided into four chambers – rumen, reticulum and abomasum. In vertebrates, the abdomen is a large cavity enclosed by the abdominal muscles and laterally, by the vertebral column dorsally.
Lower ribs can enclose ventral and lateral walls. The abdominal cavity is upper part of the pelvic cavity, it is attached to the thoracic cavity by the diaphragm. Structures such as the aorta, inferior vena cava and esophagus pass through the diaphragm. Both the abdominal and pelvic cavities are lined by a serous membrane known as the parietal peritoneum; this membrane is continuous with the visceral peritoneum lining the organs. The abdomen in vertebrates contains a number of organs belonging, for instance, to the digestive tract and urinary system. There are three layers of the abdominal wall, they are, from the outside to the inside: external oblique, internal oblique, transverse abdominal. The first three layers extend between the vertebral column, the lower ribs, the iliac crest and pubis of the hip. All of their fibers merge towards the midline and surround the rectus abdominis in a sheath before joining up on the opposite side at the linea alba. Strength is gained by the criss-crossing of fibers, such that the external oblique are downward and forward, the internal oblique upward and forward, the transverse abdominal horizontally forward.
The transverse abdominal muscle is triangular, with its fibers running horizontally. It lies between the underlying transverse fascia, it originates from Poupart's ligament, the inner lip of the ilium, the lumbar fascia and the inner surface of the cartilages of the six lower ribs. It inserts into the linea alba behind the rectus abdominis; the rectus abdominis muscles are flat. The muscle is crossed by three fibrous bands called the tendinous intersections; the rectus abdominis is enclosed in a thick sheath formed, as described above, by fibers from each of the three muscles of the lateral abdominal wall. They originate at the pubis bone, run up the abdomen on either side of the linea alba, insert into the cartilages of the fifth and seventh ribs. In the region of the groin, the inguinal canal, a passage through the layers; this gap is where the testes can drop through the wall and where the fibrous cord from the uterus in the female runs. This is where weakness can form, cause inguinal hernias.
The pyramidalis muscle is triangular. It is located in the lower abdomen in front of the rectus abdominis, it is inserted into the linea alba halfway up to the navel. Functionally, the human abdomen is where most of the alimentary tract is placed and so most of the absorption and digestion of food occurs here; the alimentary tract in the abdomen consists of the lower esophagus, the stomach, the duodenum, the jejunum, the cecum and the appendix, the ascending and descending colons, the sigmoid colon and the rectum. Other vital organs inside the abdomen include the kidneys, the pancreas and the spleen; the abdominal wall is split into the posterior and anterior walls. The abdominal muscles have different important functions, they assist in the breathing process as accessory muscles of respiration. Moreover, these muscles serve as protection for the inner organs. Furthermore, together with the back muscles they provide postural support and are important in defining the form; when the glottis is closed and the thorax and pelvis are fixed, they are integral in the cough, defecation, childbirth and singing functions.
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Digestion
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism, divided into two processes based on how food is broken down: mechanical and chemical digestion; the term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. In chemical digestion, enzymes break down food into the small molecules. In the human digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication, a form of mechanical digestion, the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus.
It will travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice contains hydrochloric acid and pepsin, it contains rennin in case of infants and toddlers. As the first two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, waves of muscular contractions that move along the stomach wall; this allows the mass of food to further mix with the digestive enzymes. After some time, the resulting thick liquid is called chyme; when the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas and bile juice from the liver and passes through the small intestine, in which digestion continues. When the chyme is digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine.
Water and minerals are reabsorbed back into the blood in the colon where the pH is acidic about 5.6 ~ 6.9. Some vitamins, such as biotin and vitamin K produced by bacteria in the colon are absorbed into the blood in the colon. Waste material is eliminated from the rectum during defecation. Digestive systems take many forms. There is a fundamental distinction between external digestion. External digestion developed earlier in evolutionary history, most fungi still rely on it. In this process, enzymes are secreted into the environment surrounding the organism, where they break down an organic material, some of the products diffuse back to the organism. Animals have a tube in which internal digestion occurs, more efficient because more of the broken down products can be captured, the internal chemical environment can be more efficiently controlled; some organisms, including nearly all spiders secrete biotoxins and digestive chemicals into the extracellular environment prior to ingestion of the consequent "soup".
