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141-433: A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals , plants , fungi and protists . Nutrients can be incorporated into cells for metabolic purposes or excreted by cells to create non-cellular structures such as hair , scales , feathers , or exoskeletons . Some nutrients can be metabolically converted into smaller molecules in
282-452: A chemical reaction form a chemical compound . All compounds are substances, but not all substances are compounds. A chemical compound can be either atoms bonded together in molecules or crystals in which atoms, molecules or ions form a crystalline lattice . Compounds based primarily on carbon and hydrogen atoms are called organic compounds , and all others are called inorganic compounds . Compounds containing bonds between carbon and
423-660: A database and is popularly known as the Chemical substances index. Other computer-friendly systems that have been developed for substance information are: SMILES and the International Chemical Identifier or InChI. Often a pure substance needs to be isolated from a mixture , for example from a natural source (where a sample often contains numerous chemical substances) or after a chemical reaction (which often gives mixtures of chemical substances). Stoichiometry ( / ˌ s t ɔɪ k i ˈ ɒ m ɪ t r i / )
564-460: A gland . When the signal is received and acted on, negative feedback is provided to the receptor that stops the need for further signaling. The cannabinoid receptor type 1 (CB1), located at the presynaptic neuron , is a receptor that can stop stressful neurotransmitter release to the postsynaptic neuron; it is activated by endocannabinoids (ECs) such as anandamide ( N -arachidonoylethanolamide; AEA) and 2-arachidonoylglycerol (2-AG) via
705-574: A mixture , referencing them in the chemical substances index allows CAS to offer specific guidance on standard naming of alloy compositions. Non-stoichiometric compounds are another special case from inorganic chemistry , which violate the requirement for constant composition. For these substances, it may be difficult to draw the line between a mixture and a compound, as in the case of palladium hydride . Broader definitions of chemicals or chemical substances can be found, for example: "the term 'chemical substance' means any organic or inorganic substance of
846-548: A retrograde signaling process in which these compounds are synthesized by and released from postsynaptic neurons, and travel back to the presynaptic terminal to bind to the CB1 receptor for modulation of neurotransmitter release to obtain homeostasis. The polyunsaturated fatty acids (PUFAs) are lipid derivatives of omega-3 (docosahexaenoic acid, DHA , and eicosapentaenoic acid, EPA ) or of omega-6 (arachidonic acid, ARA ) are synthesized from membrane phospholipids and used as
987-618: A "ligand". However, the center does not need to be a metal, as exemplified by boron trifluoride etherate BF 3 OEt 2 , where the highly Lewis acidic , but non-metallic boron center takes the role of the "metal". If the ligand bonds to the metal center with multiple atoms, the complex is called a chelate . In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called isomers . Isomers usually have substantially different chemical properties, and often may be isolated without spontaneously interconverting. A common example
1128-416: A chemical reaction takes place and a new substance is formed, the compound iron(II) sulfide , with chemical formula FeS. The resulting compound has all the properties of a chemical substance and is not a mixture. Iron(II) sulfide has its own distinct properties such as melting point and solubility , and the two elements cannot be separated using normal mechanical processes; a magnet will be unable to recover
1269-494: A constant and sufficient supply of the micronutrient while simultaneously avoiding excess levels. If an insufficient amount of copper is ingested for a short period of time, copper stores in the liver will be depleted. Should this depletion continue, a copper health deficiency condition may develop. If too much copper is ingested, an excess condition can result. Both of these conditions, deficiency and excess, can lead to tissue injury and disease. However, due to homeostatic regulation,
1410-495: A deficiency state that compromises growth, survival and reproduction. Consumer advisories for dietary nutrient intakes such as the United States Dietary Reference Intake , are based on the amount required to prevent deficiency and provide macronutrient and micronutrient guides for both lower and upper limits of intake. In many countries, regulations require that food product labels display information about
1551-576: A dietary inadequacy". In setting human nutrient guidelines, government organizations do not necessarily agree on amounts needed to avoid deficiency or maximum amounts to avoid the risk of toxicity. For example, for vitamin C , recommended intakes range from 40 mg/day in India to 155 mg/day for the European Union. The table below shows U.S. Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for vitamins and minerals, PRIs for
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#17329162696441692-593: A dozen minerals absorbed through roots, plus carbon dioxide and oxygen absorbed or released through leaves. All organisms obtain all their nutrients from the surrounding environment. Plants absorb carbon, hydrogen, and oxygen from air and soil as carbon dioxide and water. Other nutrients are absorbed from soil (exceptions include some parasitic or carnivorous plants). Counting these, there are 17 important nutrients for plants: these are macronutrients; nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), oxygen(O) and hydrogen (H), and
1833-403: A high plasma pH stimulates the kidneys to secrete hydrogen ions into the blood and to excrete bicarbonate into the urine. The hydrogen ions combine with the excess bicarbonate ions in the plasma, once again forming an excess of carbonic acid which can be exhaled, as carbon dioxide, in the lungs, keeping the plasma bicarbonate ion concentration, the partial pressure of carbon dioxide and, therefore,
1974-400: A hormone released into the blood by the kidney in response to high PTH levels in the blood, the duodenum and jejunum . Parathyroid hormone (in high concentrations in the blood) causes bone resorption , releasing calcium into the plasma. This is a very rapid action which can correct a threatening hypocalcemia within minutes. High PTH concentrations cause the excretion of phosphate ions via
2115-400: A lowering of the plasma sodium concentration, or to a fall in the arterial blood pressure, the juxtaglomerular cells release renin into the blood. Renin is an enzyme which cleaves a decapeptide (a short protein chain, 10 amino acids long) from a plasma α-2-globulin called angiotensinogen . This decapeptide is known as angiotensin I . It has no known biological activity. However, when
2256-436: A metal are called organometallic compounds . Compounds in which components share electrons are known as covalent compounds. Compounds consisting of oppositely charged ions are known as ionic compounds, or salts . Coordination complexes are compounds where a dative bond keeps the substance together without a covalent or ionic bond. Coordination complexes are distinct substances with distinct properties different from
2397-605: A nutrient does harm to an organism. In the United States and Canada, recommended dietary intake levels of essential nutrients are based on the minimum level that "will maintain a defined level of nutriture in an individual", a definition somewhat different from that used by the World Health Organization and Food and Agriculture Organization of a "basal requirement to indicate the level of intake needed to prevent pathologically relevant and clinically detectable signs of
2538-462: A particular molecular identity, including – (i) any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature". In the field of geology , inorganic solid substances of uniform composition are known as minerals . When two or more minerals are combined to form mixtures (or aggregates ), they are defined as rocks . Many minerals, however, mutually dissolve into solid solutions , such that
2679-399: A precursor for endocannabinoids (ECs) mediate significant effects in the fine-tuning adjustment of body homeostasis. The word homeostasis ( / ˌ h oʊ m i oʊ ˈ s t eɪ s ɪ s / ) uses combining forms of homeo- and -stasis , Neo-Latin from Greek : ὅμοιος homoios , "similar" and στάσις stasis , "standing still", yielding the idea of "staying the same". The concept of
2820-402: A process of acclimatisation . Homeostasis does not govern every activity in the body. For instance, the signal (be it via neurons or hormones ) from the sensor to the effector is, of necessity, highly variable in order to convey information about the direction and magnitude of the error detected by the sensor. Similarly, the effector's response needs to be highly adjustable to reverse
2961-435: A range of blood pressure values by vasoconstriction and vasodilation of the arteries. High pressure receptors called baroreceptors in the walls of the aortic arch and carotid sinus (at the beginning of the internal carotid artery ) monitor the arterial blood pressure . Rising pressure is detected when the walls of the arteries stretch due to an increase in blood volume . This causes heart muscle cells to secrete
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#17329162696443102-437: A separate chemical substance. However, the polymer may be defined by a known precursor or reaction(s) and the molar mass distribution . For example, polyethylene is a mixture of very long chains of -CH 2 - repeating units, and is generally sold in several molar mass distributions, LDPE , MDPE , HDPE and UHMWPE . The concept of a "chemical substance" became firmly established in the late eighteenth century after work by
3243-516: A significant impact on health. Dietary fiber is not absorbed in the human digestive tract. Soluble fiber is metabolized to butyrate and other short-chain fatty acids by bacteria residing in the large intestine. Soluble fiber is marketed as serving a prebiotic function with claims for promoting "healthy" intestinal bacteria. Ethanol (C 2 H 5 OH) is not an essential nutrient, but it does supply approximately 29 kilojoules (7 kilocalories) of food energy per gram. For spirits (vodka, gin, rum, etc.)
