Urea known as carbamide, is an organic compound with chemical formula CO2. This amide has two –NH2 groups joined by a carbonyl functional group. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals, it is a colorless, odorless solid soluble in water, non-toxic. Dissolved in water, it is neither alkaline; the body uses it in most notably nitrogen excretion. The liver forms it by combining two ammonia molecules with a carbon dioxide molecule in the urea cycle. Urea is used in fertilizers as a source of nitrogen and is an important raw material for the chemical industry. Friedrich Wöhler's discovery in 1828 that urea can be produced from inorganic starting materials was an important conceptual milestone in chemistry, it showed for the first time that a substance known only as a byproduct of life could be synthesized in the laboratory without biological starting materials thereby contradicting the held doctrine of vitalism.
More than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has the lowest transportation costs per unit of nitrogen nutrient. Many soil bacteria possess the enzyme urease, which catalyzes conversion of urea to ammonia or ammonium ion and bicarbonate ion, thus urea fertilizers transform to the ammonium form in soils. Among the soil bacteria known to carry urease, some ammonia-oxidizing bacteria, such as species of Nitrosomonas, can assimilate the carbon dioxide the reaction releases to make biomass via the Calvin cycle, harvest energy by oxidizing ammonia to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria Nitrobacter, oxidize nitrite to nitrate, mobile in soils because of its negative charge and is a major cause of water pollution from agriculture. Ammonium and nitrate are absorbed by plants, are the dominant sources of nitrogen for plant growth.
Urea is used in many multi-component solid fertilizer formulations. Urea is soluble in water and is therefore very suitable for use in fertilizer solutions, e.g. in'foliar feed' fertilizers. For fertilizer use, granules are preferred over prills because of their narrower particle size distribution, an advantage for mechanical application; the most common impurity of synthetic urea is biuret. Urea is spread at rates of between 40 and 300 kg/ha but rates vary. Smaller applications incur lower losses due to leaching. During summer, urea is spread just before or during rain to minimize losses from volatilization; because of the high nitrogen concentration in urea, it is important to achieve an spread. The application equipment must be calibrated and properly used. Drilling must not occur on contact with or close to seed, due to the risk of germination damage. Urea dissolves in water for application through irrigation systems. In grain and cotton crops, urea is applied at the time of the last cultivation before planting.
In high rainfall areas and on sandy soils and where good in-season rainfall is expected, urea can be side- or top-dressed during the growing season. Top-dressing is popular on pasture and forage crops. In cultivating sugarcane, urea is side-dressed after planting, applied to each ratoon crop. In irrigated crops, urea can be applied dry to the soil, or dissolved and applied through the irrigation water. Urea dissolves in its own weight in water, but becomes difficult to dissolve as the concentration increases. Dissolving urea in water is endothermic—the solution temperature falls when urea dissolves; as a practical guide, when preparing urea solutions for fertigation, dissolve no more than 3 g urea per 1 L water. In foliar sprays, urea concentrations of between 0.5% and 2.0% are used in horticultural crops. Low-biuret grades of urea are indicated. Urea absorbs moisture from the atmosphere and therefore is stored either in closed or sealed bags on pallets or, if stored in bulk, under cover with a tarpaulin.
As with most solid fertilizers, storage in a cool, well-ventilated area is recommended. Overdose or placing urea near seed is harmful. Urea is a raw material for the manufacture of two main classes of materials: urea-formaldehyde resins and urea-melamine-formaldehyde used in marine plywood. Urea can be used to make urea nitrate, a high explosive, used industrially and as part of some improvised explosive devices, it is a stabilizer in nitrocellulose explosives. Urea is used in SNCR and SCR reactions to reduce the NOx pollutants in exhaust gases from combustion from Diesel, dual fuel, lean-burn natural gas engines; the BlueTec system, for example, injects a water-based urea solution into the exhaust system. The ammonia produced by the hydrolysis of the urea reacts with the nitrogen oxide emissions and is converted into nitrogen and water within the catalytic converter. Trucks and cars using these catalytic converters need to carry a supply of diesel exhaust fluid, a solution of urea in water. Urea in concentrations up to 10 M is a powerful protein denaturant as it disrupts the noncovalent bonds in the proteins.