In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient. Bacteria use several systems to obtain nutrients from other organisms in the environments. In a channel transupport system, several proteins form a contiguous channel traversing the inner and outer membranes of the bacteria, it is a simple system, which consists of only three protein subunits: the ABC protein, membrane fusion protein, outer membrane protein. This secretion system transports various molecules, from drugs, to proteins of various sizes; the molecules secreted vary in size from the small Escherichia coli peptide colicin V, to the Pseudomonas fluorescens cell adhesion protein LapA of 900 kDa. A type III secretion system means that a molecular syringe is used through which a bacterium can inject nutrients into protist cells. One such mechanism was first discovered in Y. pestis and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than be secreted into the extracellular medium.
The conjugation machinery of some bacteria is capable of transporting proteins. It was discovered in Agrobacterium tumefaciens, which uses this system to introduce the Ti plasmid and proteins into the host, which develops the crown gall; the VirB complex of Agrobacterium tumefaciens is the prototypic system. The nitrogen fixing Rhizobia are an interesting case, wherein conjugative elements engage in inter-kingdom conjugation; such elements as the Agrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and transform the plant cells into factories for the production of opines, which the bacteria use as carbon and energy sources. Infected plant cells form crown root tumors; the Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts of the infected plant. The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an inde
Peristaltic pump
A peristaltic pump is a type of positive displacement pump used for pumping a variety of fluids, they are commonly known as roller pumps. The fluid is contained within a flexible tube fitted inside a circular pump casing. A rotor with a number of "rollers", "shoes", "wipers", or "lobes" attached to the external circumference of the rotor compresses the flexible tube; as the rotor turns, the part of the tube under compression is pinched closed thus forcing the fluid to be pumped to move through the tube. Additionally, as the tube opens to its natural state after the passing of the cam fluid flow is induced to the pump; this process is called peristalsis and is used in many biological systems such as the gastrointestinal tract. There will be two or more rollers, or wipers, occluding the tube, trapping between them a body of fluid; the body of fluid is transported, at ambient pressure, toward the pump outlet. Peristaltic pumps may run continuously, or they may be indexed through partial revolutions to deliver smaller amounts of fluid.
The peristaltic pump was first patented in the United States by Rufus Porter and J. D. Bradley in 1855 as a well pump, by Eugene Allen in 1881 for blood transfusions, it was developed by heart surgeon Dr. Michael DeBakey for blood transfusions while he was a medical student in 1932 and used by him for cardiopulmonary bypass systems. A specialized nonocclusive roller pump using soft flat tubing was developed in 1992 for cardiopulmonary bypass systems. Peristaltic pumps are used to pump clean/sterile or aggressive fluids without exposing those fluids to contamination from exposed pump components; some common applications include pumping IV fluids through an infusion device, aggressive chemicals, high solids slurries and other materials where isolation of the product from the environment, the environment from the product, are critical. It is used in heart-lung machines to circulate blood during a bypass surgery, in hemodialysis systems, as the pump does not cause significant hemolysis. Peristaltic pumps are used in a wide variety of industrial applications agriculture as they are well suited for common agricultural chemicals.
Their unique design makes roller pumps suited to pumping abrasives and viscous fluids. The ideal peristaltic pump should have an infinite diameter of the pump head and the largest possible diameter of the rollers; such an ideal peristaltic pump would offer the longest possible tubing lifetime and provide a constant and pulsation-free flow rate. Such an ideal peristaltic pump can not be constructed in reality. However, peristaltic pumps can be designed to approach these ideal peristaltic pump parameters. One example of a possible construction is depicted; the exceptional design of the few peristaltic pumps offer constant accurate flow rates for several weeks together with a long tubing lifetime without the risk of tubing rupture. The pumped fluid contacts only the inside surface of the tubing thereby negating concern for other valves, O-rings or seals that might be incompatible with fluid being pumped. Therefore, only the composition of the tubing that the pumped medium travels through is considered for chemical compatibility.