3384-400: A simple mixture. Typically these have a metal, such as a copper ion, in the center and a nonmetals atom, such as the nitrogen in an ammonia molecule or oxygen in water in a water molecule, forms a dative bond to the metal center, e.g. tetraamminecopper(II) sulfate [Cu(NH 3 ) 4 ]SO 4 ·H 2 O. The metal is known as a "metal center" and the substance that coordinates to the center is called
3525-479: A single chemical substance ( allotropes ). For instance, oxygen exists as both diatomic oxygen (O 2 ) and ozone (O 3 ). The majority of elements are classified as metals . These are elements with a characteristic lustre such as iron , copper , and gold . Metals typically conduct electricity and heat well, and they are malleable and ductile . Around 14 to 21 elements, such as carbon , nitrogen , and oxygen , are classified as non-metals . Non-metals lack
3666-430: A single manufacturing process. For example, charcoal is an extremely complex, partially polymeric mixture that can be defined by its manufacturing process. Therefore, although the exact chemical identity is unknown, identification can be made with a sufficient accuracy. The CAS index also includes mixtures. Polymers almost always appear as mixtures of molecules of multiple molar masses, each of which could be considered
3807-723: A single rock is a uniform substance despite being a mixture in stoichiometric terms. Feldspars are a common example: anorthoclase is an alkali aluminum silicate, where the alkali metal is interchangeably either sodium or potassium. In law, "chemical substances" may include both pure substances and mixtures with a defined composition or manufacturing process. For example, the EU regulation REACH defines "monoconstituent substances", "multiconstituent substances" and "substances of unknown or variable composition". The latter two consist of multiple chemical substances; however, their identity can be established either by direct chemical analysis or reference to
3948-725: A standard serving in the United States is 44 millilitres ( 1 + 1 ⁄ 2 US fluid ounces), which at 40% ethanol (80 proof) would be 14 grams and 410 kJ (98 kcal). At 50% alcohol, 17.5 g and 513 kJ (122.5 kcal). Wine and beer contain a similar amount of ethanol in servings of 150 and 350 mL (5 and 12 US fl oz), respectively, but these beverages also contribute to food energy intake from components other than ethanol. A 150 mL (5 US fl oz) serving of wine contains 420 to 540 kJ (100 to 130 kcal). A 350 mL (12 US fl oz) serving of beer contains 400 to 840 kJ (95 to 200 kcal). According to
4089-424: A subsequent increase in hemoglobin that increases the oxygen carrying capacity. This is the mechanism whereby high altitude dwellers have higher hematocrits than sea-level residents, and also why persons with pulmonary insufficiency or right-to-left shunts in the heart (through which venous blood by-passes the lungs and goes directly into the systemic circulation) have similarly high hematocrits. Regardless of
4230-495: A technical specification instead of particular chemical substances. For example, gasoline is not a single chemical compound or even a particular mixture: different gasolines can have very different chemical compositions, as "gasoline" is primarily defined through source, properties and octane rating . Every chemical substance has one or more systematic names , usually named according to the IUPAC rules for naming . An alternative system
4371-442: A yellow-grey mixture. No chemical process occurs, and the material can be identified as a mixture by the fact that the sulfur and the iron can be separated by a mechanical process, such as using a magnet to attract the iron away from the sulfur. In contrast, if iron and sulfur are heated together in a certain ratio (1 atom of iron for each atom of sulfur, or by weight, 56 grams (1 mol ) of iron to 32 grams (1 mol) of sulfur),
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4512-436: Is glucose vs. fructose . The former is an aldehyde , the latter is a ketone . Their interconversion requires either enzymatic or acid-base catalysis . However, tautomers are an exception: the isomerization occurs spontaneously in ordinary conditions, such that a pure substance cannot be isolated into its tautomers, even if these can be identified spectroscopically or even isolated in special conditions. A common example
4653-704: Is glucose , which has open-chain and ring forms. One cannot manufacture pure open-chain glucose because glucose spontaneously cyclizes to the hemiacetal form. All matter consists of various elements and chemical compounds, but these are often intimately mixed together. Mixtures contain more than one chemical substance, and they do not have a fixed composition. Butter , soil and wood are common examples of mixtures. Sometimes, mixtures can be separated into their component substances by mechanical processes, such as chromatography , distillation , or evaporation . Grey iron metal and yellow sulfur are both chemical elements, and they can be mixed together in any ratio to form
4794-592: Is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmuted into another element through a nuclear reaction . This is because all of the atoms in a sample of an element have the same number of protons , though they may be different isotopes , with differing numbers of neutrons . As of 2019, there are 118 known elements, about 80 of which are stable – that is, they do not change by radioactive decay into other elements. Some elements can occur as more than
4935-440: Is a component of Vitamin B 12 which is essential. There are other minerals which are essential for some plants and animals, but may or may not be essential for humans, such as boron and silicon . Choline is an essential nutrient. The cholines are a family of water-soluble quaternary ammonium compounds . Choline is the parent compound of the cholines class, consisting of ethanolamine having three methyl substituents attached to
5076-557: Is a deficiency. Deficiencies can be due to several causes, including an inadequacy in nutrient intake, called a dietary deficiency, or any of several conditions that interfere with the utilization of a nutrient within an organism. Some of the conditions that can interfere with nutrient utilization include problems with nutrient absorption, substances that cause a greater-than-normal need for a nutrient, conditions that cause nutrient destruction, and conditions that cause greater nutrient excretion. Nutrient toxicity occurs when excess consumption of
5217-787: Is a nutrient required for normal physiological function that cannot be synthesized in the body – either at all or in sufficient quantities – and thus must be obtained from a dietary source. Apart from water , which is universally required for the maintenance of homeostasis in mammals, essential nutrients are indispensable for various cellular metabolic processes and for the maintenance and function of tissues and organs. The nutrients considered essential for humans comprise nine amino acids, two fatty acids, thirteen vitamins , fifteen minerals and choline . In addition, there are several molecules that are considered conditionally essential nutrients since they are indispensable in certain developmental and pathological states. An essential amino acid
5358-910: Is a unique form of matter with constant chemical composition and characteristic properties . Chemical substances may take the form of a single element or chemical compounds . If two or more chemical substances can be combined without reacting , they may form a chemical mixture . If a mixture is separated to isolate one chemical substance to a desired degree, the resulting substance is said to be chemically pure . Chemical substances can exist in several different physical states or phases (e.g. solids , liquids , gases , or plasma ) without changing their chemical composition. Substances transition between these phases of matter in response to changes in temperature or pressure . Some chemical substances can be combined or converted into new substances by means of chemical reactions . Chemicals that do not possess this ability are said to be inert . Pure water
