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Carbonyl group
From Wikipedia, the free encyclopedia
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For carboxyl as a ligand, see Metal carbonyl.
A compound containing a carbonyl group (C=O)
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a carbonyl group is often referred to as a carbonyl compound.
The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex (a metal carbonyl, e.g. nickel carbonyl).
The remainder of this article concerns itself with the organic chemistry definition of carbonyl, where carbon and oxygen share a double bond.
Contents
[hide]
1 Carbonyl compounds
2 Reactivity
2.1 Carbonyl resonance chemistry
2.2 Carbonyl reduction
2.3 Carbonyl alkylation
2.4 Carbonyl chemoselectivity
2.5 Carbonyl specialty reactions
3 α,β-Unsaturated carbonyl compounds
4 Spectroscopy
5 See also
6 References
7 Further reading
Carbonyl compounds[edit]
A carbonyl group characterizes the following types of compounds:
Compound Aldehyde Ketone Carboxylic acid Carboxylate Ester Amide
Structure
General formula RCHO RCOR' RCOOH RCOOR' RCONR'R''
Compound Enone Acyl halide Acid anhydride Imide
Structure
General formula RC(O)C(R')CR''R''' RCOX (RCO)2O RC(O)N(R')C(O)R''
Note that the most specific labels are usually employed. For example, R(CO)O(CO)R' structures are known as acid anhydride rather than the more generic ester, even though the ester motif is present.
Carbon dioxide
Other organic carbonyls are urea and the carbamates, the derivatives of acyl chlorides chloroformates and phosgene, carbonate esters, thioesters, lactones, lactams, hydroxamates, and isocyanates. Examples of inorganic carbonyl compounds are carbon dioxide and carbonyl sulfide.
A special group of carbonyl compounds are 1,3-dicarbonyl compounds that have acidic protons in the central methylene unit. Examples are Meldrum's acid, diethyl malonate and acetylacetone.
Reactivity[edit]
Carbonyl resonance chemistry[edit]
Because oxygen is more electronegative than carbon, carbonyl compounds often have resonance structures which affect their reactivity. This relative electronegativity draws electron density away from carbon, increasing the bond's polarity, therefore making carbon an electrophile (i.e. slightly positive). Carbon can then be attacked by nucleophiles (e.g. negatively charged ions, like the cyanide ion) or a negatively charged part of another molecule (e.g. the lone pair electrons of nitrogen in the ammonia molecule). During the reaction, the carbon-oxygen double bond is broken, and the carbonyl group may experience addition reactions. This reaction is known as addition-elimination (because a water molecule is often lost) or condensation.[1] The electronegative oxygen also can react with an electrophile; for example a proton in an acidic solution or with Lewis acids to form an oxocarbenium ion.
A carbonyl compound
The polarity of oxygen also makes the alpha hydrogens of carbonyl compounds much more acidic (roughly 1030 times more acidic) than typical sp3 C-H bonds, such as those in methane. For example, the pKa values of acetaldehyde and acetone are 16.7 and 19 respectively,[2] while the pKa value of methane is extrapolated to be approximately 50.[3] This is because a carbonyl is in tautomeric resonance with an enol. The deprotonation of the enol with a strong base produces an enolate, which is a powerful nucleophile and can alkylate electrophiles such as other carbonyls.
Amides are the most stable of the carbonyl couplings due to their high resonance stabilization between the nitrogen-carbon and carbon-oxygen bonds.
Carbonyl reduction[edit]
Main article: Carbonyl reduction
Carbonyl groups can be reduced by reaction with hydride reagents such as NaBH4 and LiAlH4, with baker's yeast, or by catalytic hydrogenation. Ketones give secondary alcohols while aldehydes, esters and carboxylic acids give primary alcohols.
Carbonyl alkylation[edit]
Carbonyls can be alkylated in nucleophilic addition reactions using organometallic compounds such as organolithium reagents, Grignard reagents, or acetylides. Carbonyls also may be alkylated by enolates as in aldol reactions. Carbonyls are also the prototypical groups with vinylogous reactivity (e.g. the Michael reaction where an unsaturated carbon in conjugation with the carbonyl is alkylated instead of the carbonyl itself).
