Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 1 Chapter 30 Acid-Base Balance

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 2 Introduction  Acid-base balance is one of the most important of the body’s homeostatic mechanisms  Acid-base balance refers to regulation of hydrogen ion concentration in body fluids  Precise regulation of pH at the cellular level is necessary for survival  Slight pH changes have dramatic effects on cellular metabolism

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 3 Mechanisms That Control pH of Body Fluids  Meaning of pH—negative logarithm of hydrogen ion concentration of solution (Figure 30-1)  Sources of pH-influencing elements  Carbonic acid—formed by aerobic glucose metabolism  Lactic acid—formed by anaerobic glucose metabolism  Sulfuric acid—formed by oxidation of sulfur-containing amino acids  Phosphoric acid—formed in breakdown of phosphoproteins and ribonucleotides  Acidic ketone bodies—formed in breakdown of fats Acetone Acetone Acetoacetic acid Acetoacetic acid Beta-hydroxybutyric acid Beta-hydroxybutyric acid

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 4 Mechanisms That Control pH of Body Fluids  Acid-forming potential of foods—determined by chloride, sulfur, and phosphorus content  Types of pH control mechanisms  Chemical—rapid action buffers Bicarbonate buffer system Bicarbonate buffer system Phosphate buffer system Phosphate buffer system Protein buffer system Protein buffer system  Physiological—delayed action buffers Respiratory response Respiratory response Renal response Renal response  Summary of pH control mechanisms Buffers Buffers Respiration Respiration Kidney excretion of acids and bases Kidney excretion of acids and bases  Effectiveness of pH control mechanisms; range of pH— extremely effective, normally maintain pH within very narrow range of 7.36 to 7.40

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 5 Mechanisms That Control pH of Body Fluids  Buffers defined  Substances that prevent marked change in pH of solution when an acid or base is added to it  Consist of weak acid (or its acid salt) and basic salt of that acid  Buffer pairs present in body fluids—mainly carbonic acid, proteins, hemoglobin, acid phosphate, and sodium and potassium salts of these weak acids

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 6 Buffer Mechanisms for Controlling pH of Body Fluids  Action of buffers to prevent marked changes in pH of body fluids  Nonvolatile acids, such as hydrochloric acid, lactic acid, and ketone bodies, buffered mainly by sodium bicarbonate  Volatile acids, chiefly carbonic acid, buffered mainly by potassium salts of hemoglobin and oxyhemoglobin (Figure 30-5)  Chloride shift makes it possible for carbonic acid to be buffered in red blood cell and then carried as bicarbonate in plasma (Figure 30-6)  Bases buffered mainly by carbonic acid (when homeostasis of pH at 7.4 exists) Ratio B HCO 3 /H 2 CO 3 =20/1  Evaluation of role of buffers in pH control—cannot maintain normal pH without adequate functioning of respiratory and urinary pH control mechanisms

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 7 Respiratory Mechanism of pH Control  Explanation of mechanism  Amount of blood carbon dioxide directly relates to amount of carbonic acid and therefore to concentration of H +  With increased respirations, less carbon dioxide remains in blood, hence less carbonic acid and fewer H + ; with decreased respirations, more carbon dioxide remains in blood, hence more carbonic acid and more H +  Adjustment of respirations to pH of arterial blood

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 8 Respiratory Mechanism of pH Control  Some principles relating respirations and pH of body fluids Acidosis → hyperventilation ↓ increases elimination of CO 2 ↓ decreases blood CO 2 ↓ decreases blood H 2 CO 3 ↓ decreases blood H+; that is, increases blood pH ↓ tends to correct acidosis; that is, to restore normal pH

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 9 Respiratory Mechanism of pH Control  Some principles relating respirations and pH of body fluids (cont.)  Prolonged hyperventilation, by decreasing blood H + excessively, may produce alkalosis  Alkalosis causes hypoventilation, which tends to correct alkalosis by increasing blood CO 2 and therefore blood H 2 CO 3 and H +  Prolonged hypoventilation, by eliminating too little CO 2, causes increase in blood H 2 CO 3 and consequently in blood H+, thereby may produce acidosis

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 10 Urinary Mechanisms of pH Control  General principles about mechanism—plays vital role in acid-base balance because kidneys can eliminate more H + from body while reabsorbing more base when pH tends toward acid side, and eliminates fewer H + while reabsorbing less base when pH tends toward alkaline side

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 11 Urinary Mechanisms of pH Control  Mechanisms that control urine pH  Secretion of H + into urine—when blood CO 2, H 2 CO 3, and H + increase above normal, distal tubules secrete more H + into urine to displace basic ion (mainly sodium) from a urine salt and then reabsorb sodium into blood in exchange for the H + excreted  Secretion of NH 3 —when blood hydrogen ion concentration increases, distal tubules secrete more NH 3, which combines with H + of urine to form ammonium ion, which displaces basic ion (mainly sodium) from a salt; basic ion then reabsorbed back into blood in exchange for ammonium ion excreted