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| | I would like to introduce myself to you, I am Andrew and my wife doesn't like it at all. Distributing production has been his occupation for some time. The preferred pastime for him and his children is to play lacross and he would never give it up. For a whilst I've been in Mississippi but now I'm considering other choices.<br><br>Feel free to surf to my site ... [http://netwk.hannam.ac.kr/xe/data_2/85669 psychic chat online] |
| {{Refimprove|date=March 2009}}
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| <!-- Deleted image removed: [[File:Oxyhemoglobin dissociation curve.png|The O<sub>2</sub>-Hb dissociation curve is a sigmoidal curve that represents the relationship between O<sub>2</sub> concentration and the percentage saturation of Hb. As the concentration increases from about 90% there is a plateau in the curve that has several important biological implications.|right|thumb|300px|{{Deletable image-caption|1=Saturday, 3 October 2009|date=May 2012}}|{{Deletable image-caption|1=Saturday, 3 October 2009|date=May 2012}}]] --><!-- Deleted image removed: [[File:Oxygen-hemoglobin dissociation curve.png|frame|alt=Puzzle globe logo|A normal oxyhemoglobin dissociation curve and curves for the case of a 50 percent anemia and the case of a 50 percent carboxyhemoglobinemia.|{{Deletable image-caption|1=Saturday, 3 October 2009|date=May 2012}}|{{Deletable image-caption|1=Saturday, 3 October 2009|date=May 2012}}]] --><!-- Deleted image removed: [[File:The Oxy-Hemoglobin Dissociation Curve.png|frame|The Oxy-Hemoglobin Dissociation Curve underpins the relationship between PaO2 and O2 saturation|{{Deletable image-caption|1=Saturday, 3 October 2009|date=May 2012}}]] -->
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| The '''oxygen–haemoglobin dissociation curve''' plots the proportion of [[hemoglobin]] in its saturated form on the vertical axis against the prevailing [[oxygen]] tension on the horizontal axis. The oxyhemoglobin dissociation curve is an important tool for understanding how our blood carries and releases oxygen. Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (sO<sub>2</sub>) and partial pressure of oxygen in the blood (pO<sub>2</sub>), and is determined by what is called "Hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.
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| ==Background==
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| Hemoglobin (Hb), a [[globular protein]], is the primary vehicle for transporting oxygen in the blood. Oxygen is also carried dissolved in the blood's plasma, but to a much lesser degree. Hemoglobin is contained in erythrocytes, more commonly referred to as red blood cells. Under certain conditions, oxygen bound to the hemoglobin is released into the blood's plasma and absorbed into the tissues. Each hemoglobin molecule has the capacity to carry four oxygen molecules. How much of that capacity is filled by oxygen at any time is called the oxygen saturation. Expressed as a percentage, the oxygen saturation is the ratio of the amount of oxygen bound to the hemoglobin, to the oxygen-carrying capacity of the hemoglobin. The oxygen-carrying capacity of hemoglobin is determined by the type of hemoglobin present in the blood.
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| The amount of oxygen bound to the hemoglobin at any time is related, in large part, to the partial pressure of oxygen to which the hemoglobin is exposed. In the lungs, at the alveolar–capillary interface, the partial pressure of oxygen is typically high, and therefore the oxygen binds readily to hemoglobin that is present. As the blood circulates to other body tissue in which the partial pressure of oxygen is less, the hemoglobin releases the oxygen into the tissue because the hemoglobin cannot maintain its full bound capacity of oxygen in the presence of lower oxygen partial pressures
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| ==Sigmoidal shape==
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| [[File:Oxyhaemoglobin dissociation curve.png|thumb|Hemoglobin saturation curve.]]It is usually a [[sigmoid function|sigmoid]] plot described by a formula of the kind:
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| :<math>S(t) = \frac{1}{1 + e^{-t}}.</math>
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| A hemoglobin molecule can bind up to four oxygen molecules in a reversible way.
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| The shape of the curve results from the interaction of bound oxygen molecules with incoming molecules. The binding of the first molecule is difficult. However, this facilitates the binding of the second, third and fourth, partly as a result of crowding of the hemoglobin molecule, partly as a natural tendency of oxygen to dissociate.
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| In its most simple form, the oxyhemoglobin dissociation curve describes the relation between the partial pressure of oxygen (x axis) and the oxygen saturation (y axis). Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape. At pressures above about 60 mmHg, the standard dissociation curve is relatively flat, which means that the oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure. To get more oxygen to the tissue would require blood transfusions to increase the hemoglobin count (and hence the oxygen-carrying capacity), or supplemental oxygen that would increase the oxygen dissolved in plasma.
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| Although binding of oxygen to hemoglobin continues to some extent for pressures about 50
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| mmHg, as oxygen partial pressures decrease in this steep area of the curve, the oxygen is unloaded to peripheral tissue readily as the hemoglobin's affinity diminishes.