This property can be exploited to increase the solubility of some proteins. A mixture of urea and choline chloride is used as
Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, resulting in a phase change from water vapor to ice as the water vapor reaches the freezing point. In temperate climates, it most appears on surfaces near the ground as fragile white crystals; the propagation of crystal formation occurs by the process of nucleation. The ice crystals of frost form as the result of fractal process development; the depth of frost crystals varies depending on the amount of time they have been accumulating, the concentration of the water vapor. Frost crystals may be clear, or white. Types of frost include crystalline frost from deposition of water vapor from air of low humidity, white frost in humid conditions, window frost on glass surfaces, advection frost from cold wind over cold surfaces, black frost without visible ice at low temperatures and low humidity, rime under supercooled wet conditions.
Plants that have evolved in warmer climates suffer damage when the temperature falls low enough to freeze the water in the cells that make up the plant tissue. The tissue damage resulting from this process is known as "frost damage". Farmers in those regions where frost damage is known to affect their crops invest in substantial means to protect their crops from such damage. If a solid surface is chilled below the dew point of the surrounding humid air and the surface itself is colder than freezing, ice will form on it. If the water deposits as a liquid that freezes, it forms a coating that may look glassy, opaque, or crystalline, depending on its type. Depending on context, that process may be called atmospheric icing; the ice it produces differs in some ways from crystalline frost, which consists of spicules of ice that project from the solid surface on which they grow. The main difference between the ice coatings and frost spicules arises from the fact that the crystalline spicules grow directly from desublimation of water vapour from air, desublimation is not a factor in icing of freezing surfaces.
For desublimation to proceed the surface must be below the frost point of the air, meaning that it is sufficiently cold for ice to form without passing through the liquid phase. The air must be humid, but not sufficiently humid to permit the condensation of liquid water, or icing will result instead of desublimation; the size of the crystals depends on the temperature, the amount of water vapor available, how long they have been growing undisturbed. As a rule, except in conditions where supercooled droplets are present in the air, frost will form only if the deposition surface is colder than the surrounding air. For instance frost may be observed around cracks in cold wooden sidewalks when humid air escapes from the warmer ground beneath. Other objects on which frost forms are those with low specific heat or high thermal emissivity, such as blackened metals; the erratic occurrence of frost in adjacent localities is due to differences of elevation, the lower areas becoming colder on calm nights.
Where static air settles above an area of ground in the absence of wind, the absorptivity and specific heat of the ground influence the temperature that the trapped air attains. Hoar frost hoarfrost, radiation frost, or pruina, refers to white ice crystals deposited on the ground or loosely attached to exposed objects, such as wires or leaves, they form on cold, clear nights when conditions are such that heat radiates out to the open air faster than it can be replaced from nearby sources, such as wind or warm objects. Under suitable circumstances, objects cool to below the frost point of the surrounding air, well below the freezing point of water; such freezing may be promoted by effects such as frost pocket. These occur when ground-level radiation losses cool air until it flows downhill and accumulates in pockets of cold air in valleys and hollows. Hoar frost may freeze in such low-lying cold air when the air temperature a few feet above ground is well above freezing; the word hoar comes from an Old English adjective that means "showing signs of old age".
In this context, it refers to the frost that makes bushes look like white hair. Hoar frost may have different names depending on where it forms: Air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, wires. Surface hoar refers to fern-like ice crystals directly deposited on snow, ice or frozen surfaces. Crevasse hoar consists of crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions. Depth hoar refers to faceted crystals that have grown large within cavities beneath the surface of banks of dry snow. Depth hoar crystals grow continuously at the expense of neighbouring smaller crystals, so are visibly stepped and have faceted hollows; when surface hoar covers sloping snowbanks, the layer of frost crystals may create an avalanche risk. Ideal conditions for hoarfrost to form on snow are cold clear nights, with light, cold air currents conveying humidity at the right rate for growth of frost crystals. Wind, too strong or war
Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. In vertebrates, it is composed of blood cells suspended in blood plasma. Plasma, which constitutes 55% of blood fluid, is water, contains proteins, mineral ions, carbon dioxide, blood cells themselves. Albumin is the main protein in plasma, it functions to regulate the colloidal osmotic pressure of blood; the blood cells are red blood cells, white blood cells and platelets. The most abundant cells in vertebrate blood are red blood cells; these contain hemoglobin, an iron-containing protein, which facilitates oxygen transport by reversibly binding to this respiratory gas and increasing its solubility in blood. In contrast, carbon dioxide is transported extracellularly as bicarbonate ion transported in plasma. Vertebrate blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated.
