Buffers, lungs & kidneys control pH
Alkalosis, an increased alkaline condition, is a circumstance where pH is elevated above 7.45 because the blood has lost too much hydrogen ion [H+]. One cause may be severe vomiting. Severe vomiting can result in loss of stomach acid H+, potassium ion [K+], and sodium ion [Na+] along with the stomach contents. When this happens kidney processes compensate for the loss of K+ and Na+ at the expense of H+, leading to a condition called metabolic alkalosis.
In contrast, there is a constant production of acid in body tissues and fluids due to normal physiological processes such as breaking down of protein molecules and muscle contraction. Acidosis occurs in adults when pH of body tissues and of blood in the arteries falls below 7.35.
In healthy people wide shifts in blood pH do not happen. Long term maintenance of neutral blood pH is accomplished by the kidney’s secretion of excess H+. Rapid response to both acidosis and alkalosis is managed by blood buffers and elimination of CO2 from the lungs.
Because ongoing cellular metabolic activity requires a steady local pH, the fast response to regional changes in pH is managed by buffers. Buffers are a mixture of molecules that are part weak acid and part base.
An acid is any molecule soluble in water that breaks apart upon solution such that one fragment is an H+.
The other fragment formed upon dissociation of an acid carries a negative charge similar to the OH– fragment of auto dissociated water. The negatively charged fragment of a dissociated acidic molecule is called a base.
There are many buffers used by the human body, but the most important one for maintaining neutral pH in blood is the carbonic acid – bicarbonate mixture.
Carbonic acid has the molecular formula H2CO3. Bicarbonate, a base, has the molecular formula HCO3–. A summary of reactions available to carbonic acid are shown by the reaction sequences below.
CO2 + H2O ↔ H2CO3 (carbonic acid)
H2CO3 ↔ HCO3– (bicarbonate) + H+ (hydrogen ion)
Arrows pointing forward and backward indicate that these reactions can proceed in either direction depending upon the availability of the components.
The hydrogen proton released by carbonic acid, one part of the buffering mixture, may combine with water to form hydronium ion [H3O+] making the solution more acidic. Or, it may attach itself to a basic molecule – either its own base fragment bicarbonate, or another basic molecule present in solution, or with OH–.
The second part of this buffering mixture, the base bicarbonate, is present in blood in greater quantity than carbonic acid. The HCO3– in solution that is in excess to that formed by the dissociation of carbonic acid comes from other compounds such as sodium bicarbonate, NaHCO3. Sodium bicarbonate is a salt, and it is dissolved in water much like NaCl.
Lung regulation of pH
The left side of the chemical equation from above displays another valuable characteristic of the carbonic acid buffering system. Carbonic acid can be reversibly converted to carbon dioxide and water.
CO2 + H2O ↔ H2CO3 (carbonic acid) ↔ HCO3– (bicarbonate) + H+ (hydrogen ion)
It is that part of the reaction sequence that makes this buffering system particularly flexible.
Because all these reactions are reversible, removing CO2 from the left side of the equation – as when CO2 is released from the lung – causes reformation of H2CO3 from HCO3– and H+ reducing the level of acid H+ in blood.
Increasing CO2 in the blood due to energy demands of tissue such as contracting muscle pushes the reaction in the opposite direction. Carbon dioxide and water combine to form carbonic acid. Part of the carbonic acid dissociates into bicarbonate and H+.
This decreases blood pH and serves as a signal to central control centers to increase the rate of breathing. Increased breathing causes release of more carbon dioxide from the lung. As carbon dioxide is released into air, blood pH increases again toward neutral because the carbonic acid reactions proceed in the reverse order and breathing rate returns to baseline.
Also, if you are interested in learning more about the physiological response to acidosis and alkalosis in a clinical setting check out Normal Acid-Base Regulation, a YouTube video created by Eric Strong MD, Stanford University.
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Margaret Thompson Reece PhD, physiologist, former Senior Scientist and Laboratory Director at academic medical centers in California, New York and Massachusetts and CSO at Serometrix LLC is now CEO at Reece Biomedical Consulting LLC.
Dr. Reece is passionate about helping students, online and in person, pursue careers in life sciences. Her books “Physiology: Custom-Designed Chemistry” (2012), “Inside the Closed World of the Brain” (2015) and the workbook (2017) companion to her online course “30-Day Challenge: Craft Your Plan for Learning Physiology” are written for those new to life science. More about her books can be found at amazon/author/margaretreece.
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