5499-597: Is addressed by requirements set for protein, which is composed of nitrogen-containing amino acids. Sulfur is essential, but again does not have a recommended intake. Instead, recommended intakes are identified for the sulfur-containing amino acids methionine and cysteine . The essential nutrient trace elements for humans, listed in order of Recommended Dietary Allowance (expressed as a mass), are potassium , chloride , sodium , calcium , phosphorus , magnesium , iron , zinc , manganese , copper , iodine , chromium , molybdenum , and selenium . Additionally, cobalt
5640-467: Is an amino acid that is required by an organism but cannot be synthesized de novo by it, and therefore must be supplied in its diet. Out of the twenty standard protein-producing amino acids, nine cannot be endogenously synthesized by humans: phenylalanine , valine , threonine , tryptophan , methionine , leucine , isoleucine , lysine , and histidine . Essential fatty acids (EFAs) are fatty acids that humans and other animals must ingest because
5781-402: Is an example of complete combustion . Stoichiometry measures these quantitative relationships, and is used to determine the amount of products and reactants that are produced or needed in a given reaction. Describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry . In the example above, reaction stoichiometry measures
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5922-417: Is an example of a chemical substance, with a constant composition of two hydrogen atoms bonded to a single oxygen atom (i.e. H 2 O). The atomic ratio of hydrogen to oxygen is always 2:1 in every molecule of water. Pure water will tend to boil near 100 °C (212 °F), an example of one of the characteristic properties that define it. Other notable chemical substances include diamond (a form of
6063-433: Is an integral part of the antioxidant enzyme copper-zinc superoxide dismutase, and has a role in iron homeostasis as a cofactor in ceruloplasmin. Changes in the levels of oxygen, carbon dioxide, and plasma pH are sent to the respiratory center , in the brainstem where they are regulated. The partial pressure of oxygen and carbon dioxide in the arterial blood is monitored by the peripheral chemoreceptors ( PNS ) in
6204-499: Is conditional, as people who get sufficient exposure to ultraviolet light, either from the sun or an artificial source, synthesize vitamin D in the skin. Minerals are the exogenous chemical elements indispensable for life. Although the four elements: carbon , hydrogen , oxygen , and nitrogen ( CHON ) are essential for life, they are so plentiful in food and drink that these are not considered nutrients and there are no recommended intakes for these as minerals. The need for nitrogen
6345-1045: Is essential to humans and some animal species but most other animals and many plants are able to synthesize it. Nutrients may be organic or inorganic: organic compounds include most compounds containing carbon, while all other chemicals are inorganic. Inorganic nutrients include nutrients such as iron , selenium , and zinc , while organic nutrients include, protein, fats, sugars and vitamins. A classification used primarily to describe nutrient needs of animals divides nutrients into macronutrients and micronutrients . Consumed in relatively large amounts ( grams or ounces ), macronutrients (carbohydrates, fats , proteins, water) are primarily used to generate energy or to incorporate into tissues for growth and repair. Micronutrients are needed in smaller amounts ( milligrams or micrograms ); they have subtle biochemical and physiological roles in cellular processes, like vascular functions or nerve conduction . Inadequate amounts of essential nutrients or diseases that interfere with absorption, result in
6486-404: Is homeostatically controlled it does not imply that its value is necessarily absolutely steady in health. Core body temperature is, for instance, regulated by a homeostatic mechanism with temperature sensors in, amongst others, the hypothalamus of the brain . However, the set point of the regulator is regularly reset. For instance, core body temperature in humans varies during the course of
6627-433: Is limited in its capacity to respond to extreme temperatures. When the core temperature falls, the blood supply to the skin is reduced by intense vasoconstriction . The blood flow to the limbs (which have a large surface area) is similarly reduced and returned to the trunk via the deep veins which lie alongside the arteries (forming venae comitantes ). This acts as a counter-current exchange system that short-circuits
6768-454: Is more of a deciding factor in the monitoring of pH. However, at high altitude (above 2500 m) the monitoring of the partial pressure of oxygen takes priority, and hyperventilation keeps the oxygen level constant. With the lower level of carbon dioxide, to keep the pH at 7.4 the kidneys secrete hydrogen ions into the blood and excrete bicarbonate into the urine. This is important in acclimatization to high altitude . The kidneys measure
6909-496: Is no change in the osmolality of the ECF, and therefore no change in the ADH concentration of the plasma. However, low aldosterone levels cause a loss of sodium ions from the ECF, which could potentially cause a change in extracellular osmolality and therefore of ADH levels in the blood. High potassium concentrations in the plasma cause depolarization of the zona glomerulosa cells' membranes in
7050-639: Is not sufficient information to set EARs and RDAs. PRI Population Reference Intake is European Union equivalent of RDA; higher for adults than for children, and may be even higher for women who are pregnant or lactating. For Thiamin and Niacin, the PRIs are expressed as amounts per megajoule (239 kilocalories) of food energy consumed. Upper Limit Tolerable upper intake levels. ND ULs have not been determined. NE EARs, PRIs or AIs have not yet been established or will not be (EU does not consider chromium an essential nutrient). Plant nutrients consist of more than
7191-402: Is stimulated to rise ( tachycardia ) when the arterial blood pressure falls, or to slow down ( bradycardia ) when the pressure rises above the set point. Thus the heart rate (for which there is no sensor in the body) is not homeostatically controlled but is one of the effector responses to errors in arterial blood pressure. Another example is the rate of sweating . This is one of the effectors in
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#17329162696447332-411: Is the relationships among the weights of reactants and products before, during, and following chemical reactions . Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations among quantities of reactants and products typically form a ratio of positive integers. This means that if
7473-439: Is the state of steady internal physical and chemical conditions maintained by living systems . This is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance , being kept within certain pre-set limits (homeostatic range). Other variables include the pH of extracellular fluid , the concentrations of sodium , potassium , and calcium ions , as well as
7614-438: Is therefore that hydrogen ions are lost in the urine when the pH of the plasma falls. The concomitant rise in the plasma bicarbonate mops up the increased hydrogen ions (caused by the fall in plasma pH) and the resulting excess carbonic acid is disposed of in the lungs as carbon dioxide. This restores the normal ratio between bicarbonate and the partial pressure of carbon dioxide and therefore the plasma pH. The converse happens when
