Unit 23 - Acid-Base Balance


    Acid-base balance (pH balance)  is another important homeostatic function of the body.  Alteration of pH can lead to rapid death.  This is because virtually all biochemical reactions that go on in the body are pH sensitive.  The systems which are principally responsible for pH homeostasis are the circulatory, respiratory, and urinary systems.


A.  Acids, bases, and pH - pH is a measure of the hydrogen ion concentration of a solution.  It is by definition equal to the reciprocal of the logarithm of the hydrogen ion concentration.  It is usually expressed as follows.


                       pH  =     1   

                                log (H+)


    1.  The value of pH is represented by a number from 0 to 14.  The smaller the number, the greater the hydrogen ion concentration.


    2.  Acid - Any substance that will yield  hydrogen ion when placed into solution.  Acids increase hydrogen ion and therefore decrease pH.  Any pH value less than 7 is considered to be acid.


    3.  Base - Any substance that will remove  hydrogen ion when placed into solution.  Bases decrease hydrogen ion and therefore increase pH.  Any pH value greater than 7 is considered to be basic (alkaline).


B.  Significance of pH - The tertiary structure of protein is directly dependent upon pH.  Alteration in pH causes changes in tertiary structure.  In the case of enzymes, this change results in a loss of activity and therefore the biochemical reactions catalyzed by these altered enzymes grind to a halt.  This is why pH changes in the body are so critical to life.


C.  Sources of hydrogen ion - In most cases the threat to pH homeostasis comes from excess hydrogen ion, acidity.  This is because that many of the end products of metabolism are acidic.  There are three major categories of acids in the body.


    1.  Volatile acids - This is one which can leave solution and enter the atmosphere.  Carbonic acid, formed by the reaction of carbon dioxide and water is the most important example in the body.


    2.  Fixed acids - These do not leave solution.  Sulfuric and phosphoric acids are the most important and are formed during the catabolism of amino acids and phosphate containing compounds.


    3.  Organic acids - Two major categories occur here.  Lactic acid from anaerobic metabolism and  ketone bodies from fat metabolism.  Large quantities of organic acids are formed daily, but under normal circumstances they are metabolized rapidly and do not accumulate.


    Threats to pH homeostasis from the alkaline side are less common.


D.  Mechanisms of pH maintenance


    1.  Buffering -  Buffers are substances that resist a change in pH. They function by converting strong acids or bases into weak acids or bases.  They therefore do not prohibit a change in pH but rather minimize  a change in pH.


        a.  The  principle buffers of the extracellular fluids are sodium bicarbonate and sodium phosphate.  Intracellular fluids are buffered by protein.


        b.  An example of buffering would be sodium bicarbonate buffering a strong acid, hydrochloric acid.  The details are as follows.


                     NaHCO3  =  Na+  + HC03-


                    HCl  =     H+  +  Cl-  


                         NaCl   +   H2C03


            Note that a strong acid, hydrochloric acid, is converted               into a weak acid, carbonic acid.


        c.  Intracellular buffering is accomplished largely by protein. Although protein buffers affect primarily the intracellular environment, they can help stabilized the extracellular fluid pH.  This is accomplished by transporting hydrogen ion into cells where it can be buffered by the protein.  Especially important in this regard is the Hemoglobin system found in erythrocytes.  During the conversion of carbon dioxide into carbonic acid inside of the RBC, hydrogen ion that is produced is buffered by the hemoglobin molecule.


        Buffers represent the first line of defense against pH change. The large amount of bicarbonate found in the plasma creates an effective alkali reserve to protect against a decrease in pH.


    2.  Respiratory control  - Respiratory control of pH can be exerted by controlling the levels of carbon dioxide in the plasma.  Carbon dioxide equates to hydrogen ion via the formation of carbonic acid.  If respiration rate is increased carbon dioxide will be decreased, hydrogen ion will be decreased, and pH will increase.  If respiration rate is decreased then carbon dioxide increases, hydrogen ion increases, and pH decreases.


    3.  Urinary control - The kidney is the last major means of regulating pH.  The kidney can secrete excess hydrogen ions and recover bicarbonate thereby raising the pH of the plasma.  In times of alkalosis, the kidney has the ability to excrete bicarbonate and retain hydrogen ions, thereby bringing down the plasma pH.


E.  Acidosis and alkalosis - Loss of pH homeostasis results in acidosis or alkalosis.


    1.  Normal plasma pH is 7.4.  Acidosis begins at 7.2.  A drop to 6.8 will result in death if not corrected quickly.  An increase to 7.6 results in alkalosis.


    2.  Acidosis is usually either due to metabolic or respiratory causes.  Metabolic acidosis results when more acid end product is produced than the homeostatic mechanisms can handle (as in diabetes mellitus or heavy exercise), or, when there is a failure in one of the pH maintenance mechanisms (as in kidney failure).  Another cause can be severe diarrhea.  Here large amounts of bicarbonate are lost from the digestive tract.  These molecules are normally reabsorbed and their loss results in acidification.


    3.  Metabolic alkalosis can result from the ingestion of alkaline material, loss of body acid such as occurs in acute vomiting, or reduced volumes of extracellular fluids.  When the latter occurs the kidney begins to increase sodium reabsorption in order to restore ECF volume.  This reabsorption of sodium occurs in the distal tubules in exchange for hydrogen ion.  Therefore hydrogen ion levels become quite low and alkalosis results.  Respiratory alkalosis may occur during hyperventilation.