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| {{refimprove|date=October 2013}}
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| The '''bicarbonate buffering system''' is an important [[buffer solution|buffer]] system in the [[acid-base homeostasis]] of living things, including humans. As a buffer, it tends to maintain a relatively constant [[blood plasma|plasma]] [[pH]] and counteract any force that would alter.
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| In this system, [[carbon dioxide]] (CO<sub>2</sub>) combines with [[water]] to form [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>), which in turn rapidly dissociates to form [[hydrogen ions]] and [[bicarbonate]] (HCO<sub>3</sub><sup>- </sup>) as shown in the reactions below. The carbon dioxide - carbonic acid equilibrium is [[catalysis|catalyzed]] by the [[enzyme]] [[carbonic anhydrase]]; the carbonic acid - bicarbonate equilibrium is simple proton dissociation/association and needs no catalyst.
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| :<math>\rm CO_2 + H_2O \rightleftarrows H_2CO_3 \rightleftarrows HCO_3^- + H^+ </math>
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| Any disturbance of the system will be compensated by a shift in the [[chemical equilibrium]] according to [[Le Chatelier's principle]]. For example, if one attempted to acidify the blood by dumping in an excess of hydrogen ions ([[acidemia]]), some of those hydrogen ions will associate with bicarbonate, forming carbonic acid, resulting in a smaller net increase of acidity than otherwise. This buffering system becomes an even more powerful regulator of acidity when it is coupled with the body's capacity for [[respiratory compensation]], in which [[breathing]] is altered to modify the amount of CO<sub>2</sub> in circulation. In the above example, increased ventilation would increase the loss of CO<sub>2</sub> to the atmosphere, driving the equilibria above to the left. The process could continue until the excess acid is all [[exhalation|exhaled]].
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| This process is extremely important in the [[physiology]] of blood-having animals. It manages the many [[acid-base imbalance|acid and base imbalances]] that can be produced by both normal and [[pathophysiology|abnormal physiology]]. It also affects the handling of carbon dioxide, the constantly produced waste product of [[cellular respiration]].
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| ==Henderson–Hasselbalch equation==
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| A modified version of the [[Henderson–Hasselbalch equation]] can be used to relate the pH of [[blood]] to constituents of the bicarbonate buffering system:<ref name=Bray1999>[http://books.google.com.my/books?id=qyHu0Iu-XOUC&pg=PA556 page 556], section "Estimating plasma pH" in: {{Cite book | last1 = Bray | first1 = John J. | title = Lecture notes on human physiolog | year = 1999 | publisher = Blackwell Science | location = Malden, Mass. | isbn = 978-0-86542-775-4 | pages = }}</ref>
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| :<math> pH = pK_{a~H_2CO_3}+ \log \left ( \frac{[HCO_3^-]}{[H_2CO_3]} \right )</math>
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| , where:
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| *pK<sub>a H<sub>2</sub>CO<sub>3</sub></sub> is the negative logarithm (base 10) of the [[acid dissociation constant]] of [[carbonic acid]]. It is equal to 6.1.
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| *[HCO<sub>3</sub><sup>-</sup>] is the concentration of [[bicarbonate]] in the blood
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| *[H<sub>2</sub>CO<sub>3</sub>] is the concentration of carbonic acid in the blood
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| This is useful in [[arterial blood gas]], but these usually state ''pCO<sub>2</sub>'', that is, the [[partial pressure]] of [[carbon dioxide]], rather than H<sub>2</sub>CO<sub>3</sub>. However, these are related by the equation:<ref name=Bray1999/>
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| :<math> [H_2CO_3] = k_{\rm H~CO_2}\, \times pCO_2 </math> | |
| , where:
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| *[H<sub>2</sub>CO<sub>3</sub>] is the concentration of carbonic acid in the blood
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| *''k<sub>H CO<sub>2</sub></sub>'' is a constant including the [[solubility]] of carbon dioxide in blood. ''k<sub>H CO<sub>2</sub></sub>'' is approximately 0.03 ([[millimole|mmol]]/[[Litre|L]])/[[mmHg]]
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| *''pCO<sub>2</sub>'' is the [[partial pressure]] of [[carbon dioxide]] in the blood
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| Taken together, the following equation can be used to relate the pH of blood to the concentration of bicarbonate and the partial pressure of carbon dioxide:<ref name=Bray1999/>
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| :<math> pH = 6.1 + \log \left ( \frac{[HCO_3^-]}{0.03 \times pCO_2} \right )</math>
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| , where:
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| *pH is the acidity in the blood
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| *[HCO<sub>3</sub><sup>-</sup>] is the concentration of bicarbonate in the blood
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| *''pCO<sub>2</sub>'' is the partial pressure of carbon dioxide in the blood
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| ==References==
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| {{reflist}}
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| ==External links==
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| * {{GeorgiaPhysiology|7/7ch12/7ch12p17}}
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| {{Renal physiology}}
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| [[Category:Electrolyte disturbances]]
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| [[Category:Buffers]]
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