Some animals, such as crustaceans and mollusks, use hemocyanin to carry oxygen, instead of hemoglobin. Insects and some mollusks use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed circulatory system. In most insects, this "blood" does not contain oxygen-carrying molecules such as hemoglobin because their bodies are small enough for their tracheal system to suffice for supplying oxygen. Jawed vertebrates have an adaptive immune system, based on white blood cells. White blood cells help to resist parasites. Platelets are important in the clotting of blood. Arthropods, using hemolymph, have hemocytes as part of their immune system. Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.
Medical terms related to blood begin with hemo- or hemato- from the Greek word αἷμα for "blood". In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen. Blood performs many important functions within the body, including: Supply of oxygen to tissues Supply of nutrients such as glucose, amino acids, fatty acids Removal of waste such as carbon dioxide and lactic acid Immunological functions, including circulation of white blood cells, detection of foreign material by antibodies Coagulation, the response to a broken blood vessel, the conversion of blood from a liquid to a semisolid gel to stop bleeding Messenger functions, including the transport of hormones and the signaling of tissue damage Regulation of core body temperature Hydraulic functions Blood accounts for 7% of the human body weight, with an average density around 1060 kg/m3 close to pure water's density of 1000 kg/m3.
The average adult has a blood volume of 5 litres, composed of plasma and several kinds of cells. These blood cells consist of erythrocytes and thrombocytes. By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, white cells about 0.7%. Whole blood exhibits non-Newtonian fluid dynamics. If all human hemoglobin were free in the plasma rather than being contained in RBCs, the circulatory fluid would be too viscous for the cardiovascular system to function effectively. One microliter of blood contains: 4.7 to 6.1 million, 4.2 to 5.4 million erythrocytes: Red blood cells contain the blood's hemoglobin and distribute oxygen. Mature red blood cells lack a nucleus and organelles in mammals; the red blood cells are marked by glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is referred to as the hematocrit, is about 45%; the combined surface area of all red blood cells of the human body would be 2,000 times as great as the body's exterior surface.
4,000–11,000 leukocytes: White blood cells are part of the body's immune system. The cancer of leukocytes is called leukemia. 200,000 -- 500,000 thrombocytes: Also called platelets. Fibrin from the coagulation cascade creates a mesh over the platelet plug. About 55% of blood is blood plasma, a fluid, the blood's liquid medium, which by itself is straw-yellow in color; the blood plasma volume totals of 2.7–3.0 liters in an average human. It is an aqueous solution containing 92% water, 8% blood plasma proteins, trace amounts of other materials. Plasma circulates dissolved nutrients, such as glucose, amino acids, fatty acids, removes waste products, such as carbon dioxide and lactic acid. Other important components include: Serum albumin Blood-clotting factors Immunoglobulins lipoprotein particles Various
The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. It can be further classified into the proximal straight tubule; the most distinctive characteristic of the proximal tubule is its luminal brush border. The luminal surface of the epithelial cells of this segment of the nephron is covered with densely packed microvilli forming a border visible under the light microscope giving the brush border cell its name; the microvilli increase the luminal surface area of the cells facilitating their resorptive function as well as putative flow sensing within the lumen. The cytoplasm of the cells is densely packed with mitochondria, which are found in the basal region within the infoldings of the basal plasma membrane; the high quantity of mitochondria gives the cells an acidophilic appearance. The mitochondria are needed in order to supply the energy for the active transport of sodium ions out of the cells to create a concentration gradient which allows more sodium ions to enter the cell from the luminal side.
Water passively follows the sodium out of the cell along its concentration gradient. Cuboidal epithelial cells lining the proximal tubule have extensive lateral interdigitations between neighboring cells, which lend an appearance of having no discrete cell margins when viewed with a light microscope. Agonal resorption of the proximal tubular contents after interruption of circulation in the capillaries surrounding the tubule leads to disturbance of the cellular morphology of the proximal tubule cells, including the ejection of cell nuclei into the tubule lumen; this has led some observers to describe the lumen of proximal tubules as occluded or "dirty-looking", in contrast to the "clean" appearance of distal tubules, which have quite different properties. The proximal tubule as a part of the nephron can be divided into two sections, pars convoluta and pars recta. Differences in cell outlines exist between these segments, therefore in function too. Regarding ultrastructure, it can be divided into three segments, oS1, S2, S3: The pars convoluta is the initial convoluted portion.
