A diuretic is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way. Alternatively, an antidiuretic, such as vasopressin, is an agent or drug which reduces the excretion of water in urine. In medicine, diuretics are used to treat heart failure, liver cirrhosis, influenza, water poisoning, certain kidney diseases; some diuretics, such as acetazolamide, help to make the urine more alkaline and are helpful in increasing excretion of substances such as aspirin in cases of overdose or poisoning. Diuretics are sometimes abused by people with an eating disorder people with bulimia nervosa, with the goal of losing weight; the antihypertensive actions of some diuretics are independent of their diuretic effect. That is, the reduction in blood pressure is not due to decreased blood volume resulting from increased urine production, but occurs through other mechanisms and at lower doses than that required to produce diuresis.
Indapamide was designed with this in mind, has a larger therapeutic window for hypertension than most other diuretics. High ceiling diuretics may cause a substantial diuresis – up to 20% of the filtered load of NaCl and water; this is large in comparison to normal renal sodium reabsorption which leaves only about 0.4% of filtered sodium in the urine. Loop diuretics have this ability, are therefore synonymous with high ceiling diuretics. Loop diuretics, such as furosemide, inhibit the body's ability to reabsorb sodium at the ascending loop in the nephron, which leads to an excretion of water in the urine, whereas water follows sodium back into the extracellular fluid. Other examples of high ceiling loop diuretics include ethacrynic torasemide. Thiazide-type diuretics such as hydrochlorothiazide act on the distal convoluted tubule and inhibit the sodium-chloride symporter leading to a retention of water in the urine, as water follows penetrating solutes. Frequent urination is due to the increased loss of water that has not been retained from the body as a result of a concomitant relationship with sodium loss from the convoluted tubule.
The short-term anti-hypertensive action is based on the fact that thiazides decrease preload, decreasing blood pressure. On the other hand, the long-term effect is due to an unknown vasodilator effect that decreases blood pressure by decreasing resistance. Carbonic anhydrase inhibitors inhibit the enzyme carbonic anhydrase, found in the proximal convoluted tubule; this results in several effects including bicarbonate accumulation in the urine and decreased sodium absorption. Drugs in this class include methazolamide; these are diuretics. The term "potassium-sparing" refers to an effect rather than a location. Aldosterone adds sodium channels in the principal cells of the collecting duct and late distal tubule of the nephron. Spironolactone prevents aldosterone from entering the principal cells, preventing sodium reabsorption. Similar agents are potassium canreonate. Epithelial sodium channel blockers: amiloride and triamterene; the term "calcium-sparing diuretic" is sometimes used to identify agents that result in a low rate of excretion of calcium.
The reduced concentration of calcium in the urine can lead to an increased rate of calcium in serum. The sparing effect on calcium can be beneficial in unwanted in hypercalcemia; the thiazides and potassium-sparing diuretics are considered to be calcium-sparing diuretics. The thiazides cause a net decrease in calcium lost in urine; the potassium-sparing diuretics cause a net increase in calcium lost in urine, but the increase is much smaller than the increase associated with other diuretic classes. By contrast, loop diuretics promote a significant increase in calcium excretion; this can increase risk of reduced bone density. Osmotic diuretics are substances that increase osmolarity but have limited tubular epithelial cell permeability, they work by expanding extracellular fluid and plasma volume, therefore increasing blood flow to the kidney the peritubular capillaries. This thus impairs the concentration of urine in the loop of Henle. Furthermore, the limited tubular epithelial cell permeability increases osmolality and thus water retention in the filtrate.