Carbonyl chemoselectivity[edit]
In case of multiple carbonyl types in one molecule, one can expect the most electrophilic carbonyl carbon to react first. Acyl chlorides and carboxylic anhydrides react fastest, followed by aldehydes and ketones. Esters react much more slowly and amides are almost completely unreactive due to resonance of the amide nitrogen towards the carbonyl group. This reactivity difference allows chemoselectivity when a reactant contains multiple carbonyl groups. An instructive example is found in the last part of the total synthesis of monensin by Kishi in 1979:[4]
The left-hand reactant possesses two potential electrophilic sites: an aldehyde (indicated in blue) and an ester (indicated in green). Only the aldehyde, which is more electrophilic, will react with the enolate of the methyl ketone in the other part of the molecule. The methyl ester remains untouched. Of course, other effects can play a role in this selectivity process, including electronic effects, steric effects, and thermodynamic versus kinetic reaction control.
Carbonyl specialty reactions[edit]
Other important reactions include:
Carbonyl Alpha-Substitution Reactions
Wittig Reaction a phosphonium ylid is used to create an alkene
Wolff-Kishner reduction into a hydrazone and further into a saturated alkane
Clemmensen reduction into a saturated alkane
Mozingo reduction into a saturated alkane
Conversion into thioacetals
Hydration to hemiacetals and hemiketals, and then to acetals and ketals
Reaction with ammonia and primary amines to form imines
Reaction with hydroxylamines to form oximes
Reaction with cyanide anion to form cyanohydrins
Oxidation with oxaziridines to acyloins
Reaction with Tebbe's reagent and phosphonium ylides to alkenes.
Perkin reaction, an aldol reaction variant
Aldol condensation, a reaction between an enolate and a carbonyl
Cannizzaro reaction, a disproportionation of aldehydes into alcohols and acids
Tishchenko reaction, another disproportionation of aldehydes that gives a dimeric ester
Nucleophilic abstraction is used to produce carbon dioxide
α,β-Unsaturated carbonyl compounds[edit]
Acrolein, an α,β-unsaturated carbonyl compound.
α,β-Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general structure (O=CR)−Cα=Cβ-R; for example enones and enals. In these compounds the carbonyl group is conjugated with an alkene (hence the adjective unsaturated), from which they derive special properties. Unlike the case for simple carbonyls, α,β-unsaturated carbonyl compounds are often attacked by nucleophiles at the β carbon. This pattern of reactivity is called vinylogous. Examples of unsaturated carbonyls are acrolein (propenal), mesityl oxide, acrylic acid, and maleic acid. Unsaturated carbonyls can be prepared in the laboratory in an aldol reaction and in the Perkin reaction.
The carbonyl group draws electrons away from the alkene, and the alkene group is, therefore, deactivated towards an electrophile, such as bromine or hydrochloric acid. As a general rule with asymmetric electrophiles, hydrogen attaches itself at the α-position in an electrophilic addition. On the other hand, these compounds are activated towards nucleophiles in nucleophilic conjugate addition.
Since α,β-unsaturated compounds are electrophiles, many α,β-unsaturated carbonyl compounds are toxic, mutagenic and carcinogenic. DNA can attack the β carbon and thus be alkylated. However, the endogenous scavenger compound glutathione naturally protects from toxic electrophiles in the body. Some drugs (amifostine, N-acetylcysteine) containing thiol groups may protect biomolecules from such harmful alkylation.
Spectroscopy[edit]
Infrared spectroscopy: the C=O double bond absorbs infrared light at wavenumbers between approximately 1600–1900 cm−1(5263 nm to 6250 nm). The exact location of the absorption is well understood with respect to the geometry of the molecule. This absorption is known as the "carbonyl stretch" when displayed on an infrared absorption spectrum.[5] In addition, the ultraviolet-visible spectra of propanone in water gives an absoption of carbonyl at 257 nm.[6]
Nuclear magnetic resonance: the C=O double-bond exhibits different resonances depending on surrounding atoms, generally a downfield shift. The 13C NMR of a carbonyl carbon is in the range of 160-220 ppm.
See also[edit]
Organic chemistry
Functional group
Bridging carbonyl
References[edit]
Jump up ^ "an introduction to aldehydes and ketones". www.chemguide.co.uk.
Jump up ^ Ouellette, R.J. and Rawn, J.D. "Organic Chemistry" 1st Ed. Prentice-Hall, Inc., 1996: New Jersey. ISBN 0-02-390171-3
Jump up ^ Claden, Johnathan; et al. Organic Chemistry. Oxford University Press. ISBN 978-0-19-850346-0.