7755-435: Is thought to be the central motivation for all organic action. All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: a receptor, a control center, and an effector. The receptor is the sensing component that monitors and responds to changes in the environment, either external or internal. Receptors include thermoreceptors and mechanoreceptors . Control centers include
7896-486: Is used by the Chemical Abstracts Service (CAS). Many compounds are also known by their more common, simpler names, many of which predate the systematic name. For example, the long-known sugar glucose is now systematically named 6-(hydroxymethyl)oxane-2,3,4,5-tetrol. Natural products and pharmaceuticals are also given simpler names, for example the mild pain-killer Naproxen is the more common name for
8037-427: The autonomic nervous system are stimulated to influence the activity of chiefly the heart and the smallest diameter arteries, called arterioles . The arterioles are the main resistance vessels in the arterial tree , and small changes in diameter cause large changes in the resistance to flow through them. When the arterial blood pressure rises the arterioles are stimulated to dilate making it easier for blood to leave
8178-406: The blood sugar level , and these need to be regulated despite changes in the environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life. Homeostasis is brought about by a natural resistance to change when already in optimal conditions, and equilibrium is maintained by many regulatory mechanisms; it
8319-433: The carotid artery and aortic arch . A change in the partial pressure of carbon dioxide is detected as altered pH in the cerebrospinal fluid by central chemoreceptors ( CNS ) in the medulla oblongata of the brainstem . Information from these sets of sensors is sent to the respiratory center which activates the effector organs – the diaphragm and other muscles of respiration . An increased level of carbon dioxide in
8460-400: The central nervous system play a homeostatic role in the balance of neuronal activity between excitation and inhibition. Inhibitory neurons using GABA , make compensating changes in the neuronal networks preventing runaway levels of excitation. An imbalance between excitation and inhibition is seen to be implicated in a number of neuropsychiatric disorders . The neuroendocrine system is
8601-448: The hypothalamus detects a hypertonic extracellular environment, it causes the secretion of an antidiuretic hormone (ADH) called vasopressin which acts on the effector organ, which in this case is the kidney . The effect of vasopressin on the kidney tubules is to reabsorb water from the distal convoluted tubules and collecting ducts , thus preventing aggravation of the water loss via the urine. The hypothalamus simultaneously stimulates
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#17329162696448742-411: The medulla oblongata of the brain indicating whether the blood pressure has fallen or risen, and by how much. The medulla oblongata then distributes messages along motor or efferent nerves belonging to the autonomic nervous system to a wide variety of effector organs, whose activity is consequently changed to reverse the error in the blood pressure. One of the effector organs is the heart whose rate
8883-408: The renal tubular fluid after it has already undergone a certain amount of modification in the proximal convoluted tubule and loop of Henle . These cells also respond to rate of blood flow through the juxtaglomerular apparatus, which, under normal circumstances, is directly proportional to the arterial blood pressure , making this tissue an ancillary arterial blood pressure sensor. In response to
9024-460: The renin–angiotensin system , control more than one variable. When the receptor senses a stimulus, it reacts by sending action potentials to a control center. The control center sets the maintenance range—the acceptable upper and lower limits—for the particular variable, such as temperature. The control center responds to the signal by determining an appropriate response and sending signals to an effector , which can be one or more muscles, an organ, or
9165-433: The respiratory center and the renin-angiotensin system . An effector is the target acted on, to bring about the change back to the normal state. At the cellular level, effectors include nuclear receptors that bring about changes in gene expression through up-regulation or down-regulation and act in negative feedback mechanisms. An example of this is in the control of bile acids in the liver . Some centers, such as
9306-416: The zona glomerulosa of the adrenal glands has an effect on particularly the epithelial cells of the distal convoluted tubules and collecting ducts of the kidneys. Here it causes the reabsorption of sodium ions from the renal tubular fluid , in exchange for potassium ions which are secreted from the blood plasma into the tubular fluid to exit the body via the urine. The reabsorption of sodium ions from
9447-501: The European Union (same concept as RDAs), followed by what three government organizations deem to be the safe upper intake. RDAs are set higher than EARs to cover people with higher-than-average needs. Adequate Intakes (AIs) are set when there is insufficient information to establish EARs and RDAs. Countries establish tolerable upper intake levels , also referred to as upper limits (ULs), based on amounts that cause adverse effects. Governments are slow to revise information of this nature. For
9588-962: The U.S. Department of Agriculture, based on NHANES 2013–2014 surveys, women ages 20 and up consume on average 6.8 grams of alcohol per day and men consume on average 15.5 grams per day. Ignoring the non-alcohol contribution of those beverages, the average ethanol contributions to daily food energy intake are 200 and 450 kJ (48 and 108 kcal), respectively. Alcoholic beverages are considered empty calorie foods because, while providing energy, they contribute no essential nutrients. By definition, phytochemicals include all nutritional and non-nutritional components of edible plants. Included as nutritional constituents are provitamin A carotenoids , whereas those without nutrient status are diverse polyphenols , flavonoids , resveratrol , and lignans that are present in numerous plant foods. Some phytochemical compounds are under preliminary research for their potential effects on human diseases and health. However,
9729-539: The U.S. values, except calcium and vitamin D, all data date from 1997 to 2004. * The daily recommended amounts of niacin and magnesium are higher than the tolerable upper limit because, for both nutrients, the ULs identify the amounts which will not increase risk of adverse effects when the nutrients are consumed as a serving of a dietary supplement. Magnesium supplementation above the UL may cause diarrhea. Supplementation with niacin above
9870-457: The UL may cause flushing of the face and a sensation of body warmth. Each country or regional regulatory agency decides on a safety margin below when symptoms may occur, so the ULs may differ based on source. EAR U.S. Estimated Average Requirements. RDA U.S. Recommended Dietary Allowances; higher for adults than for children, and may be even higher for women who are pregnant or lactating. AI U.S. Adequate Intake; AIs established when there
10011-399: The alpha cells into the blood. This inhibits the uptake of glucose from the blood by the liver, fats cells, and muscle. Instead the liver is strongly stimulated to manufacture glucose from glycogen (through glycogenolysis ) and from non-carbohydrate sources (such as lactate and de-aminated amino acids ) using a process known as gluconeogenesis . The glucose thus produced is discharged into
