View source for Acid-base homeostasis

{{Acids and bases}}
'''Acid-base homeostasis''' is the part of [[human homeostasis]] concerning the proper balance between [[acid]]s and [[Chemical base|bases]], in other words, the [[pH]]. The body is very sensitive to its pH level, so strong mechanisms exist to maintain it. Outside the acceptable range of pH, [[protein]]s are denatured and digested, [[enzyme]]s lose their ability to function, and [[death]] may occur.

==Mechanism==
[[File:Acid-base_nomogram.svg|thumb]]

The body's acid-base balance is tightly regulated. Several [[buffering agent]]s that reversibly bind hydrogen ions and impede any change in pH exist. [[Extracellular]] buffers include [[bicarbonate]] and [[ammonia]], whereas [[protein]]s and [[phosphate]] act as [[intracellular]] buffers. The [[bicarbonate buffering system]] is especially key, as [[carbon dioxide]] (CO<sub>2</sub>) can be shifted through [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>) to hydrogen ions and [[bicarbonate]] (HCO<sub>3</sub><sup>-</sup>) as shown below.
:<center><math>\rm HCO_3^- + H^+ \leftrightarrow H_2CO_3 \leftrightarrow CO_2 + H_2O</math></center>

Acid-base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of [[Ventilation (physiology)|ventilation]]. This alters the concentration of [[carbon dioxide]] in the blood, shifting the above reaction according to [[Le Chatelier's principle]], which in turn alters the pH. For instance, if the blood pH drops too low (''acidemia''), the body will compensate by increasing breathing, expelling CO<sub>2</sub>, and shifting the above reaction to the right such that less hydrogen ions are free; thus the pH will rise back to normal. For ''alkalemia'', the opposite occurs.

The [[kidneys]] are slower to compensate, but [[renal physiology]] has several powerful mechanisms to control pH by the excretion of excess acid or base. In responses to acidosis, [[renal tubule|tubular]] cells reabsorb more bicarbonate from the tubular fluid, [[collecting duct]] cells secrete more hydrogen and generate more bicarbonate, and [[ammoniagenesis]] leads to increased formation of the NH<sub>3</sub> buffer. In responses to alkalosis, the kidney may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of [[glutamine]] metabolism and ammonia excretion.

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==Imbalance==
[[Acid-base imbalance]] occurs when a significant insult causes the blood pH to shift out of the normal range (7.35 to 7.45). An excess of acid is called [[acidosis]] (pH less than 7.35) and an excess of base is called [[alkalosis]] (pH greater than 7.45). The process that causes the imbalance is classified based on the [[etiology]] of the disturbance (respiratory or metabolic) and the direction of change in pH (acidosis or alkalosis). There are four basic processes: [[metabolic acidosis]], [[respiratory acidosis]], [[metabolic alkalosis]], and [[respiratory alkalosis]]. One or a combination may occur at any given time.

==External links==
* [http://www.acidbase.org Stewart's original text at acidbase.org]
* [http://www.anaesthesiamcq.com/AcidBaseBook/ABindex.php On-line text at AnaesthesiaMCQ.com]
* [http://www2.kumc.edu/ki/physiology/course/nine/9_1.htm Overview at kumc.edu]
* [http://www.acid-base.com/physiology.php Tutorial at acid-base.com]
* [http://www.anaesthesiamcq.com/AcidBaseBook/ABindex.php On-line Acid-Base Physiology text]
* [http://www.lakesidepress.com/pulmonary/ABG/MixedAB.htm Diagnoses at lakesidepress.com]
* [http://www.nda.ox.ac.uk/wfsa/html/u16/u1602_01.htm Interpretation at nda.ox.ac.uk]

{{Renal physiology}}
{{Human homeostasis}}

[[Category:Human homeostasis]]
[[Category:Acid-base physiology]]
[[Category:Acid-base disturbances]]
[[Category:Equilibrium chemistry]]

[[bs:Acido-Bazna Ravnoteža]]
[[de:Säure-Basen-Haushalt]]
[[el:Οξεοβασική ισορροπία]]
[[fr:Métabolisme acido-basique]]
[[it:Equilibrio acido base]]
[[ro:Echilibru acido-bazic sanguin]]
[[uk:Буферні системи]]
[[Category:2Fix]]