It was believed that the primary mechanism of osmotic diuretics such as mannitol is that they are filtered in the glomerulus, but cannot be reabsorbed. Thus their presence leads to an increase in the osmolarity of the filtrate and to maintain osmotic balance, water is retained in the urine. Glucose, like mannitol, is a sugar that can behave as an osmotic diuretic. Unlike mannitol, glucose is found in the blood. However, in certain conditions, such as diabetes mellitus, the concentration of glucose in the blood exceeds the maximum reabsorption capacity of the kidney; when this happens, glucose remains in the filtrate, leading to the osmotic retention of water in the urine. Glucosuria causes a loss of hypotonic water and Na+, leading to a hypertonic state wit
A vulture is a scavenging bird of prey. The two types of vultures are the New World vultures, including the Californian and Andean condors, the Old World vultures, including the birds that are seen scavenging on carcasses of dead animals on African plains; some traditional Old World vultures are not related to the others, why the vultures are to be subdivided into three taxa rather than two. New World vultures are found in South America. A particular characteristic of many vultures is a bald head, devoid of normal feathers. Although it has been believed to help keep the head clean when feeding, the bare skin may play an important role in thermoregulation. Vultures have been observed to hunch their bodies and tuck in their heads in the cold, open their wings and stretch their necks in the heat. Vultures use urine as a way to keep themselves cool by urinating on themselves. A group of vultures is called a committee or wake; the term kettle refers to vultures in flight, while committee refers to vultures resting on the ground or in trees.
Wake is reserved for a group of vultures. The word Geier does not have a precise meaning in ornithology; the Old World vultures found in Africa and Europe belong to the family Accipitridae, which includes eagles, kites and hawks. Old World vultures find carcasses by sight; the 16 species in 9 genera are: Cinereous vulture, Aegypius monachus Griffon vulture, Gyps fulvus White-rumped vulture, Gyps bengalensis Rüppell's vulture, Gyps rueppelli Indian vulture, Gyps indicus Slender-billed vulture, Gyps tenuirostris Himalayan vulture, Gyps himalayensis White-backed vulture, Gyps africanus Cape vulture, Gyps coprotheres Hooded vulture, Necrosyrtes monachus Red-headed vulture, Sarcogyps calvus Lappet-faced vulture, Torgos tracheliotos White-headed vulture, Trigonoceps occipitalis Bearded vulture, Gypaetus barbatus Egyptian vulture, Neophron percnopterus Palm-nut vulture, Gypohierax angolensis The New World vultures and condors found in warm and temperate areas of the Americas are not related to the similar Accipitridae, but belong in the family Cathartidae, once considered to be related to the storks.
However, recent DNA evidence suggests that they should be included among the Accipitriformes, along with other birds of prey. However, they are still not related to the other vultures. Several species have a good sense of smell, unusual for raptors, are able to smell dead animals from great heights, up to a mile away; the seven species are: Black vulture Coragyps atratus in South America and north to the US Turkey vulture Cathartes aura throughout the Americas to southern Canada Lesser yellow-headed vulture Cathartes burrovianus in South America and north to Mexico Greater yellow-headed vulture Cathartes melambrotus in the Amazon Basin of tropical South America California condor Gymnogyps californianus in California widespread in the mountains of western North America Andean condor Vultur gryphus in the Andes King vulture Sarcoramphus papa from southern Mexico to northern Argentina Vultures are scavengers, meaning they eat dead animals. They attack healthy animals, but may kill the wounded or sick.
When a carcass has too thick a hide for its beak to open, it waits for a larger scavenger to eat first. Vast numbers have been seen upon battlefields, they gorge themselves when prey is abundant, until their crops bulge, sit, sleepy or half torpid, to digest their food. These birds disgorge it from their crops; the mountain-dwelling bearded vulture is the only vertebrate to specialize in eating bones, does carry bones to the nest for the young, it hunts some live prey. Vultures are of great value as scavengers in hot regions. Vulture stomach acid is exceptionally corrosive, allowing them to safely digest putrid carcasses infected with botulinum toxin, hog cholera bacteria, anthrax bacteria that would be lethal to other scavengers and remove these bacteria from the environment. New World vultures vomit when threatened or approached. Contrary to some accounts, they do not "projectile vomit" on their attacker as a deliberate defense, but it does lighten their stomach load to make take-off easier, the vomited meal residue may distract a predator, allowing the bird to escape.