In relation to the morphology of the kidney as a whole, the convoluted segments of the proximal tubules are confined to the renal cortex. Some investigators on the basis of particular functional differences have divided the convoluted part into two segments designated S1 and S2; the pars recta is the following straight portion. Straight segments descend into the outer medulla, they terminate at a remarkably uniform level and it is their line of termination that establishes the boundary between the inner and outer stripes of the outer zone of the renal medulla. As a logical extension of the nomenclature described above, this segment is sometimes designated as S3; the proximal tubule efficiently regulates the pH of the filtrate by exchanging hydrogen ions in the interstitium for bicarbonate ions in the filtrate. Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries; this is driven by sodium transport from the lumen into the blood by the Na+/K+ ATPase in the basolateral membrane of the epithelial cells.
Sodium reabsorption is driven by this P-type ATPase. 60-70% of the filtered sodium load is reabsorbed in the proximal tubule through active transport, solvent drag, paracellular electrodiffusion. Active transport is through the sodium/hydrogen antiporter NHE3. Paracellular transport increases transport efficiency, as determined by oxygen consumed per unit of Na+ reabsorbed, thus playing a part in maintaining renal oxygen homeostasis. Many types of medications are secreted in the proximal tubule. Further reading: Table of medication secreted in kidney Most of the ammonium, excreted in the urine is formed in the proximal tubule via the breakdown of glutamine to alpha-ketoglutarate; this takes place in two steps, each of which generates an ammonium anion: the conversion of glutamine to glutamate and the conversion of glutamate to alpha-ketoglutarate. The alpha-ketoglutarate generated in this process is further broken down to form two bicarbonate anions, which are pumped out of the basolateral portion of the tubule cell by co-transport with sodium ions.
Proximal tubular epithelial cells have a pivotal role in kidney disease. Two mammalian cell lines are used as models of the proximal tubule: porcine LLC-PK1 cells and marsupial OK cells. Most renal cell carcinoma, the most common form of kidney cancer, arises from the convoluted tubules. Acute tubular necrosis occurs when PTECs are directly damaged by toxins such as antibiotics and sepsis. Renal tubular acidosis occurs when the PTECs are unable to properly reabsorb glomerular filtrate so that there is increased loss of bicarbonate, amino acids, phosphate. PTECs participate in the progression of tubulointerstitial injury due to glomerulonephritis, interstitial nephritis, vascular injury, diabetic nephropathy. In these situations, PTECs may be directly affected by glucose, or cytokines. There are several ways in which PTECs may respond: producing cytokines and collagen. Urinary pole Brush border This
Uremic frost is a colloquial description for crystallized urea deposits that can be found on the skin of those affected by chronic kidney disease. In states of prolonged kidney failure and subsequent uremia, the high level of urea in the bloodstream leads to high levels of urea secreted by eccrine sweat glands as a component of sweat; as water evaporates off of the skin, it results in crystallization of the remaining urea. This condition is more common in severe, untreated uremia and is associated with serum BUN levels >200. It is becoming rare in people with chronic kidney disease managed on long-term hemodialysis, with estimated prevalence between 0.8 and 3%
Parenchyma is the bulk of a substance. In animals, a parenchyma comprises the functional parts of an organ and in plants parenchyma is the ground tissue of nonwoody structures; the term "parenchyma" is New Latin from word Greek παρέγχυμα parenchyma, "visceral flesh" from παρεγχεῖν parenkhein, "to pour in" from παρα- para-, "beside", ἐν en-, "in" and χεῖν khein, "to pour". Erasistratus and other anatomists used it to refer to certain human tissues, it was applied to some plant tissues by Nehemiah Grew. The parenchyma is the functional parts of an organ in the body; this is in contrast to the stroma, which refers to the structural tissue of organs, the connective tissues. In the brain, the parenchyma refers to the functional tissue in the brain, made up of the two types of brain cell and glial cells. Damage or trauma to the brain parenchyma results in a loss of cognitive ability or death. Lung parenchyma is the substance of the lung outside of the circulation system, involved with gas exchange and includes the alveoli and respiratory bronchioles.
