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*Increases indicate an alkalaemia.
*Increases indicate an alkalaemia.
*Decreases indicate an acidemia.
*Decreases indicate an acidemia.
==Acid / Base==
It is essential that 50-100meq of acid is excreted by the kidneys every day. This is achieved by secretion of H<sup>+</sup> in two regions of the nephron, the proximal tubule and the collecting ducts, and is essential for maintaining the acid base ratio, within the body, at the correct levels. If there is a net gain or loss of H<sup>+</sup> within the body then the kidneys will compensate for it. The H<sup>+</sup> ions cannot be secreted as free ions, however virtually all filtered HCO<sub>3</sub><sup>-</sup> must be reabsorbed. The result is that the H<sup>+</sup> ions bind to other filtered buffers which are not fully reabsorbed such as ammonia or phosphate. Extracellular pH is th main physiological regulator affecting how much acid is secreted. In pathological states circulating volume, aldosterone and plasma potassium affect it.
===The Role of the Kidneys in Acid Base===
[[Image:buffereg1ap.jpg|right|thumb|200px|<small><center>The HPO<sub>4</sub><sup>-</sup>Buffer System</center></small>]]
[[Image:ammoniumproxitubap.jpg|right|thumb|200px|<small><center>The Role of Ammonium in the Proximal Tubule</center></small>]]
[[Image:ammoniumcollductap.jpg|right|thumb|200px|<small><center>The Role of Ammonium in the Collecting Duct</center></small>]]
The kidneys work with the respiratory system to regulate H<sup>+</sup>. Where as the respiratory systems quickly compensates for a problem it is left to the kidneys to actually remove the problem and restore a proper balance. They do this by altering the plasma concentration of HCO<sub>3</sub><sup>-</sup>.
=====Alkalosis=====
In a situation of alkalosis the kidneys allow more HCO<sub>3</sub><sup>-</sup> to be excreted. This results in an increase in an increase of un-buffered H<sup>+</sup> and thus returns the pH towards normal.
=====Acidosis=====
In a situation of increased H<sup>+</sup> levels the body is said to be in a state of acidosis and the kidneys stop excreting HCO<sub>3</sub><sup>-</sup> and the tubular cells produce more bicarbonate. This results in more H<sup>+</sup> being buffered and the pH increases back to normal
===Buffers of H<sup>+</sup> in Urine===
* Weak acids are filtered and act as buffers
* Ability depends upon pKa<sup>+</sup> and concentration
* Once all the bicarbonate has been reabsorbed the secreted H<sup>+</sup> combine with these instead.
* The H<sup>+</sup> ion is then excreted
=====The Role of Ammonium in the Proximal Tubule=====
* The body is able to excrete H<sup>+</sup> as ammonium NH<sub>4</sub><sup>+</sup>. This is very useful as:
* It adds flexibility to renal acid base regulation and can help regulate NH<sub>4</sub><sup>+</sup>
* It is ionised, fat insoluble and trapped therefore is excreted
* It is easily replaced so is quite a good method
* Under physiological control
=====The Role of Ammonium in the Collecting Ducts=====
* Very different mechanism
* No bicarbonate produced
* See diagram
===The Effect of The Buffers on Body pH===
* When H<sup>+</sup> is binding to buffers other than bicarbonate it is excreted. This means that the reabsorption of bicarbonate represents a net gain of bicarbonate not just a replacement of what was filtered. This results in an increase in plasma pH.
* Bicarbonate is also produced as part of the secretion of ammonium in the proximal tubule
===Regulation===
* H<sup>+</sup> is important for both HCO<sub>3</sub><sup>-</sup> reabsorption and generation of new HCO<sub>3</sub><sup>-</sup>
* However the rate of secretion must be carefully regulated
* There must be enough H<sup>+</sup> secreted to reabsorb all the filtered HCO<sub>3</sub><sup>-</sup>
==Oxygen==
==Oxygen==