10152-832: The amino function. Healthy humans fed artificially composed diets that are deficient in choline develop fatty liver, liver damage, and muscle damage. Choline was not initially classified as essential because the human body can produce choline in small amounts through phosphatidylcholine metabolism. Conditionally essential nutrients are certain organic molecules that can normally be synthesized by an organism, but under certain conditions in insufficient quantities. In humans, such conditions include premature birth , limited nutrient intake, rapid growth, and certain disease states. Inositol , taurine , arginine , glutamine and nucleotides are classified as conditionally essential and are particularly important in neonatal diet and metabolism. Non-essential nutrients are substances within foods that can have
10293-675: The amount of any macronutrients and micronutrients present in the food in significant quantities. Nutrients in larger quantities than the body needs may have harmful effects. Edible plants also contain thousands of compounds generally called phytochemicals which have unknown effects on disease or health including a diverse class with non-nutrient status called polyphenols which remain poorly understood as of 2024. Macronutrients are defined in several ways. Macronutrients provide energy: Micronutrients are essential dietary elements required in varying quantities throughout life to serve metabolic and physiological functions . An essential nutrient
10434-532: The amounts of the separate reactants are known, then the amount of the product can be calculated. Conversely, if one reactant has a known quantity and the quantity of the products can be empirically determined, then the amount of the other reactants can also be calculated. This is illustrated in the image here, where the balanced equation is: Here, one molecule of methane reacts with two molecules of oxygen gas to yield one molecule of carbon dioxide and two molecules of water . This particular chemical equation
10575-498: The application, but higher tolerance of impurities is usually expected in the production of bulk chemicals. Thus, the user of the chemical in the US might choose between the bulk or "technical grade" with higher amounts of impurities or a much purer "pharmaceutical grade" (labeled "USP", United States Pharmacopeia ). "Chemicals" in the commercial and legal sense may also include mixtures of highly variable composition, as they are products made to
10716-436: The arteries, thus deflating them, and bringing the blood pressure down, back to normal. At the same time, the heart is stimulated via cholinergic parasympathetic nerves to beat more slowly (called bradycardia ), ensuring that the inflow of blood into the arteries is reduced, thus adding to the reduction in pressure, and correcting the original error. Low pressure in the arteries, causes the opposite reflex of constriction of
10857-412: The arterioles, and a speeding up of the heart rate (called tachycardia ). If the drop in blood pressure is very rapid or excessive, the medulla oblongata stimulates the adrenal medulla , via "preganglionic" sympathetic nerves , to secrete epinephrine (adrenaline) into the blood. This hormone enhances the tachycardia and causes severe vasoconstriction of the arterioles to all but the essential organ in
10998-407: The biological term of homeostasis. The metabolic processes of all organisms can only take place in very specific physical and chemical environments. The conditions vary with each organism, and with whether the chemical processes take place inside the cell or in the interstitial fluid bathing the cells. The best-known homeostatic mechanisms in humans and other mammals are regulators that keep
11139-464: The blood circulates through the lungs a pulmonary capillary endothelial enzyme called angiotensin-converting enzyme (ACE) cleaves a further two amino acids from angiotensin I to form an octapeptide known as angiotensin II . Angiotensin II is a hormone which acts on the adrenal cortex , causing the release into the blood of the steroid hormone , aldosterone . Angiotensin II also acts on the smooth muscle in
11280-399: The blood correcting the detected error ( hypoglycemia ). The glycogen stored in muscles remains in the muscles, and is only broken down, during exercise, to glucose-6-phosphate and thence to pyruvate to be fed into the citric acid cycle or turned into lactate . It is only the lactate and the waste products of the citric acid cycle that are returned to the blood. The liver can take up only
11421-419: The blood, or a decreased level of oxygen, will result in a deeper breathing pattern and increased respiratory rate to bring the blood gases back to equilibrium. Too little carbon dioxide, and, to a lesser extent, too much oxygen in the blood can temporarily halt breathing, a condition known as apnea , which freedivers use to prolong the time they can stay underwater. The partial pressure of carbon dioxide
11562-401: The blood, the latter combines with the excess hydrogen ions in the plasma that stimulated the kidneys to perform this operation. The resulting reaction in the plasma is the formation of carbonic acid which is in equilibrium with the plasma partial pressure of carbon dioxide. This is tightly regulated to ensure that there is no excessive build-up of carbonic acid or bicarbonate. The overall effect
11703-598: The blood. This combination (high blood insulin levels and low glucagon levels) act on effector tissues, the chief of which is the liver , fat cells , and muscle cells . The liver is inhibited from producing glucose , taking it up instead, and converting it to glycogen and triglycerides . The glycogen is stored in the liver, but the triglycerides are secreted into the blood as very low-density lipoprotein (VLDL) particles which are taken up by adipose tissue , there to be stored as fats. The fat cells take up glucose through special glucose transporters ( GLUT4 ), whose numbers in
11844-499: The body (especially the heart, lungs, and brain). These reactions usually correct the low arterial blood pressure ( hypotension ) very effectively. The plasma ionized calcium (Ca ) concentration is very tightly controlled by a pair of homeostatic mechanisms. The sensor for the first one is situated in the parathyroid glands , where the chief cells sense the Ca level by means of specialized calcium receptors in their membranes. The sensors for
11985-1003: The body requires them for good health but cannot synthesize them. Only two fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid ) and linoleic acid (an omega-6 fatty acid ). Vitamins occur in a variety of related forms known as vitamers . The vitamers of a given vitamin perform the functions of that vitamin and prevent symptoms of deficiency of that vitamin. Vitamins are those essential organic molecules that are not classified as amino acids or fatty acids. They commonly function as enzymatic cofactors , metabolic regulators or antioxidants . Humans require thirteen vitamins in their diet, most of which are actually groups of related molecules (e.g. vitamin E includes tocopherols and tocotrienols ): vitamins A, C, D, E, K, thiamine (B 1 ), riboflavin (B 2 ), niacin (B 3 ), pantothenic acid (B 5 ), pyridoxine (B 6 ), biotin (B 7 ), folate (B 9 ), and cobalamin (B 12 ). The requirement for vitamin D
12126-407: The brain. Homeostasis is an almost exclusively biological term, referring to the concepts described by Bernard and Cannon, concerning the constancy of the internal environment in which the cells of the body live and survive. The term cybernetics is applied to technological control systems such as thermostats , which function as homeostatic mechanisms but are often defined much more broadly than
12267-612: The cell wall are increased as a direct effect of insulin acting on these cells. The glucose that enters the fat cells in this manner is converted into triglycerides (via the same metabolic pathways as are used by the liver) and then stored in those fat cells together with the VLDL-derived triglycerides that were made in the liver. Muscle cells also take glucose up through insulin-sensitive GLUT4 glucose channels, and convert it into muscle glycogen. A fall in blood glucose, causes insulin secretion to be stopped, and glucagon to be secreted from