New World vultures urinate straight down their legs. Vultures in south Asia in India and Nepal, have declined since the early 1990s, it has been found that this decline was caused by residues of the veterinary drug Diclofenac in animal carcasses. The government of India has taken late cognizance of this fact and has banned the drug for animals. However, it may take decades for vultures to come back to their earlier population level, if they do: without vultures to pick corpses clean, rabies-carrying dogs have multiplied, feeding on the carrion, age-old practices like the sky burials of the Parsees are coming to an end, permanently reducing the supply of corpses; the same problem is seen in Nepal where government has taken some late steps to conserve remaining vultures. In Central Africa there has been efforts to conserve the remaining vultures and bring their population numbers back up; this is due to
A drug is any substance that, when inhaled, smoked, absorbed via a patch on the skin, or dissolved under the tongue causes a physiological change in the body. In pharmacology, a drug is a chemical substance of known structure, other than a nutrient of an essential dietary ingredient, when administered to a living organism, produces a biological effect. A pharmaceutical drug called a medication or medicine, is a chemical substance used to treat, prevent, or diagnose a disease or to promote well-being. Traditionally drugs were obtained through extraction from medicinal plants, but more also by organic synthesis. Pharmaceutical drugs may be used for a limited duration, or on a regular basis for chronic disorders. Pharmaceutical drugs are classified into drug classes—groups of related drugs that have similar chemical structures, the same mechanism of action, a related mode of action, that are used to treat the same disease; the Anatomical Therapeutic Chemical Classification System, the most used drug classification system, assigns drugs a unique ATC code, an alphanumeric code that assigns it to specific drug classes within the ATC system.
Another major classification system is the Biopharmaceutics Classification System. This classifies drugs according to their permeability or absorption properties. Psychoactive drugs are chemical substances that affect the function of the central nervous system, altering perception, mood or consciousness, they include alcohol, a depressant, the stimulants nicotine and caffeine. These three are the most consumed psychoactive drugs worldwide and are considered recreational drugs since they are used for pleasure rather than medicinal purposes. Other recreational drugs include hallucinogens and amphetamines and some of these are used in spiritual or religious settings; some drugs can cause addiction and all drugs can have side effects. Excessive use of stimulants can promote stimulant psychosis. Many recreational drugs are illicit and international treaties such as the Single Convention on Narcotic Drugs exist for the purpose of their prohibition. In English, the noun "drug" is thought to originate from Old French "drogue" deriving into "droge-vate" from Middle Dutch meaning "dry barrels", referring to medicinal plants preserved in them.
The transitive verb "to drug" arose and invokes the psychoactive rather than medicinal properties of a substance. A medication or medicine is a drug taken to cure or ameliorate any symptoms of an illness or medical condition; the use may be as preventive medicine that has future benefits but does not treat any existing or pre-existing diseases or symptoms. Dispensing of medication is regulated by governments into three categories—over-the-counter medications, which are available in pharmacies and supermarkets without special restrictions. In the United Kingdom, behind-the-counter medicines are called pharmacy medicines which can only be sold in registered pharmacies, by or under the supervision of a pharmacist; these medications are designated by the letter P on the label. The range of medicines available without a prescription varies from country to country. Medications are produced by pharmaceutical companies and are patented to give the developer exclusive rights to produce them; those that are not patented are called generic drugs since they can be produced by other companies without restrictions or licenses from the patent holder.
Pharmaceutical drugs are categorised into drug classes. A group of drugs will share a similar chemical structure, or have the same mechanism of action, the same related mode of action or target the same illness or related illnesses; the Anatomical Therapeutic Chemical Classification System, the most used drug classification system, assigns drugs a unique ATC code, an alphanumeric code that assigns it to specific drug classes within the ATC system. Another major classification system is the Biopharmaceutics Classification System; this groups drugs according to their permeability or absorption properties. Some religions ethnic religions are based on the use of certain drugs, known as entheogens, which are hallucinogens,—psychedelics, dissociatives, or deliriants; some drugs used as entheogens include kava which can act as a stimulant, a sedative, a euphoriant and an anesthetic. The roots of the kava plant are used to produce a drink, consumed throughout the cultures of the Pacific Ocean; some shamans from different cultures use entheogens, defined as "generating the divine within" to achieve religious ecstasy.
