Transcript Chapter_030
Chapter 30 Acid-Base Balance Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 1 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 2 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 • Acetoacetic acid • Beta-hydroxybutyric acid Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 3 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 • Phosphate buffer system • Protein buffer system Physiological—delayed action buffers • Respiratory response • Renal response Summary of pH control mechanisms • Buffers • Respiration • 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 4 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 5 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 • HCO3/H2CO3=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 6 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 7 Respiratory Mechanism of pH Control Some principles relating respirations and pH of body fluids Acidosis → hyperventilation ↓ increases elimination of CO2 ↓ decreases blood CO2 ↓ decreases blood H2CO3 ↓ 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 8 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 CO2 and therefore blood H2CO3 and H+ Prolonged hypoventilation, by eliminating too little CO2, causes increase in blood H2CO3 and consequently in blood H+, thereby may produce acidosis Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 9 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 10 Urinary Mechanisms of pH Control Mechanisms that control urine pH Secretion of H+ into urine—when blood CO2, H2CO3, 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 NH3—when blood hydrogen ion concentration increases, distal tubules secrete more NH3, 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 Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 11