12408-442: The cell, into the interstitial fluid and two potassium ions into the cell from the interstitial fluid. This creates an ionic concentration gradient which results in the reabsorption of sodium (Na ) ions from the tubular fluid into the blood, and secreting potassium (K ) ions from the blood into the urine (lumen of collecting duct). The total amount of water in the body needs to be kept in balance. Fluid balance involves keeping
12549-408: The chemical combination of the known chemical elements. As of Feb 2021, about "177 million organic and inorganic substances" (including 68 million defined-sequence biopolymers) are in the scientific literature and registered in public databases. The names of many of these compounds are often nontrivial and hence not very easy to remember or cite accurately. Also, it is difficult to keep track of them in
12690-451: The chemical compound (S)-6-methoxy-α-methyl-2-naphthaleneacetic acid. Chemists frequently refer to chemical compounds using chemical formulae or molecular structure of the compound. There has been a phenomenal growth in the number of chemical compounds being synthesized (or isolated), and then reported in the scientific literature by professional chemists around the world. An enormous number of chemical compounds are possible through
12831-460: The chemical industry, manufactured "chemicals" are chemical substances, which can be classified by production volume into bulk chemicals, fine chemicals and chemicals found in research only: The cause of the difference in production volume is the complexity of the molecular structure of the chemical. Bulk chemicals are usually much less complex. While fine chemicals may be more complex, many of them are simple enough to be sold as "building blocks" in
12972-402: The chemical substances of which fruits and vegetables, for example, are naturally composed even when growing wild are not called "chemicals" in general usage. In countries that require a list of ingredients in products, the "chemicals" listed are industrially produced "chemical substances". The word "chemical" is also often used to refer to addictive, narcotic, or mind-altering drugs. Within
13113-419: The chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate . He deduced that, "All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound." This is now known as the law of constant composition . Later with the advancement of methods for chemical synthesis particularly in
13254-607: The composition of the extracellular fluid (or the "internal environment") constant, especially with regard to the temperature , pH , osmolality , and the concentrations of sodium , potassium , glucose , carbon dioxide , and oxygen . However, a great many other homeostatic mechanisms, encompassing many aspects of human physiology , control other entities in the body. Where the levels of variables are higher or lower than those needed, they are often prefixed with hyper- and hypo- , respectively such as hyperthermia and hypothermia or hypertension and hypotension . If an entity
13395-427: The day (i.e. has a circadian rhythm ), with the lowest temperatures occurring at night, and the highest in the afternoons. Other normal temperature variations include those related to the menstrual cycle . The temperature regulator's set point is reset during infections to produce a fever. Organisms are capable of adjusting somewhat to varied conditions such as temperature changes or oxygen levels at altitude, by
13536-411: The distal convoluted tubules and collecting ducts is impermeable to water in the absence of antidiuretic hormone (ADH) in the blood. ADH is part of the control of fluid balance . Its levels in the blood vary with the osmolality of the plasma, which is measured in the hypothalamus of the brain. Aldosterone's action on the kidney tubules prevents sodium loss to the extracellular fluid (ECF). So there
13677-413: The earlier reactions are insufficient to correct the hypothermia . When core temperature rises are detected by thermoreceptors , the sweat glands in the skin are stimulated via cholinergic sympathetic nerves to secrete sweat onto the skin, which, when it evaporates, cools the skin and the blood flowing through it. Panting is an alternative effector in many vertebrates, which cools the body also by
13818-531: The element carbon ), table salt (NaCl; an ionic compound ), and refined sugar (C 12 H 22 O 11 ; an organic compound ). In addition to the generic definition offered above, there are several niche fields where the term "chemical substance" may take alternate usages that are widely accepted, some of which are outlined in the sections below. Chemical Abstracts Service (CAS) lists several alloys of uncertain composition within their chemical substance index. While an alloy could be more closely defined as
13959-429: The error – in fact it should be very nearly in proportion (but in the opposite direction) to the error that is threatening the internal environment. For instance, arterial blood pressure in mammals is homeostatically controlled and measured by stretch receptors in the walls of the aortic arch and carotid sinuses at the beginnings of the internal carotid arteries . The sensors send messages via sensory nerves to
14100-444: The evaporation of water, but this time from the mucous membranes of the throat and mouth. Blood sugar levels are regulated within fairly narrow limits. In mammals, the primary sensors for this are the beta cells of the pancreatic islets . The beta cells respond to a rise in the blood sugar level by secreting insulin into the blood and simultaneously inhibiting their neighboring alpha cells from secreting glucagon into
14241-414: The excess water in the body. Urinary water loss, when the body water homeostat is intact, is a compensatory water loss, correcting any water excess in the body. However, since the kidneys cannot generate water, the thirst reflex is the all-important second effector mechanism of the body water homeostat, correcting any water deficit in the body. The plasma pH can be altered by respiratory changes in
14382-424: The fact that water losses from the body, (through unavoidable water loss through the skin which is not entirely waterproof and therefore always slightly moist, water vapor in the exhaled air , sweating , vomiting , normal feces and especially diarrhea ) are all hypotonic , meaning that they are less salty than the body fluids (compare, for instance, the taste of saliva with that of tears. The latter has almost
14523-400: The fluid volume stabilized, and also keeping the levels of electrolytes in the extracellular fluid stable. Fluid balance is maintained by the process of osmoregulation and by behavior. Osmotic pressure is detected by osmoreceptors in the median preoptic nucleus in the hypothalamus . Measurement of the plasma osmolality to give an indication of the water content of the body, relies on
14664-567: The gut. Another example are the most well-characterised endocannabinoids like anandamide ( N -arachidonoylethanolamide; AEA) and 2-arachidonoylglycerol (2-AG), whose synthesis occurs through the action of a series of intracellular enzymes activated in response to a rise in intracellular calcium levels to introduce homeostasis and prevention of tumor development through putative protective mechanisms that prevent cell growth and migration by activation of CB1 and/or CB2 and adjoining receptors . The homeostatic mechanism which controls
14805-412: The homeostatic control of body temperature, and therefore highly variable in rough proportion to the heat load that threatens to destabilize the body's core temperature, for which there is a sensor in the hypothalamus of the brain. Mammals regulate their core temperature using input from thermoreceptors in the hypothalamus , brain, spinal cord , internal organs , and great veins. Apart from