The tubing needs to be elastomeric to maintain the circular cross section after millions of cycles of squeezing in the pump. This requirement eliminates a variety of non-elastomeric polymers that have compatibility with a wide range of chemicals, such as PTFE, polyolefins, PVDF, etc. from consideration as material for pump tubing. The popular elastomers for pump tubing are nitrile, Viton, silicone, PVC, EPDM, EPDM+polypropylene and natural rubber. Of these materials, natural rubber has the best fatigue resistance, EPDM and Hypalon have the best chemical compatibility. Silicone is popular with water-based fluids, such as in bio-pharma industry, but have limited range of chemical compatibility in other industries. Extruded fluoropolymer tubes such as FKM have good compatibility with acids and petroleum fuels, but have insufficient fatigue resistance to achieve an effective tube life. There are a couple of newer tubing developments that offer a broad chemical compatibility using lined tubing and fluoroelastomers.
With lined tubing, the thin inside liner is made of a chemically resistant material such as poly-olefin and PTFE that form a barrier for the rest of the tubing wall from coming in contact with the pumped fluid. These liners are materials that are not elastomeric, therefore the entire tube wall cannot be made with this material for peristaltic pump applications; this tubing provides adequate chemical compatibility and life to be used in chemically challenging applications. There are a few things to keep in mind when using these tubes - any pin holes in the liner during manufacturing could render the tubing vulnerable to chemical attack. In the case of stiff plastic liners like the polyolefins, with repeated flexing in the peristaltic pump they can develop cracks, rendering the bulk material again vulnerable to chemical attack. A common issue with all lined tubing is delamination of the liner with repeated flexing that signals the end of the tube's life. For those with need for chemically compatible tubing, these lined tubing offer a good solution.
With fluoroelastomer tubing, the elastomer itself has the chemical resistance. In the case of e.g. Chem-Sure, it is made of a perfluoroelastomer, that has the broadest chemical compatibility of all elastomers; the two fluoroelastomer tubes listed above combine the chemical compatibility with a long tu
Annelid
The annelids known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extant species including ragworms and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, yet others in moist terrestrial environments; the annelids are bilaterally symmetrical, coelomate, invertebrate organisms. They have parapodia for locomotion. Most textbooks still use the traditional division into polychaetes and leech-like species. Cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora and Sipuncula regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that includes molluscs, brachiopods and nemerteans; the basic annelid form consists of multiple segments.
Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly defined or absent in others, Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates within blood vessels, the vessels in segments near the front ends of these species are built up with muscles that act as hearts; the septa of such species enable them to change the shapes of individual segments, which facilitates movement by peristalsis or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment. Earthworms are oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil.
The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as bait. Scientists observe annelids to monitor the quality of fresh water. Although blood-letting is used less by doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength. Since annelids are soft-bodied, their fossils are rare – jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil, identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago.
Palaeontologists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, the earliest indisputable fossils of the group appear in the Tertiary period, which began 66 million years ago. There are over 22,000 living annelid species, ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus, which can both grow up to 3 metres long. Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids, most textbooks use the traditional classification into the following sub-groups: Polychaetes; as their name suggests, they have multiple chetae per segment. Polychaetes have parapodia that function as limbs, nuchal organs that are thought to be chemosensors. Most are marine animals, although a few species live in fresh water and fewer on land. Clitellates; these have few or no chetae per segment, no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch or, in moniligastrids, yolky eggs that provide nutrition for the embyros.
