Editing Homeostasis (section)

===Blood pH===
[[File:2714 Respiratory Regulation of Blood.jpg|class=skin-invert-image|border|right|364x364px]]
{{Main|Acid–base homeostasis |Acid-base imbalance}}
The [[pH#Living systems|plasma pH]] can be altered by respiratory changes in the partial pressure of carbon dioxide; or altered by metabolic changes in the [[carbonic acid]] to [[bicarbonate ion]] ratio. The [[bicarbonate buffer system]] regulates the ratio of carbonic acid  to bicarbonate to be equal to 1:20, at which ratio the blood pH is 7.4 (as explained in the [[Henderson–Hasselbalch equation]]). A change in the plasma pH gives an [[acid–base imbalance]].
In [[acid–base homeostasis]] there are two mechanisms that can help regulate the pH. [[Respiratory compensation]] a mechanism of the [[respiratory center]], adjusts the [[PCO2|partial pressure of carbon dioxide]] by changing the rate and depth of breathing, to bring the pH back to normal. The partial pressure of carbon dioxide also determines the concentration of carbonic acid, and the bicarbonate buffer system can also come into play. Renal compensation can help the bicarbonate buffer system.
The sensor for the plasma bicarbonate concentration is not known for certain. It is very probable that the renal tubular cells of the distal convoluted tubules are themselves sensitive to the pH of the plasma.{{citation needed |date=November 2017}} The metabolism of these cells produces carbon dioxide, which is rapidly converted to hydrogen and bicarbonate through the action of [[carbonic anhydrase]].<ref name=tortora1 /> When the ECF pH falls (becoming more acidic) the renal tubular cells excrete hydrogen ions into the tubular fluid to leave the body via urine. Bicarbonate ions are simultaneously secreted into the blood that decreases the carbonic acid, and consequently raises the plasma pH.<ref name=tortora1>{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |url= https://archive.org/details/principlesofan1987tort |url-access= registration |pages=[https://archive.org/details/principlesofan1987tort/page/581 581]–582, 675–676|edition=  Fifth |location= New York |publisher= Harper & Row, Publishers|date= 1987 |isbn= 978-0-06-350729-6 }}</ref>  The converse happens when the plasma pH rises above normal: bicarbonate ions are excreted into the urine, and hydrogen ions released into the plasma.

When hydrogen ions are excreted into the urine, and bicarbonate into the blood, the latter combines with the excess hydrogen ions in the plasma that stimulated the kidneys to perform this operation. The resulting reaction in the plasma is the formation of carbonic acid which is in equilibrium with the plasma partial pressure of carbon dioxide.  This is tightly regulated to ensure that there is no excessive build-up of carbonic acid or bicarbonate. The overall effect is therefore that hydrogen ions are lost in the urine when the pH of the plasma falls. The concomitant rise in the plasma bicarbonate mops up the increased hydrogen ions (caused by the fall in plasma pH) and the resulting excess carbonic acid is disposed of in the lungs as carbon dioxide. This restores the normal ratio between bicarbonate and the partial pressure of carbon dioxide and therefore the plasma pH.
The converse happens when a high plasma pH stimulates the kidneys to secrete hydrogen ions into the blood and to excrete bicarbonate into the urine.  The hydrogen ions combine with the excess bicarbonate ions in the plasma, once again forming an excess of carbonic acid which can be exhaled, as carbon dioxide, in the lungs, keeping the plasma bicarbonate ion concentration, the partial pressure of carbon dioxide and, therefore, the plasma pH, constant.