Jump up ^ Nicolaou, Kyriacos Costa; E. J. Sorensen (1996). Classics in Total Synthesis: Targets, Strategies, Methods. Wiley-VCH. pp. 230–232. ISBN 3-527-29231-4.
Jump up ^ Mayo D.W., Miller F.A and Hannah R.W “Course Notes On The Interpretation of Infrared and Raman Spectra” 1st Ed. John Wiley & Sons Inc, 2004: New Jersey. ISBN 0-471-24823-1.
Jump up ^ "Archived copy" (PDF). Archived from the original (PDF) on 2015-08-24. Retrieved 2015-07-11.
Further reading[edit]
Wikimedia Commons has media related to Carbonyl compounds.
Wikiquote has quotations related to: Carbonyl group
L.G. Wade, Jr. Organic Chemistry, 5th ed. Prentice Hall, 2002. ISBN 0-13-033832-X
The Frostburg State University Chemistry Department. Organic Chemistry Help (2000).
Advanced Chemistry Development, Inc. IUPAC Nomenclature of Organic Chemistry (1997).
William Reusch. tara VirtualText of Organic Chemistry (2004).
Purdue Chemistry Department [1] (retrieved Sep 2006). Includes water solubility data.
William Reusch. (2004) Aldehydes and Ketones Retrieved 23 May 2005.
ILPI. (2005) The MSDS Hyperglossary- Anhydride.
[show]
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See also chemical classification, chemical nomenclature (inorganic, organic)
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text:
Speedometer
From Wikipedia, the free encyclopedia
Jump to: navigation, search
An animation of an electronic Aston Martin speedometer's self-test routine, showing how an analogue speedometer needle may indicate the vehicle's speed.
A Ford speedometer
A modern Toyota Corolla Speedometer
Dashboard_Speedometers of a jeep
A speedometer or a speed meter is a gauge that measures and displays the instantaneous speed of a vehicle. Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards.[1] Speedometers for other vehicles have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.
Charles Babbage is credited with creating an early type of a speedometer, which were usually fitted to locomotives.[2][3]
The electric speedometer was invented by the Croatian Josip Belušić[4] in 1888, and was originally called a velocimeter.
Contents
[hide]
1 Operation
1.1 Electronic
1.1.1 Bicycle speedometers
2 Error
2.1 International agreements
2.2 Australia
2.3 United Kingdom
2.4 United States
3 Regulation in the US
4 GPS
5 See also
6 References
7 External links
Operation[edit]
Originally patented by Otto Schultze on October 7, 1902,[5] it uses a rotating flexible cable usually driven by gearing linked to the output of the vehicle's transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.
When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, which then turns the speedometer mechanism itself. A small permanent magnet affixed to the speedometer cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet exerts a torque on the cup, "dragging" it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.[1]
The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup increases with the speed of rotation of the magnet. Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup is balanced by the opposing torque of the spring, and then stop. Given the torque on the cup is proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed, so that equally spaced markers on the dial can be used for gaps in speed. At a given speed the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.
The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires.
One of the key disadvantages of the eddy current speedometer is that it cannot show the vehicle speed when running in reverse gear since the cup would turn in the opposite direction - in this scenario the needle would be driven against its mechanical stop pin on the zero position.
Electronic[edit]
See also: Vehicle speed sensor
Many modern speedometers are electronic. In designs derived from earlier eddy-current models, a rotation sensor mounted in the transmission delivers a series of electronic pulses whose frequency corresponds to the (average) rotational speed of the driveshaft, and therefore the vehicle's speed, assuming the wheels have full traction. The sensor is typically a set of one or more magnets mounted on the output shaft or (in transaxles) differential crownwheel, or a toothed metal disk positioned between a magnet and a magnetic field sensor. As the part in question turns, the magnets or teeth pass beneath the sensor, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.[1] Alternatively, in more recent designs, some manufactures rely on pulses coming from the ABS wheel sensors. Most modern electronic speedometers have the additional ability over the eddy current type to show the vehicle speed when moving in reverse gear.
A computer converts the pulses to a speed and displays this speed on an electronically controlled, analog-style needle or a digital display. Pulse information is also used for a variety of other purposes by the ECU or full-vehicle control system, e.g. triggering ABS or traction control, calculating average trip speed, or to increment the odometer in place of it being turned directly by the speedometer cable.
Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission. The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity. The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.
Bicycle speedometers[edit]
Main article: Cyclocomputer
Typical bicycle speedometers measure the time between each wheel revolution, and give a readout on a small, handlebar-mounted digital display. The sensor is mounted on the bike at a fixed location, pulsing when the spoke-mounted magnet passes by. In this way, it is analogous to an electronic car speedometer using pulses from an ABS sensor, but with a much cruder time/distance resolution - typically one pulse/display update per revolution, or as seldom as once every 2–3 seconds at low speed with a 26-inch (2.07m circumference, without tire) wheel. However, this is rarely a critical problem, and the system provides frequent updates at higher road speeds where the information is of more importance. The low pulse frequency also has little impact on measurement accuracy, as these digital devices can be programmed by wheel size, or additionally by wheel or tire circumference in order to make distance measurements more accurate and precise than a typical motor vehicle gauge. However these devices carry some minor disadvantage in requiring power from batteries that must be replaced every so often in the receiver (AND sensor, for wireless models), and, in wired models, the signal being carried by a thin cable that is much less robust than that used for brakes, gears, or cabled speedometers.
Other, usually older bicycle speedometers are cable driven from one or other wheel, as in the motorcycle speedometers described above. These do not require battery power, but can be relatively bulky and heavy, and may be less accurate. The turning force at the wheel may be provided either from a gearing system at the hub (making use of the presence of e.g. a hub brake, cylinder gear or dynamo) as per a typical motorcycle, or with a friction wheel device that pushes against the outer edge of the rim (same position as rim brakes, but on the opposite edge of the fork) or the sidewall of the tyre itself. The former type are quite reliable and low maintenance but need a gauge and hub gearing properly matched to the rim and tyre size, whereas the latter require little or no calibration for a moderately accurate readout (with standard tyres, the "distance" covered in each wheel rotation by a friction wheel set against the rim should scale fairly linearly with wheel size, almost as if it were rolling along the ground itself) but are unsuitable for off-road use, and must be kept properly tensioned and clean of road dirt to avoid slipping or jamming.
Error[edit]
Most speedometers have tolerances of some ±10%, mainly due to variations in tire diameter.[citation needed] Sources of error due to tire diameter variations are wear, temperature, pressure, vehicle load, and nominal tire size. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits.[citation needed]
Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the error is
Percentage error = 100 × ( 1 − new diameter / standard diameter ) {\displaystyle {\mbox{Percentage error}}=100\times (1-{\mbox{new diameter}}/{\mbox{standard diameter}})}
Nearly all tires now have their size shown as "T/A_W" on the side of the tire (See: Tire code), and the tire's
Diameter in millimetres = 2 × T × A / 100 + W × 25.4 {\displaystyle {\mbox{Diameter in millimetres}}=2\times T\times A/100+W\times 25.4}
Diameter in inches = T × A / 1270 + W {\displaystyle {\mbox{Diameter in inches}}=T\times A/1270+W}
For example, a standard tire is "185/70R14" with diameter = 2*185*(70/100)+(14*25.4) = 614.6 mm (185x70/1270 + 14 = 24.20 in). Another is "195/50R15" with 2*195*(50/100)+(15*25.4) = 576.0 mm (195x50/1270 + 15 = 22.68 in). Replacing the first tire (and wheels) with the second (on 15" = 381 mm wheels), a speedometer reads 100 * (1-(576/614.6)) = 100 * (1 - 22.68/24.20) = 6.28% higher than the actual speed. At an actual speed of 100 km/h (60 mph), the speedometer will indicate 100 x 1.0628 = 106.28 km/h (60 * 1.0628 = 63.77 mph), approximately.
In the case of wear, a new "185/70R14" tyre of 620 mm (24.4 inch) diameter will have ~8 mm tread depth, at legal limit this reduces to 1.6 mm, the difference being 12.8 mm in diameter or 0.5 inches which is 2% in 620 mm (24.4 inches).
International agreements[edit]
In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe (UNECE) Regulation 39,[6] which covers those aspects of vehicle type approval that relate to speedometers. The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country where it is sold.
European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers[7][8][9] are similar to the UNECE regulation in that they specify that:
The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.
The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature and road surface. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.