The clitellates are sub-divided into: Oligochaetes. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or decomposed organic materials. Hirudinea, whose name means "leech-shaped" and whose best known members are leeches. Marine species are blood-sucking parasites on fish, while most freshwater species are predators, they have suckers at both ends of their bodies, use these to move rather like inchworms. The Archiannelida, minute annelids that live in the spaces between grains of marine sediment, were treated as a separate class because of their simple body structure, but are now regarded as polychaetes; some other groups of animals have been classified in various ways, but are now regarded as annelids: Pogonophora / Siboglinidae were first discovered in 1914, their lack of a recognizable gut made it difficult to classify them. They have been classified as a separate phylum, Pogonophora, or as two phyla and Vestimentifera. More they have been re-classified as a family, Siboglinidae
Seta
In biology, setae are any of a number of different bristle- or hair-like structures on living organisms. Annelid setae are stiff, they help, for example, earthworms to attach to the surface and prevent backsliding during peristaltic motion. These hairs make it difficult to pull a worm straight from the ground. Setae in oligochaetes are composed of chitin, they are classified according to the limb. Crustaceans have mechano- and chemosensory setae. Setae are present on the mouthparts of crustaceans and can be found on grooming limbs. In some cases, setae are modified into scale like structures. Setae on the legs of krill and other small crustaceans help them to gather phytoplankton, it allows them to be eaten. Setae on the integument of insects are unicellular, meaning that each is formed from a single epidermal cell of a type called a trichogen meaning "bristle generator", they are at first hollow and in most forms remain hollow. They grow through and project through a secondary or accessory cell of a type called a tormogen, which generates the special flexible membrane that connects the base of the seta to the surrounding integument.
Depending on their form and function, setae may be called hairs, chaetae, or scales. The setal membrane is not cuticularized and movement is possible; some insects, such as Eriogaster lanestris larvae, use setae as a defense mechanism, as they can cause dermatitis when they come into contact with skin. The pads on a gecko's feet are small hair-like processes that play a role in the animal's ability to cling to vertical surfaces; the micrometer-scale setae branch into nanometer-scale projections called spatulae. Gekko's seta: According to Kellar Autumn, "Two front feet of a tokay gecko can withstand 20.1 N of force parallel to the surface with 227 mm2 of pad area. The foot of a tokay bears 3600 tetrads of setae per mm2, or 14,400 setae per mm2 - Consequently, a single seta should produce an average force of 6-2 pN, an average shear stress of 0-090 N mm−l. However, single setae proved both much less sticky and much more sticky than predicted by whole animal measurements, under varying experimental conditions, implying that attachment and detachment in gecko setae are mechanically controlled."
In mycology, "setae" refer to dark brown, thick-walled, thorn-like cystidia found in corticioid and poroid fungi in the family Hymenochaetaceae. Though microscopic, the setae of some species may be sufficiently prominent to be visible with a hand lens. In botany, "seta" refers to the stalk supporting the capsule of a moss or liverwort, supplying it with nutrients; the seta is part of the sporophyte and has a short foot embedded in the gametophyte on which it is parasitic. Setae are not present in all mosses, but in some species they may reach 15 to 20 centimeters in height. In the diatom family Chaetocerotaceae, "seta" refers to the hairlike outgrowths of the valve, i.e. of the face of the cells. These setae have a different structure than the valve; such setae may prevent rapid sinking and protect the cells from grazing. Synthetic setae are a class of synthetic adhesives that detach at will, sometimes called resetable adhesives, yet display substantial stickiness; the development of such synthetic materials is a matter of current research.
Chaeta Synthetic setae Van der Waals force
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two subgroups. Within single-unit cells, the whole bundle or sheet contracts as a syncytium. Smooth muscle cells are found in the walls of hollow organs, including the stomach, urinary bladder and uterus, in the walls of passageways, such as the arteries and veins of the circulatory system, the tracts of the respiratory and reproductive systems; these cells are present in the eyes and are able to change the size of the iris and alter the shape of the lens. In the skin, smooth muscle cells cause hair to stand erect in response to cold fear. Most smooth muscle is of the single-unit variety, that is, either the whole muscle contracts or the whole muscle relaxes, but there is multiunit smooth muscle in the trachea, the large elastic arteries, the iris of the eye. Single unit smooth muscle, however, is most common and lines blood vessels, the urinary tract, the digestive tract. However, the terms single- and multi-unit smooth muscle represents an oversimplification.