14946-400: The hormone atrial natriuretic peptide (ANP) into the blood. This acts on the kidneys to inhibit the secretion of renin and aldosterone causing the release of sodium, and accompanying water into the urine, thereby reducing the blood volume. This information is then conveyed, via afferent nerve fibers , to the solitary nucleus in the medulla oblongata . From here motor nerves belonging to
15087-591: The human body is capable of balancing a wide range of copper intakes for the needs of healthy individuals. Many aspects of copper homeostasis are known at the molecular level. Copper's essentiality is due to its ability to act as an electron donor or acceptor as its oxidation state fluxes between Cu ( cuprous ) and Cu ( cupric ). As a component of about a dozen cuproenzymes, copper is involved in key redox (i.e., oxidation-reduction) reactions in essential metabolic processes such as mitochondrial respiration, synthesis of melanin , and cross-linking of collagen . Copper
15228-398: The idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers . An element
15369-495: The internal regulation of temperature, a process called allostasis can come into play that adjusts behaviour to adapt to the challenge of very hot or cold extremes (and to other challenges). These adjustments may include seeking shade and reducing activity, seeking warmer conditions and increasing activity, or huddling. Behavioral thermoregulation takes precedence over physiological thermoregulation since necessary changes can be affected more quickly and physiological thermoregulation
15510-414: The iron, since there is no metallic iron present in the compound. While the term chemical substance is a precise technical term that is synonymous with chemical for chemists, the word chemical is used in general usage to refer to both (pure) chemical substances and mixtures (often called compounds ), and especially when produced or purified in a laboratory or an industrial process. In other words,
15651-424: The lactate, and, by the process of energy-consuming gluconeogenesis , convert it back to glucose. Controlling iron levels in the body is a critically important part of many aspects of human health and disease. In humans iron is both necessary to the body and potentially harmful. Copper is absorbed, transported, distributed, stored, and excreted in the body according to complex homeostatic processes which ensure
15792-425: The large number of chemical substances reported in chemistry literature need to be indexed. Isomerism caused much consternation to early researchers, since isomers have exactly the same composition, but differ in configuration (arrangement) of the atoms. For example, there was much speculation about the chemical identity of benzene , until the correct structure was described by Friedrich August Kekulé . Likewise,
15933-409: The literature. Several international organizations like IUPAC and CAS have initiated steps to make such tasks easier. CAS provides the abstracting services of the chemical literature, and provides a numerical identifier, known as CAS registry number to each chemical substance that has been reported in the chemical literature (such as chemistry journals and patents ). This information is compiled as
16074-411: The metallic properties described above, they also have a high electronegativity and a tendency to form negative ions . Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids . A chemical compound is a chemical substance that is composed of a particular set of atoms or ions . Two or more elements combined into one substance through
16215-636: The micronutrients; iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo) and nickel (Ni). In addition to carbon, hydrogen, and oxygen, nitrogen , phosphorus , and sulfur are also needed in relatively large quantities. Together, the " Big Six " are the elemental macronutrients for all organisms . They are sourced from inorganic matter (for example, carbon dioxide , water , nitrates , phosphates , sulfates , and diatomic molecules of nitrogen and, especially, oxygen) and organic matter ( carbohydrates , lipids , proteins ). Chemical substance A chemical substance
16356-469: The nearby thirst center causing an almost irresistible (if the hypertonicity is severe enough) urge to drink water. The cessation of urine flow prevents the hypovolemia and hypertonicity from getting worse; the drinking of water corrects the defect. Hypo-osmolality results in very low plasma ADH levels. This results in the inhibition of water reabsorption from the kidney tubules, causing high volumes of very dilute urine to be excreted, thus getting rid of
16497-435: The one hand, and calcitonin on the other can very rapidly correct any impending error in the plasma ionized calcium level by either removing calcium from the blood and depositing it in the skeleton, or by removing calcium from it. The skeleton acts as an extremely large calcium store (about 1 kg) compared with the plasma calcium store (about 180 mg). Longer term regulation occurs through calcium absorption or loss from
16638-427: The outer layer of the adrenal cortex . This causes the release of aldosterone into the blood. Aldosterone acts primarily on the distal convoluted tubules and collecting ducts of the kidneys, stimulating the excretion of potassium ions into the urine. It does so, however, by activating the basolateral Na /K pumps of the tubular epithelial cells. These sodium/potassium exchangers pump three sodium ions out of
16779-454: The oxygen content rather than the partial pressure of oxygen in the arterial blood. When the oxygen content of the blood is chronically low, oxygen-sensitive cells secrete erythropoietin (EPO) into the blood. The effector tissue is the red bone marrow which produces red blood cells (RBCs, also called erythrocytes ). The increase in RBCs leads to an increased hematocrit in the blood, and
16920-425: The pH. Respiratory compensation a mechanism of the respiratory center , adjusts the partial pressure of carbon dioxide by changing the rate and depth of breathing, to bring the pH back to normal. The partial pressure of carbon dioxide also determines the concentration of carbonic acid, and the bicarbonate buffer system can also come into play. Renal compensation can help the bicarbonate buffer system. The sensor for
17061-515: The partial pressure of carbon dioxide; or altered by metabolic changes in the carbonic acid to bicarbonate ion ratio. The bicarbonate buffer system regulates the ratio of carbonic acid to bicarbonate to be equal to 1:20, at which ratio the blood pH is 7.4 (as explained in the Henderson–Hasselbalch equation ). A change in the plasma pH gives an acid–base imbalance . In acid–base homeostasis there are two mechanisms that can help regulate
17202-529: The partial pressure of oxygen in the blood, the amount of oxygen that can be carried, depends on the hemoglobin content. The partial pressure of oxygen may be sufficient for example in anemia , but the hemoglobin content will be insufficient and subsequently as will be the oxygen content. Given enough supply of iron, vitamin B12 and folic acid , EPO can stimulate RBC production, and hemoglobin and oxygen content restored to normal. The brain can regulate blood flow over
17343-446: The plasma bicarbonate concentration is not known for certain. It is very probable that the renal tubular cells of the distal convoluted tubules are themselves sensitive to the pH of the plasma. The metabolism of these cells produces carbon dioxide, which is rapidly converted to hydrogen and bicarbonate through the action of carbonic anhydrase . When the ECF pH falls (becoming more acidic) the renal tubular cells excrete hydrogen ions into