Amazonian shamans use ayahuasca a hallucinogenic brew for this purpose. Mazatec shamans have a long and continuous tradition of religious use of Salvia divinorum a psychoactive plant, its use is to facilitate visionary states of consciousness during spiritual healing sessions. Silene undulata is used as an entheogen, its root is traditionally used to induce vivid lucid dreams during the initiation process of shamans, classifying it a occurring oneirogen similar to the more well-known dream herb Calea ternifolia. Peyote a small spineless cactus has been a
In vitro studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology and its subdisciplines are traditionally done in labware such as test tubes, Petri dishes, microtiter plates. Studies conducted using components of an organism that have been isolated from their usual biological surroundings permit a more detailed or more convenient analysis than can be done with whole organisms. In contrast to in vitro experiments, in vivo studies are those conducted in animals, including humans, whole plants. In vitro studies are conducted using components of an organism that have been isolated from their usual biological surroundings, such as microorganisms, cells, or biological molecules. For example, microorganisms or cells can be studied in artificial culture media, proteins can be examined in solutions. Colloquially called "test-tube experiments", these studies in biology and their subdisciplines are traditionally done in test tubes, Petri dishes, etc.
They now involve the full range such as the omics. In contrast, studies conducted in living beings are called in vivo. Examples of in vitro studies include: the isolation and identification of cells derived from multicellular organisms in. Viruses, which only replicate in living cells, are studied in the laboratory in cell or tissue culture, many animal virologists refer to such work as being in vitro to distinguish it from in vivo work in whole animals. Polymerase chain reaction is a method for selective replication of specific DNA and RNA sequences in the test tube. Protein purification involves the isolation of a specific protein of interest from a complex mixture of proteins obtained from homogenized cells or tissues. In vitro fertilization is used to allow spermatozoa to fertilize eggs in a culture dish before implanting the resulting embryo or embryos into the uterus of the prospective mother. In vitro diagnostics refers to a wide range of medical and veterinary laboratory tests that are used to diagnose diseases and monitor the clinical status of patients using samples of blood, cells, or other tissues obtained from a patient.
In vitro testing has been used to characterize specific adsorption, distribution and excretion processes of drugs or general chemicals inside a living organism. These ADME process parameters can be integrated into so called "physiologically based pharmacokinetic models" or PBPK. In vitro studies permit a species-specific, more convenient, more detailed analysis than can be done with the whole organism. Just as studies in whole animals more and more replace human trials, so are in vitro studies replacing studies in whole animals. Living organisms are complex functional systems that are made up of, at a minimum, many tens of thousands of genes, protein molecules, RNA molecules, small organic compounds, inorganic ions, complexes in an environment, spatially organized by membranes, in the case of multicellular organisms, organ systems; these myriad components interact with each other and with their environment in a way that processes food, removes waste, moves components to the correct location, is responsive to signalling molecules, other organisms, sound, taste and balance.
This complexity makes it difficult to identify the interactions between individual components and to explore their basic biological functions. In vitro work simplifies the system under study, so the investigator can focus on a small number of components. For example, the identity of proteins of the immune system, the mechanism by which they recognize and bind to foreign antigens would remain obscure if not for the extensive use of in vitro work to isolate the proteins, identify the cells and genes that produce them, study the physical properties of their interaction with antigens, identify how those interactions lead to cellular signals that activate other components of the immune system. Another advantage of in vitro methods is that human cells can be studied without "extrapolation" from an experimental animal's cellular response. In vitro methods can be miniaturized and automated, yielding high-throughput screening methods for testing molecules in pharmacology or toxicology The primary disadvantage of in vitro experimental studies is that it may be challenging to extrapolate from the results of in vitro work back to the biology of the intact organism.
Investigators doing in vitro work must be careful to avoid over-interpretation of their results, which can lead to erroneous conclusions about organismal and systems biology. For example, scientists developing a new viral drug to treat an infection with a pathogenic virus may find that a candidate drug functions to prevent viral repl
Nonsteroidal anti-inflammatory drug
Nonsteroidal anti-inflammatory drugs are a drug class that reduce pain, decrease fever, prevent blood clots and, in higher doses, decrease inflammation. Side effects depend on the specific drug, but include an increased risk of gastrointestinal ulcers and bleeds, heart attack and kidney disease; the term nonsteroidal distinguishes these drugs from steroids, which while having a similar eicosanoid-depressing, anti-inflammatory action, have a broad range of other effects. First used in 1960, the term served to distance these medications from steroids, which where stigmatised at the time due to the connotations with anabolic steroid abuse. NSAIDs work by inhibiting the activity of cyclooxygenase enzymes. In cells, these enzymes are involved in the synthesis of key biological mediators, namely prostaglandins which are involved in inflammation, thromboxanes which are involved in blood clotting. There are two types of NSAID available: COX-2 selective. Most NSAIDs are non-selective, inhibit the activity of both COX-1 and COX-2.