In cancer, the parenchyma refers to "The portion of a tissue that lies outside the circulatory system and is responsible for carrying out the specialized functions of the tissue". In plants, "parenchyma" is one of the three main types of ground tissue, the most common, it can be distinguished through their thin cell wall as compared to other cells. Parenchyma cells make up the bulk of the soft parts of plants, including the insides of leaves and fruits; the dictionary definition of parenchyma at Wiktionary
Nitrogen is a chemical element with symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is accorded the credit because his work was published first; the name nitrogène was suggested by French chemist Jean-Antoine-Claude Chaptal in 1790, when it was found that nitrogen was present in nitric acid and nitrates. Antoine Lavoisier suggested instead the name azote, from the Greek ἀζωτικός "no life", as it is an asphyxiant gas. Nitrogen is the lightest member of group 15 of the periodic table called the pnictogens; the name comes from the Greek πνίγειν "to choke", directly referencing nitrogen's asphyxiating properties. It is a common element in the universe, estimated at about seventh in total abundance in the Milky Way and the Solar System. At standard temperature and pressure, two atoms of the element bind to form dinitrogen, a colourless and odorless diatomic gas with the formula N2.
Dinitrogen forms about 78 % of Earth's atmosphere. Nitrogen occurs in all organisms in amino acids, in the nucleic acids and in the energy transfer molecule adenosine triphosphate; the human body contains about 3% nitrogen by mass, the fourth most abundant element in the body after oxygen and hydrogen. The nitrogen cycle describes movement of the element from the air, into the biosphere and organic compounds back into the atmosphere. Many industrially important compounds, such as ammonia, nitric acid, organic nitrates, cyanides, contain nitrogen; the strong triple bond in elemental nitrogen, the second strongest bond in any diatomic molecule after carbon monoxide, dominates nitrogen chemistry. This causes difficulty for both organisms and industry in converting N2 into useful compounds, but at the same time means that burning, exploding, or decomposing nitrogen compounds to form nitrogen gas releases large amounts of useful energy. Synthetically produced ammonia and nitrates are key industrial fertilisers, fertiliser nitrates are key pollutants in the eutrophication of water systems.
Apart from its use in fertilisers and energy-stores, nitrogen is a constituent of organic compounds as diverse as Kevlar used in high-strength fabric and cyanoacrylate used in superglue. Nitrogen is a constituent including antibiotics. Many drugs are mimics or prodrugs of natural nitrogen-containing signal molecules: for example, the organic nitrates nitroglycerin and nitroprusside control blood pressure by metabolizing into nitric oxide. Many notable nitrogen-containing drugs, such as the natural caffeine and morphine or the synthetic amphetamines, act on receptors of animal neurotransmitters. Nitrogen compounds have a long history, ammonium chloride having been known to Herodotus, they were well known by the Middle Ages. Alchemists knew nitric acid as aqua fortis, as well as other nitrogen compounds such as ammonium salts and nitrate salts; the mixture of nitric and hydrochloric acids was known as aqua regia, celebrated for its ability to dissolve gold, the king of metals. The discovery of nitrogen is attributed to the Scottish physician Daniel Rutherford in 1772, who called it noxious air.
Though he did not recognise it as an different chemical substance, he distinguished it from Joseph Black's "fixed air", or carbon dioxide. The fact that there was a component of air that does not support combustion was clear to Rutherford, although he was not aware that it was an element. Nitrogen was studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, Joseph Priestley, who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as "mephitic air" or azote, from the Greek word άζωτικός, "no life". In an atmosphere of pure nitrogen, animals died and flames were extinguished. Though Lavoisier's name was not accepted in English, since it was pointed out that all gases are mephitic, it is used in many languages and still remains in English in the common names of many nitrogen compounds, such as hydrazine and compounds of the azide ion, it led to the name "pnictogens" for the group headed by nitrogen, from the Greek πνίγειν "to choke".
The English word nitrogen entered the language from the French nitrogène, coined in 1790 by French chemist Jean-Antoine Chaptal, from the French nitre and the French suffix -gène, "producing", from the Greek -γενής. Chaptal's meaning was that nitrogen is the essential part of nitric acid, which in turn was produced from nitre. In earlier times, niter had been confused with Egyptian "natron" – called νίτρον in Greek – which, despite the name, contained no nitrate; the earliest military and agricultural applications of nitrogen compounds used saltpeter, most notably in gunpowder, as fertiliser. In 1910, Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced "active nitrogen", a monatomic allotrope of nitrogen; the "whirling cloud of brilliant yellow light