The UNECE regulation relaxes the requirements for vehicles mass-produced following type approval. At Conformity of Production Audits the upper limit on indicated speed is increased to 110 percent plus 6 km/h for cars, buses, trucks and similar vehicles, and 110 percent plus 8 km/h for two- or three-wheeled vehicles that have a maximum speed above 50 km/h (or a cylinder capacity, if powered by a heat engine, of more than 50 cm³). European Union Directive 2000/7/EC, which relates to two- and three-wheeled vehicles, provides similar slightly relaxed limits in production.
Australia[edit]
There were no Australian Design Rules in place for speedometers in Australia prior to July 1988. They had to be introduced when speed cameras were first used. This means there are no legally accurate speedometers for these older vehicles. All vehicles manufactured on or after 1 July 2007, and all models of vehicle introduced on or after 1 July 2006, must conform to UNECE Regulation 39.[10]
The speedometers in vehicles manufactured before these dates but after 1 July 1995 (or 1 January 1995 for forward control passenger vehicles and off-road passenger vehicles) must conform to the previous Australian design rule. This specifies that they need only display the speed to an accuracy of +/- 10% at speeds above 40 km/h, and there is no specified accuracy at all for speeds below 40 km/h. All vehicles manufactured in Australia or imported for supply to the Australian market must comply with the Australian Design Rules.
[11] The state and territory governments may set policies for the tolerance of speed over the posted speed limits that may be lower than the 10% in the earlier versions of the Australian Design Rules permitted, such as in Victoria.[12] This has caused some controversy since it would be possible for a driver to be unaware that he is speeding should his vehicle be fitted with an under-reading speedometer.[13]
United Kingdom[edit]
A speedometer showing mph and km/h along with an odometer and a separate "trip" odometer (both showing distance traveled in miles).
The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[14]
The Motor Vehicles (Approval) Regulations 2001[15] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.
United States[edit]
Federal standards in the United States allow a maximum 5 mph error at a speed of 50 mph on speedometer readings for commercial vehicles.[16] Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.
This section's factual accuracy may be compromised due to out-of-date information. Please update this article to reflect recent events or newly available information. (June 2017)
Regulation in the US[edit]
On September 1, 1979 the NHTSA required speedometers to have special emphasis on 55 mph and display no more than a maximum speed of 85 mph. On March 25, 1982 the NHTSA revoked the rule because no "significant safety benefits" could come from maintaining the standard.[17]
GPS[edit]
This section's factual accuracy is disputed. Relevant discussion may be found on Talk:Speedometer. Please help to ensure that disputed statements are reliably sourced. (September 2015) (Learn how and when to remove this template message)
Main article: Automotive navigation system
GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error. Some GPS devices do not take into account the vertical position of the car so will under report the speed by the road's gradient.
As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed.
See also[edit]
GM Instrument Cluster Settlement
Hubometer
List of vehicle instruments
Taximeter
References[edit]
^ Jump up to: a b c Harris, William (10 July 2007). "How Speedometers Work". How stuff works. Retrieved 30 January 2015.
Jump up ^ "Charles Babbage and the Difference Engine - NewMyths.com". google.com. Retrieved 30 January 2015.
Jump up ^ "Udini → Flow". proquest.com. Retrieved 30 January 2015.
Jump up ^ Sobey, Ed (2009). A Field Guide to Automotive Technology. Chicago Review Press. p. 78. ISBN 9781556528125. Retrieved 30 January 2015.
Jump up ^ "Speedometer". Siemens. 26 April 2005. Retrieved 30 January 2015.
Jump up ^ "UNECE Transport Division - Vehicle Regulations - Addenda to 1958 agreement - Regulations 21-40". UN Economic Commission for Europe. Retrieved 30 January 2015.
Jump up ^ "Regulation No 39 of the Economic Commission for Europe of the United Nations (UN/ECE) — Uniform provisions concerning the approval of vehicles with regard to the speedometer equipment including its installation". European Commission. Retrieved 2017-04-08.
Jump up ^ "Commission Directive 97/39/EC of 24 June 1997 adapting to technical progress Council Directive 75/443/EEC of 26 June 1975 relating to the reverse and speedometer equipment of motor vehicles". European Commission. Retrieved 2007-01-07.
Jump up ^ "Directive 2000/7/EC - speedometers for two- or three-wheel motor vehicles". European Commission. Retrieved 2007-01-07.