These NSAIDs, while reducing inflammation inhibit platelet aggregation and increase the risk of gastrointestinal ulcers/bleeds. COX-2 selective inhibitors have less gastrointestinal side effects, but promote thrombosis and increase the risk of heart attack; as a result, COX-2 selective inhibitors are contraindicated due to the high risk of undiagnosed vascular disease. These differential effects are due to the different roles and tissue localisations of each COX isoenzyme. By inhibiting physiological COX activity, all NSAIDs increase the risk of kidney disease and, through a related mechanism, heart attack; the most prominent NSAIDs are aspirin and naproxen, all available over the counter in most countries. Paracetamol is not considered an NSAID because it has only minor anti-inflammatory activity, it treats pain by blocking COX-2 in the central nervous system, but not much in the rest of the body. NSAIDs are used for the treatment of acute or chronic conditions where pain and inflammation are present.
NSAIDs are used for the symptomatic relief of the following conditions: Aspirin, the only NSAID able to irreversibly inhibit COX-1, is indicated for antithrombosis through inhibition of platelet aggregation. This is useful for the management of arterial thrombosis and prevention of adverse cardiovascular events like heart attacks. Aspirin inhibits platelet aggregation by inhibiting the action of thromboxane A2. In a more specific application, the reduction in prostaglandins is used to close a patent ductus arteriosus in neonates if it has not done so physiologically after 24 hours. NSAIDs are useful in the management of post-operative dental pain following invasive dental procedures such as dental extraction; when not contra-indicated they are favoured over the use of paracetamol alone due to the anti-inflammatory effect they provide. When used in combination with paracetamol the analgesic effect has been proven to be improved. There is weak evidence suggesting that taking pre-operative analgesia can reduce the length of post operative pain associated with placing orthodontic spacers under local anaesthetic.
Combination of NSAIDs with pregabalin as preemptive analgesia has shown promising results for decreasing post operative pain intensity. The effectiveness of NSAID's for treating non-cancer chronic pain and cancer-related pain in children and adolescents is not clear. There have not been sufficient numbers of high-quality randomized controlled trials conducted. NSAIDs may be used with caution by people with the following conditions: Irritable bowel syndrome Persons who are over age 50, who have a family history of GI problems Persons who have had past GI problems from NSAID useNSAIDs should be avoided by people with the following conditions: The widespread use of NSAIDs has meant that the adverse effects of these drugs have become common. Use of NSAIDs increases risk of a range of gastrointestinal problems, kidney disease and adverse cardiovascular events; as used for post-operative pain, there is evidence of increased risk of kidney complications. Their use following gastrointestinal surgery remains controversial, given mixed evidence of increased risk of leakage from any bowel anastomosis created.
An estimated 10–20% of NSAID patients experience dyspepsia. In the 1990s high doses of prescription NSAIDs were associated with serious upper gastrointestinal adverse events, including bleeding. Over the past decade, deaths associated with gastric bleeding have declined. NSAIDs, like all drugs, may interact with other medications. For example, concurrent use of NSAIDs and quinolones may increase the risk of quinolones' adverse central nervous system effects, including seizure. There is an argument over the benefits and risks of NSAIDs for treating chronic musculoskeletal pain; each drug has a benefit-risk profile and balancing the risk of no treatment with the competing potential risks of various therapies is the clinician's responsibility. If a COX-2 inhibitor is taken, a traditional NSAID should not be taken at the same time. In addition, people on daily aspirin therapy must be careful if they use other NSAIDs, as these may inhibit the cardioprotective effects of aspirin. Rofecoxib was shown to produce fewer gastrointestinal adverse drug reactions compared with naproxen.