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| The partial pressure of oxygen in the blood at which the hemoglobin is 50% saturated, typically about 26.6 mmHg for a healthy person, is known as the P50. The P50 is a conventional measure of hemoglobin affinity for oxygen. In the presence of disease or other conditions that change the hemoglobin's oxygen affinity and, consequently, shift the curve to the right or left, the P50 changes accordingly. An increased P50 indicates a rightward shift of the standard curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation. This indicates a decreased affinity. Conversely, a lower P50 indicates a leftward shift and a higher affinity.
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| The 'plateau' portion of the oxyhemoglobin dissociation curve is the range that exists at the pulmonary capillaries (minimal reduction of oxygen transported until the p(O<sub>2</sub>) falls 50 mmHg).
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| The 'steep' portion of the oxyhemoglobin dissociation curve is the range that exists at the systemic capillaries (a small drop in systemic capillary p(O<sub>2</sub>) can result in the release of large amounts of oxygen for the metabolically active cells).
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| To see the relative affinities of each successive oxygen as you remove/add oxygen from/to the hemoglobin from the curve compare the relative increase/decrease in p(O<sub>2</sub>) needed for the corresponding increase/decrease in s(O<sub>2</sub>).
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| ==Factors that affect the standard dissociation curve==
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| The strength with which oxygen binds to hemoglobin is affected by several factors. These factors shift or reshape the oxyhemoglobin dissociation curve. A rightward shift indicates that the hemoglobin under study has a decreased affinity for oxygen. This makes it more difficult for hemoglobin to bind to oxygen (requiring a higher partial pressure of oxygen to achieve the same oxygen saturation), but it makes it easier for the hemoglobin to release oxygen bound to it. The effect of this rightward shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, such as during exercise, or hemorrhagic shock. In contrast, the curve is shifted to the left by the opposite of these conditions. This leftward shift indicates that the hemoglobin under study has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly.
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| Left shift of the curve is a sign of hemoglobin's increased affinity for oxygen (e.g. at the lungs). Similarly, right shift shows decreased affinity, as would appear with an increase in body temperature, hydrogen ion, [[2,3-diphosphoglycerate]] (also known as bisphosphoglycerate) or carbon dioxide concentration (the [http://en.wikipedia.org/wiki/Bohr_effect Bohr effect]).
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| {| class="wikitable"
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| ! Control factors
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| ! decrease
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| ! increase
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| |-
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| | Temperature
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| | left shift
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| | right shift
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| |-
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| | [[2,3-bisphosphoglycerate|2.3-BPG]]
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| | left shift
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| | right shift
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| |-
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| | p([[carbon dioxide|CO<sub>2</sub>]])
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| | left shift
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| | right shift
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| |-
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| | [[pH]] ([[Bohr effect]])
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| | right shift ([[acidosis]])
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| | left shift ([[alkalosis]])
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| |-
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| |}
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| Note:
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| * Left shift: high O<sub>2</sub> affinity
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| * Right shift: low O<sub>2</sub> affinity
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| * fetal hemoglobin has higher O<sub>2</sub> than adult hemoglobin.
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| The causes of shift to right can be remembered using the [[mnemonic]], "'''CADET''', face Right!" for '''C'''O2, '''A'''cid, 2,3-'''D'''PG, '''E'''xercise and '''T'''emperature.<ref name="LHC">{{cite web | url = http://mc.lifehugger.com/moc/67/oxygen-hemoglobin-dissociation-curve-causes-shift-right | title = Medical mnemonics | publisher = LifeHugger | accessdate = 2009-12-19}}</ref> | |
| Factors that move the oxygen dissociation curve to the right are those physiological states where tissues need more oxygen. For example during exercise, muscles have a higher metabolic rate, and consequently need more oxygen, produce more carbon dioxide and lactic acid, and their temperature rises.
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| === Variation of the hydrogen ion concentration ===
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| A decrease in pH (increase in {{H+}} ion concentration) shifts the standard curve to the right, while an increase shifts it to the left. The reason for this is that H<sup>+</sup> and O<sub>2</sub> both compete for binding to the hemoglobin molecule. Therefore, with increased acidity, the hemoglobin binds less O2 for a given P<sub>O2</sub> (and more H<sup>+</sup>). This is known as the [[Bohr effect]].<ref name="Jacquez">{{cite book | last=Jacquez | first=John | title=Respiratory Physiology | publisher=McGraw-Hill | year=1979 pages=156–175}}</ref> A reduction in the total binding capacity of hemoglobin to oxygen (i.e. shifting the curve down, not just to the right) due to reduced pH is called the [[root effect]]. This is seen in bony fish.
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| === Effects of carbon dioxide ===
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| [[Carbon dioxide]] affects the curve in two ways: first, it influences [[intracellular pH]] (the Bohr effect), and second, CO<sub>2</sub> accumulation causes carbamino compounds to be generated through chemical interactions, which bind to hemoglobin forming [[carbaminohemoglobin]]. Low levels of carbamino compounds have the effect of shifting the curve to the left, while higher levels cause a rightward shift. However, this isn't the overriding effect of CO<sub>2</sub> accumulation. Only about 5–10% of the total CO<sub>2</sub> content of blood is transported as carbamino compounds. Most of the CO<sub>2</sub> content (80–90%) is transported as bicarbonate ions. The formation of a bicarbonate ion will release a proton into the plasma. Hence, the elevated CO2 content creates a respiratory acidosis and shifts the oxygen–hemoglobin dissociation curve to the right.