This is due to the fact that smooth muscles for the most part are controlled and influenced by a combination of different neural elements. In addition, it has been observed that most of the time there will be some cell to cell communication and activators/ inhibitors produced locally; this leads to a somewhat coordinated response in multiunit smooth muscle. Smooth muscle is fundamentally different from skeletal muscle and cardiac muscle in terms of structure, regulation of contraction, excitation-contraction coupling. Smooth muscle cells known as myocytes, have a fusiform shape and, like striated muscle, can tense and relax. However, smooth muscle tissue tends to demonstrate greater elasticity and function within a larger length-tension curve than striated muscle; this ability to stretch and still maintain contractility is important in organs like the intestines and urinary bladder. In the relaxed state, each cell is 20 -- 500 micrometers in length. A substantial portion of the volume of the cytoplasm of smooth muscle cells are taken up by the molecules myosin and actin, which together have the capability to contract, through a chain of tensile structures, make the entire smooth muscle tissue contract with them.
Myosin is class II in smooth muscle. Myosin II contains two heavy chains which constitute the tail domains; each of these heavy chains contains the N-terminal head domain, while the C-terminal tails take on a coiled-coil morphology, holding the two heavy chains together. Thus, myosin II has two heads. In smooth muscle, there is a single gene that codes for the heavy chains myosin II, but there are splice variants of this gene that result in four distinct isoforms. Smooth muscle may contain MHC, not involved in contraction, that can arise from multiple genes. Myosin II contains 4 light chains, resulting in 2 per head, weighing 20 and 17 kDa; these bind the heavy chains in the "neck" region between the head and tail. The MLC20 is known as the regulatory light chain and participates in muscle contraction. Two MLC20 isoforms are found in smooth muscle, they are encoded by different genes, but only one isoform participates in contraction; the MLC17 is known as the essential light chain. Its exact function is unclear, but it's believed that it contributes to the structural stability of the myosin head along with MLC20.
Two variants of MLC17 exist as a result of alternative splicing at the MLC17 gene. Different combinations of heavy and light chains allow for up to hundreds of different types of myosin structures, but it is unlikely that more than a few such combinations are used or permitted within a specific smooth muscle bed. In the uterus, a shift in myosin expression has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle; the thin filaments that form part of the contractile machinery are predominantly composed of α- and γ-actin. Smooth muscle α-actin is the predominant isoform within smooth muscle. There are lots of actin that does not take part in contraction, but that polymerizes just below the plasma membrane in the presence of a contractile stimulant and may thereby assist in mechanical tension. Alpha actin is expressed as distinct genetic isoforms such as smooth muscle, cardiac muscle and skeletal muscle specific isoforms of alpha actin.
The ratio of actin to myosin is between 10:1 in smooth muscle. Conversely, from a mass ratio standpoint, myosin is the dominant protein in striated skeletal muscle with the actin to myosin ratio falling in the 1:2 to 1:3 range. A typical value for healthy young adults is 1:2.2.. Tropomyosin is present in smooth muscle, spanning seven actin monomers and is laid out end to end over the entire length of the thin filaments. In striated muscle, tropomyosin serves to block actin–myosin interactions until calcium is present, but in smooth muscle, its function is unknown. Calponin molecules may exist in equal number as actin, has been proposed to be a load-bearing protein. Caldesmon has been suggested to be involved in tethering actin and tropomyosin, thereby enhance the ability of smooth muscle to maintain tension. All three of these proteins may have a role in inhibiting the ATPase activity of the m
Pepsin
Pepsin is an endopeptidase that breaks down proteins into smaller peptides. It is produced in the stomach and is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. Pepsin has a three-dimensional structure, of which one or more polypeptide chains twist and fold, bringing together a small number of amino acids to form the active site, or the location on the enzyme where the substrate binds and the reaction takes place. Pepsin is an aspartic protease, it is one of three principal proteases in the human digestive system, the other two being chymotrypsin and trypsin. During the process of digestion, these enzymes, each of, specialized in severing links between particular types of amino acids, collaborate to break down dietary proteins into their components, i.e. peptides and amino acids, which can be absorbed by the small intestine. Pepsin is most efficient in cleaving peptide bonds between hydrophobic and preferably aromatic amino acids such as phenylalanine and tyrosine.