Jump up ^ "Australian Design Rule 18/03 - Instrumentation" (PDF). Road Vehicle Certification System. Retrieved 2008-01-07.
Jump up ^ "Australian Design Rule 18/02 - Instrumentation". Commonwealth of Australia Law. Retrieved 2008-01-14.
Jump up ^ Leslie Felix (2004). "Vehicle Speed Measurement II". National Motorists Association Australia. Retrieved 2008-01-14.
Jump up ^ "3.6 Accuracy of speedometers". Victoria Road Safety Committee, Inquiry Into the Demerit Points Scheme. November 1994. Retrieved 2008-01-14.
Jump up ^ "Speedometer Accuracy". Written Answers, Hansard (UK Parliament proceedings) Monday, 12th March 2001. Retrieved 2008-01-07.
Jump up ^ "The Motor Vehicles (Approval) Regulations 2001 : Schedule 3". Office of Public Sector Information. Retrieved 2007-12-19.
Jump up ^ "eCFR — Code of Federal Regulations". gpoaccess.gov. Retrieved 30 January 2015.
Jump up ^ http://www.iihs.org/externaldata/srdata/docs/sr1705.pdf
External links[edit]
Wikimedia Commons has media related to:
Speedometer (category)
Look up speedometer in Wiktionary, the free dictionary.
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Lacteal
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Lacteal
Transverse section of a villus, from the human intestine. X 350.
a. Basement membrane, here somewhat shrunken away from the epithelium.
b. Lacteal.
c. Columnar epithelium.
d. Its striated border.
e. Goblet cells.
f. Leucocytes in epithelium.
f’. Leucocytes below epithelium.
g. Bloodvessels.
h. Muscle cells cut across.
Villi of small intestine, showing bloodvessels and lymphatic vessels.
Dorlands
/Elsevier
l_01/12474659
Anatomical terminology
[edit on Wikidata]
A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine.
Triglycerides are emulsified by bile and hydrolyzed by the enzyme lipase, resulting in a mixture of fatty acids, di- and monoglycerides.[1] These then pass from the intestinal lumen into the enterocyte, where they are re-esterified to form triacylglycerol. The triacylglycerol is then combined with phospholipids, cholesterol ester, and apolipoprotein B48 to form chylomicrons. These chylomicrons then pass into the lacteals, forming a milky substance known as chyle. The lacteals merge to form larger lymphatic vessels that transport the chyle to the thoracic duct where it is emptied into the bloodstream at the subclavian vein.[2][3]
At this point, the fats are in the bloodstream in the form of chylomicrons. Once in the blood, chylomicrons are subject to delipidation by lipoprotein lipase. Eventually, enough lipid has been lost and additional apolipoproteins gained, that the resulting particle (now referred to as a chylomicron remnant) can be taken up by the liver. From the liver, the fat released from chylomicron remnants can be re-exported to the blood as the triglyceride component of very low-density lipoproteins. Very low-density lipoproteins, also subject to delipidation by vascular lipoprotein lipase, delivers fats to tissues throughout the body and, in particular, the released fatty acids can be stored in adipose cells as triglycerides. As triglycerides are lost from very low-density lipoproteins, the lipoprotein particle becomes smaller and denser (since protein is denser than lipid) and ultimately becomes low-density lipoproteins. A great deal has been written about low-density lipoproteins because they are thought to be atherogenic.
Note that in contrast to any other route of absorption from the small intestine, the lymphatic system avoids first pass metabolism.
Notes and references[edit]
Jump up ^ Young, Barbara & Wheater, Paul, eds. (2006). Wheater's functional histology: a text and colour atlas. Elsevier Health Sciences. p. 279. ISBN 978-0-443-06850-8. CS1 maint: Uses editors parameter (link)
Jump up ^ Newsholme, Eric & Leech, Anthony (2009). Functional Biochemistry in Health and Disease. John Wiley & Sons. p. 79. ISBN 978-0-471-98820-5. CS1 maint: Uses authors parameter (link)
Jump up ^ Montague, Susan E. et al, eds. (2005). Physiology for nursing practice. Elsevier. p. 502. ISBN 978-0-7020-2676-8. CS1 maint: Uses editors parameter (link)
External links[edit]
Histology image: 11705loa – Histology Learning System at Boston University - "117. Digestive System: Alimentary Canal jejunum, central lacteals "
Biology portal
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