This study, the VIGOR trial, raised the issue of the cardiovascular safety of the coxibs. A statistically significant increase in the incidence of myocardial infarctions was observed in patients on rofecoxib. Further data, from the APPROVe trial, s
A joint or articulation is the connection made between bones in the body which link the skeletal system into a functional whole. They are constructed to allow for different types of movement; some joints, such as the knee and shoulder, are self-lubricating frictionless, are able to withstand compression and maintain heavy loads while still executing smooth and precise movements. Other joints such as sutures between the bones of the skull permit little movement in order to protect the brain and the sense organs; the connection between a tooth and the jawbone is called a joint, is described as a fibrous joint known as a gomphosis. Joints are classified both functionally. Joints are classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of movement between the articulating bones. In practice, there is significant overlap between the two types of classifications. Monoarticular – concerning one joint oligoarticular or pauciarticular – concerning 2–4 joints polyarticular – concerning 5 or more joints Structural classification names and divides joints according to the type of binding tissue that connects the bones to each other.
There are four structural classifications of joints: fibrous joint – joined by dense regular connective tissue, rich in collagen fibers cartilaginous joint – joined by cartilage. There are two types: primary cartilaginous joints composed of hyaline cartilage, secondary cartilaginous joints composed of hyaline cartilage covering the articular surfaces of the involved bones with fibrocartilage connecting them. Synovial joint – not directly joined – the bones have a synovial cavity and are united by the dense irregular connective tissue that forms the articular capsule, associated with accessory ligaments. Facet joint – joint between two articular processes between two vertebrae. Joints can be classified functionally according to the type and degree of movement they allow: Joint movements are described with reference to the basic anatomical planes. Synarthrosis – permits little or no mobility. Most synarthrosis joints are fibrous joints. Amphiarthrosis – permits slight mobility. Most amphiarthrosis joints are cartilaginous joints.
Synovial joint – movable. Synovial joints can in turn be classified into six groups according to the type of movement they allow: plane joint and socket joint, hinge joint, pivot joint, condyloid joint and saddle joint. Joints can be classified, according to the number of axes of movement they allow, into nonaxial, monoaxial and multiaxial. Another classification is according to the degrees of freedom allowed, distinguished between joints with one, two or three degrees of freedom. A further classification is according to the number and shapes of the articular surfaces: flat and convex surfaces. Types of articular surfaces include trochlear surfaces. Joints can be classified based on their anatomy or on their biomechanical properties. According to the anatomic classification, joints are subdivided into simple and compound, depending on the number of bones involved, into complex and combination joints: Simple joint: two articulation surfaces Compound joint: three or more articulation surfaces Complex joint: two or more articulation surfaces and an articular disc or meniscus The joints may be classified anatomically into the following groups: Joints of hand Elbow joints Wrist joints Axillary articulations Sternoclavicular joints Vertebral articulations Temporomandibular joints Sacroiliac joints Hip joints Knee joints Articulations of footUnmyelinated nerve fibers are abundant in joint capsules and ligaments as well as in the outer part of intraarticular menisci.
These nerve fibers are responsible for pain perception. Damaging the cartilage of joints or the bones and muscles that stabilize the joints can lead to joint dislocations and osteoarthritis. Swimming is a great way to exercise the joints with minimal damage. A joint disorder is termed arthropathy, when involving inflammation of one or more joints the disorder is called arthritis. Most joint disorders involve arthritis, but joint damage by external physical trauma is not termed arthritis. Arthropathies are called polyarticular when involving many joints and monoarticular when involving only a single joint. Arthritis is the leading cause of disability in people over the age of 55. There are many different forms of arthritis; the most common form of arthritis, occurs following trauma to the joint, following an infection of the joint or as a result of aging and the deterioration of articular cartilage. Furthermore, there is emerging evidence that abnormal anatomy may contribute to early development of osteoarthritis.
Other forms of arthritis are rheumatoid arthritis and psoriatic arthritis, which are autoimmune diseases in which the body is attacking itself. Septic arthritis is caused by joint infection. Gouty arthritis is caused by deposition of uric acid crystals in the joint that results in subsequent inflammation. Additionally, there is a less common form of gout, caused by the formation of rhomboidal-shaped crystals of calcium pyrophosphate; this form of gout is known as pseudogout. Temporomandibular joint syndrome involves the jaw joints and can cause facial p
Cholesterol is an organic molecule. It is a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell membranes. Cholesterol serves as a precursor for the biosynthesis of steroid hormones, bile acid and vitamin D. Cholesterol is the principal sterol synthesized by all animals. In vertebrates, hepatic cells produce the greatest amounts, it is absent among prokaryotes, although there are some exceptions, such as Mycoplasma, which require cholesterol for growth. François Poulletier de la Salle first identified cholesterol in solid form in gallstones in 1769. However, it was not until 1815 that chemist Michel Eugène Chevreul named the compound "cholesterine". There is only one kind of cholesterol. There is no "good cholesterol" or "bad cholesterol"; the system that transports cholesterol where it is needed in the human body uses LDL and HDL to do so. Those are proteins, not lipids like cholesterol, neither of them are "bad", both are necessary to human health.