Pepsin's proenzyme, pepsinogen, is released by the chief cells in the stomach wall, upon mixing with the hydrochloric acid of the gastric juice, pepsinogen activates to become pepsin. Pepsin was one of the first enzymes to be discovered, is polypeptidic in nature, it was discovered in 1836 by Theodor Schwann. Schwann coined its name from the Greek word πέψις pepsis, meaning "digestion". Scientists around this time began discovering many biochemical compounds that play a significant role in biological processes, pepsin was one of them. An acidic substance, able to convert nitrogen-based foods into water-soluble material was determined to be pepsin. In 1928, it became one of the first enzymes to be crystallized when John H. Northrop crystallized it using dialysis and cooling. Pepsin is expressed as a zymogen called pepsinogen, whose primary structure has an additional 44 amino acids. In the stomach, chief cells release pepsinogen; this zymogen is activated by hydrochloric acid, released from parietal cells in the stomach lining.
The hormone gastrin and the vagus nerve trigger the release of both pepsinogen and HCl from the stomach lining when food is ingested. Hydrochloric acid creates an acidic environment, which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin. Pepsin cleaves the 44 amino acids from pepsinogen to create more pepsin. Pepsin is most active in acidic environments between 37 °C and 42 °C. Accordingly, its primary site of synthesis and activity is in the stomach. Pepsin will digest up to 20% of ingested amide bonds by cleaving preferentially at the C-terminal side of aromatic amino acids such as phenylalanine and tyrosine. Pepsin exhibits preferential cleavage for hydrophobic, preferably aromatic, residues in P1 and P1' positions. Increased susceptibility to hydrolysis occurs if there is a sulfur-containing amino acid close to the peptide bond, which has an aromatic amino acid. Pepsin cleaves Phe1Val, Gln4His, Glu13Ala, Ala14Leu, Leu15Tyr, Tyr16Leu, Gly23Phe, Phe24 in the insulin B chain.
Pepsin exhibits maximal activity at pH 2.0 and is inactive at pH 6.5 and above, however pepsin is not denatured or irreversibly inactivated until pH 8.0. Therefore, pepsin in solution of up to pH 8.0 can be reactivated upon re-acidification. The stability of pepsin at high pH has significant implications on disease attributed to laryngopharyngeal reflux. Pepsin remains in the larynx following a gastric reflux event. At the mean pH of the laryngopharynx pepsin would be inactive but could be reactivated upon subsequent acid reflux events resulting in damage to local tissues. Pepsin is one of the primary causes of mucosal damage during laryngopharyngeal reflux. Pepsin remains in the larynx following a gastric reflux event. While enzymatically inactive in this environment, pepsin would remain stable and could be reactivated upon subsequent acid reflux events. Exposure of laryngeal mucosa to enzymatically active pepsin, but not irreversibly inactivated pepsin or acid, results in reduced expression of protective proteins and thereby increases laryngeal susceptibility to damage.
Pepsin may cause mucosal damage during weakly acidic or non-acid gastric reflux. Weak or non-acid reflux is correlated with mucosal injury. Under non-acid conditions, pepsin is internalized by cells of the upper airways such as the larynx and hypopharynx by a process known as receptor-mediated endocytosis; the receptor by which pepsin is endocytosed is unknown. Upon cellular uptake, pepsin is stored in intracellular vesicles of low pH at which its enzymatic activity would be restored. Pepsin is retained within the cell for up to 24 hours; such exposure to pepsin at neutral pH and endocyctosis of pepsin causes changes in gene expression associated with inflammation, which underlies signs and symptoms of reflux, tumor progression. This and other research implicates pepsin in carcinogenesis attributed to gastric reflux. Pepsin in airway specimens is considered to be a sensitive and specific marker for laryngopharyngeal reflux. Research to develop new pepsin-targeted therapeutic and diagnostic tools for gastric reflux is ongoing.
A rapid non-invasive pepsin diagnostic called Peptest is now available which determines the presence of pepsin in saliva samples. Pepsins should be stored at low temperatures to prevent autolysis. Pepsin may be inhibited by inhibitor compounds. Pepstatin is a low molecular weight compound and potent in