17484-417: The plasma pH, constant. Cerebrospinal fluid (CSF) allows for regulation of the distribution of substances between cells of the brain, and neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system. For example, high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and syncope . Inhibitory neurons in
17625-415: The plasma sodium concentration is rather more complex than most of the other homeostatic mechanisms described on this page. The sensor is situated in the juxtaglomerular apparatus of kidneys, which senses the plasma sodium concentration in a surprisingly indirect manner. Instead of measuring it directly in the blood flowing past the juxtaglomerular cells , these cells respond to the sodium concentration in
17766-701: The process of releasing energy such as for carbohydrates , lipids , proteins and fermentation products ( ethanol or vinegar ) leading to end-products of water and carbon dioxide . All organisms require water. Essential nutrients for animals are the energy sources, some of the amino acids that are combined to create proteins , a subset of fatty acids , vitamins and certain minerals . Plants require more diverse minerals absorbed through roots, plus carbon dioxide and oxygen absorbed through leaves. Fungi live on dead or living organic matter and meet nutrient needs from their host. Different types of organisms have different essential nutrients. Ascorbic acid ( vitamin C )
17907-403: The qualification for nutrient status of compounds with poorly defined properties in vivo is that they must first be defined with a Dietary Reference Intake level to enable accurate food labeling, a condition not established for most phytochemicals that are claimed to provide antioxidant benefits. See Vitamin , Mineral (nutrient) , Protein (nutrient) An inadequate amount of a nutrient
18048-410: The realm of organic chemistry ; the discovery of many more chemical elements and new techniques in the realm of analytical chemistry used for isolation and purification of elements and compounds from chemicals that led to the establishment of modern chemistry , the concept was defined as is found in most chemistry textbooks. However, there are some controversies regarding this definition mainly because
18189-415: The regulation of the internal environment was described by French physiologist Claude Bernard in 1849, and the word homeostasis was coined by Walter Bradford Cannon in 1926. In 1932, Joseph Barcroft a British physiologist, was the first to say that higher brain function required the most stable internal environment. Thus, to Barcroft homeostasis was not only organized by the brain—homeostasis served
18330-514: The relationship between the quantities of methane and oxygen that react to form carbon dioxide and water. Because of the well known relationship of moles to atomic weights , the ratios that are arrived at by stoichiometry can be used to determine quantities by weight in a reaction described by a balanced equation. This is called composition stoichiometry . Homeostasis In biology , homeostasis ( British also homoeostasis ; / h ɒ m i oʊ ˈ s t eɪ s ɪ s , - m i ə -/ )
18471-436: The release of renin from the juxtaglomerular apparatus is halted, ceasing the production of angiotensin II, and its consequent aldosterone-release into the blood. The kidneys respond by excreting sodium ions into the urine, thereby normalizing the plasma sodium ion concentration. The low angiotensin II levels in the blood lower the arterial blood pressure as an inevitable concomitant response. The reabsorption of sodium ions from
18612-422: The renal tubular fluid halts further sodium ion losses from the body, and therefore preventing the worsening of hyponatremia . The hyponatremia can only be corrected by the consumption of salt in the diet. However, it is not certain whether a "salt hunger" can be initiated by hyponatremia, or by what mechanism this might come about. When the plasma sodium ion concentration is higher than normal ( hypernatremia ),
18753-440: The same salt content as the extracellular fluid, whereas the former is hypotonic with respect to the plasma. Saliva does not taste salty, whereas tears are decidedly salty). Nearly all normal and abnormal losses of body water therefore cause the extracellular fluid to become hypertonic . Conversely, excessive fluid intake dilutes the extracellular fluid causing the hypothalamus to register hypotonic hyponatremia conditions. When
18894-413: The second are the parafollicular cells in the thyroid gland . The parathyroid chief cells secrete parathyroid hormone (PTH) in response to a fall in the plasma ionized calcium level; the parafollicular cells of the thyroid gland secrete calcitonin in response to a rise in the plasma ionized calcium level. The effector organs of the first homeostatic mechanism are the bones , the kidney , and, via
19035-454: The synthesis of more complex molecules targeted for single use, as named above. The production of a chemical includes not only its synthesis but also its purification to eliminate by-products and impurities involved in the synthesis. The last step in production should be the analysis of batch lots of chemicals in order to identify and quantify the percentages of impurities for the buyer of the chemicals. The required purity and analysis depends on
19176-407: The tubular fluid as a result of high aldosterone levels in the blood does not, of itself, cause renal tubular water to be returned to the blood from the distal convoluted tubules or collecting ducts . This is because sodium is reabsorbed in exchange for potassium and therefore causes only a modest change in the osmotic gradient between the blood and the tubular fluid. Furthermore, the epithelium of
19317-402: The tubular fluid to leave the body via urine. Bicarbonate ions are simultaneously secreted into the blood that decreases the carbonic acid, and consequently raises the plasma pH. The converse happens when the plasma pH rises above normal: bicarbonate ions are excreted into the urine, and hydrogen ions released into the plasma. When hydrogen ions are excreted into the urine, and bicarbonate into
19458-455: The upper small intestine, increasing their capacity to absorb calcium from the gut contents into the blood. The second homeostatic mechanism, with its sensors in the thyroid gland, releases calcitonin into the blood when the blood ionized calcium rises. This hormone acts primarily on bone, causing the rapid removal of calcium from the blood and depositing it, in insoluble form, in the bones. The two homeostatic mechanisms working through PTH on
19599-407: The urine. Since phosphates combine with calcium ions to form insoluble salts (see also bone mineral ), a decrease in the level of phosphates in the blood, releases free calcium ions into the plasma ionized calcium pool. PTH has a second action on the kidneys. It stimulates the manufacture and release, by the kidneys, of calcitriol into the blood. This steroid hormone acts on the epithelial cells of
19740-495: The walls of the arterioles causing these small diameter vessels to constrict, thereby restricting the outflow of blood from the arterial tree, causing the arterial blood pressure to rise. This, therefore, reinforces the measures described above (under the heading of "Arterial blood pressure"), which defend the arterial blood pressure against changes, especially hypotension . The angiotensin II-stimulated aldosterone released from
19881-472: The warmth from the arterial blood directly into the venous blood returning into the trunk, causing minimal heat loss from the extremities in cold weather. The subcutaneous limb veins are tightly constricted, not only reducing heat loss from this source but also forcing the venous blood into the counter-current system in the depths of the limbs. The metabolic rate is increased, initially by non-shivering thermogenesis , followed by shivering thermogenesis if
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