Cholesterol is essential for all animal life, with each cell capable of synthesizing it by way of a complex 37-step process. This begins with the mevalonate or HMG-CoA reductase pathway, the target of statin drugs, which encompasses the first 18 steps; this is followed by 19 additional steps to convert the resulting lanosterol into cholesterol. A human male weighing 68 kg synthesizes about 1 gram of cholesterol per day, his body contains about 35 g contained within the cell membranes. Typical daily cholesterol dietary intake for a man in the United States is 307 mg. Most ingested cholesterol is esterified; the body compensates for absorption of ingested cholesterol by reducing its own cholesterol synthesis. For these reasons, cholesterol in food, seven to ten hours after ingestion, has little, if any effect on concentrations of cholesterol in the blood. However, during the first seven hours after ingestion of cholesterol, as absorbed fats are being distributed around the body within extracellular water by the various lipoproteins, the concentrations increase.
Cholesterol is recycled in the body. The liver excretes it in a non-esterified form into the digestive tract. About 50% of the excreted cholesterol is reabsorbed by the small intestine back into the bloodstream. Plants make cholesterol in small amounts. Plants manufacture phytosterols, which can compete with cholesterol for reabsorption in the intestinal tract, thus reducing cholesterol reabsorption; when intestinal lining cells absorb phytosterols, in place of cholesterol, they excrete the phytosterol molecules back into the GI tract, an important protective mechanism. The intake of occurring phytosterols, which encompass plant sterols and stanols, ranges between ~200–300 mg/day depending on eating habits. Specially designed vegetarian experimental diets have been produced yielding upwards of 700 mg/day. Cholesterol, given that it composes about 30% of all animal cell membranes, is required to build and maintain membranes and modulates membrane fluidity over the range of physiological temperatures.
The hydroxyl group of each cholesterol molecule interacts with the water molecules surrounding the membrane as do the polar heads of the membrane phospholipids and sphingolipids, while the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolar fatty-acid chain of the other lipids. Through the interaction with the phospholipid fatty-acid chains, cholesterol increases membrane packing, which both alters membrane fluidity and maintains membrane integrity so that animal cells do not need to build cell walls; the membrane remains stable and durable without being rigid, allowing animal cells to change shape and animals to move. The structure of the tetracyclic ring of cholesterol contributes to the fluidity of the cell membrane, as the molecule is in a trans conformation making all but the side chain of cholesterol rigid and planar. In this structural role, cholesterol reduces the permeability of the plasma membrane to neutral solutes, hydrogen ions, sodium ions.
Within the cell membrane, cholesterol functions in intracellular transport, cell signaling and nerve conduction. Cholesterol is essential for the structure and function of invaginated caveolae and clathrin-coated pits, including caveola-dependent and clathrin-dependent endocytosis; the role of cholesterol in endocytosis of these types can be investigated by using methyl beta cyclodextrin to remove cholesterol from the plasma membrane. Recent studies show that cholesterol is implicated in cell signaling processes, assisting in the formation of lipid rafts in the plasma membrane, which brings receptor proteins in close proximity with high concentrations of second messenger molecules. In multiple layers and phospholipids, both electrical insulators, can facilitate speed of transmission of electrical impulses along nerve tissue. For many neuron fibers, a myelin sheath, rich in cholesterol since it is derived from compacted layers of Schwann cell membrane, provides insulation for more efficient conduction of impulses.
Demyelination is believed to be part of the basis for multiple sclerosis. Within cells, cholesterol is a precursor molecule for several biochemical pathways. For example, it is the precursor molecule for the synthesis of vitamin D and all steroid hormones, including the adrenal gland ho