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| === Effects of 2,3-BPG ===
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| 2,3-Bisphosphoglycerate or 2,3-BPG (also 2,3-Diphosphoglycerate or 2,3-DPG) is an organophosphate, which is created in erythrocytes during glycolysis. The production of 2,3-DPG is likely an important adaptive mechanism, because the production increases for several conditions in the presence of diminished peripheral tissue O<sub>2</sub> availability, such as hypoxaemia, chronic lung disease, anaemia, and congestive heart failure, among others. High levels of 2,3-DPG shift the curve to the right (as in childhood), while low levels of 2,3-DPG cause a leftward shift, seen in states such as septic shock, and hypophosphataemia.<ref name="Jacquez"/> In the absence of 2,3 BPG, Hemoglobin's affinity for oxygen increases. 2,3 BPG as a heteroallosteric effector of hemoglobin, lowers hemoglobin's affinity for oxygen by binding preferentially to deoxyhemoglobin. An increased concentration of BPG in red blood cells favours formation of the T, low-affinity state of hemoglobin and so the oxygen-binding curve will shift to the right.
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| === Temperature ===
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| Temperature does not have such a dramatic effect compared to the previous factors, but hyperthermia causes a rightward shift, while hypothermia causes a leftward shift.<ref>{{Cite pmid|2089610}}</ref> Increasing temperature will weaken and denature the bond between an oxygen and a hemoglobin which in turn decreases the concentration of the oxyhemoglobin.<ref>{{Cite isbn | 0521570980 }}</ref>
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| === Carbon monoxide ===
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| Hemoglobin binds with [[carbon monoxide]] 200–250 times more readily than with oxygen.<ref name="Jacquez"/> The presence of carbon monoxide on one of the 4 [[heme|haem]] sites causes the oxygen on the other haem sites to bind with greater affinity.{{Citation needed|date=June 2013}} Moreover, because of hemoglobin's affinity for carbon monoxide over oxygen, carbon monoxide is a highly successful competitor. Even at minuscule partial pressures, carbon monoxide can displace oxygen and results in a shift of the curve to the left.<ref name="Marieb">[Human Anatomy & Physiology Eighth Edition]</ref> With an increased level of carbon monoxide, a person can suffer from severe tissue hypoxia while maintaining a normal pO<sub>2</sub>.
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| === Effects of methemoglobinaemia ===
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| [[Methemoglobinemia|Methemoglobinaemia]] is a form of abnormal hemoglobin where ferrous (Fe<sup>2+</sup>), which is normally found in hemoglobin, is converted to the ferric (Fe<sup>3+</sup>) state. This causes a leftward shift in the curve as methemoglobin does not unload O2 from Hb. However, methemoglobin has increased rbc affinity for cyanide, and is therefore useful in the treatment of cyanide poisoning. Nitrites can be used to oxidize hemoglobin to methemoglobin which can then bind cyanide. This restores cytochrome oxidase's ability to function. Furthermore, thiosulfate can be given to bind cyanide, forming thiocyanate, which can be excreted by the kidneys.
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| == Fetal hemoglobin ==
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| [[File:Hb.jpg|thumb|250px|Fetal hemoglobin saturation curve]] | |
| Fetal hemoglobin (HbF) is structurally different from normal adult hemoglobin (HbA), giving HbF a higher affinity for oxygen than HbA. HbF is composed of two alpha and two gamma chains whereas HbA is composed of two alpha and two beta chains. The fetal dissociation curve is shifted to the left relative to the curve for the normal adult because of these structural differences.
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| Typically, fetal arterial oxygen pressures are lower than adult arterial oxygen pressures. Hence higher affinity to bind oxygen is required at lower levels of partial pressure in the fetus to allow diffusion of oxygen across the [[placenta]]. At the placenta. there is a higher concentration of 2,3-DPG formed, and 2,3-DPG binds readily to beta chains rather than to alpha chains. As a result, 2,3-DPG binds more strongly to adult hemoglobin, causing HbA to release more oxygen for uptake by the fetus, whose HbF is unaffected by the 2,3-DPG.<ref name=Lippincotts>{{cite book|title=Lippincott's Illustrated Review: Biochemistry 4th edition|year=2007|publisher=Lippincott, Williams, and Wilkins|location=North America|isbn=978-0-7817-6960-0|pages=24–35}}</ref> HbF then delivers that bound oxygen to tissues that have even lower partial pressures where it can be released.
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| ==See also==
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| [[Automated analyzer]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| * {{GeorgiaPhysiology|4/4ch5/s4ch5_18}}
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| {{Respiratory physiology}}
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| {{DEFAULTSORT:Oxygen-hemoglobin dissociation curve}}
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| [[Category:Respiratory physiology]]
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| [[Category:Chemical pathology]]
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| [[Category:Hematology]]
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| [[Category:Oxygen]]
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