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ACID BASE BALANCE

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  1. ACID BASE BALANCE URINARY BLOCK 313 Dr. ShaikhMujeeb Ahmed Assistant Professor AlMaarefa College

  2. Objectives • Identify the normal range of pH values, and the upper and lower limits compatible with life. • Identify the role of kidney in regulation of acid base balance • Explains the mechanism of reabsorption of HCO3- and secretion of H+ by nephron • Describe the adjustments in filtered load and HCO3 reabsorption (H+ secretion) by alterations in systemic acid-base balance. • Describe net acid excretion by the kidneys, treatable acid, the importance of urinary buffers, and the production and excretion of ammonium.

  3. Acid-Base Balance • Refers to precise regulation of free H+ concentration in body fluids • Acids • Group of H+ containing substances that dissociate in solution to release free H+ and anions(H2CO3) • Bases • Substance that can combine with free H+ and remove it from solution(HCO3) • pH • Designation used to express the concentration of H+ • pH 7 – neutral • pH less than 7 → acidic • pH greater than 7 → basic

  4. Acid-Base Balance • Arterial pH less than 6.8 or greater than 8.0 is not compatible with life • Acidosis • Exists when blood pH falls below 7.35 • Alkalosis • Occurs when blood pH is above 7.45

  5. Acid-Base Balance • Consequences of fluctuations in pH • Changes in excitability of nerve and muscle cells • Marked influence on enzyme activity • Changes influence K+ levels in body

  6. The body produces more acids than bases • Sources of H+ in the body • Volatile acid • CO2 • Carbonic acid formation CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3- • Non volatile acids (fixed Acids) • Inorganic acids produced during breakdown of nutrients( sulfuric acid & phosphoric acid ) • Organic acids resulting from intermediary metabolism( lactic acid)

  7. Lines of Defense Against pH Changes • Chemical buffers :function almost immediately (seconds to minutes). • Respiratory mechanisms :take minutes to hours. • Renal mechanisms: may take hours to days.

  8. First line of defense Lines of Defense Against pH Changes Buffer systems do not eliminate H+ from or add them to the body but only keep them tied up until balance can be reestablished by other mechanisms.

  9. Buffer Systems in the Body Bicarbonate: most important ECF buffer Phosphate: important ICF and renal tubular buffer HPO4-- + H+ H2PO4 - Ammonia: important renal tubular buffer NH3 + H+ NH4+ Proteins: important ICF and ECF buffers Largest buffer store in the body Albumins and globulins, such as Hb H2O + CO2 H2CO3 H+ + HCO3-

  10. Importance of Buffer Systems Normal H+ concentration = 0.00004 mmol/L Amount of non-volatile acid produced ~ 80 mmol/day 80 mmol /42 L = 1.9 mmol/L = 47,500 times > normal H+ concentration

  11. Respiratory System • Second line of defense again changes in pH • Acts at a moderate speed • Regulates pH by controlling rate of CO2 removal

  12. Kidneys • Third line of defense against change in hydrogen ion concentration • Kidneys require hours to days to compensate for changes in body-fluid pH • Control pH of body fluids by adjusting • H+ excretion • HCO3- excretion/ reabsorption • Production of new HCO3- • Ammonia secretion

  13. H+ secretion • In luminal membrane • H+ ATPase pump • Na – H + Antiporter

  14. Mechanism of HCO3- Reabsorption and Na+ - H+ Exchange

  15. H+ secretion in Distal & Collecting Tubule

  16. H+ secretion and excretion couples with addition of HCO3 to plasma

  17. Renal handling of H+ during acidosis and alkalosis • Under normal circumstances proximal tubular cells and alpha intercalated cells promotes • Net H+ secretion • HCO3- reabsorption • This pattern of activity adjusted when pH deviates

  18. Control of rate of tubular secretion & H+ reabsorption

  19. Other Urinary Buffers • the minimal urine pH is about 4.5. • In order to excrete more H+, the acid must be buffered in tubular lumen. • H+ secreted into the tubular tubule and combines with HPO4-2 or NH3. • HPO4-2 + H+ H2PO4-2 • NH3 + H+ NH4+

  20. Buffering of Secreted H+ by Filtered phosphate (NaHPO4-) and Generation of “New” HCO3- “New” HCO3-

  21. H++NH3 Production and Secretion of NH4+ and HCO3- by Proximal, Thick Loop of Henle, and Distal Tubules “New” HCO3-

  22. Buffering of Hydrogen Ion Secretion by Ammonia (NH3) in the Collecting Tubules “New” HCO3- Figure 30-9; Guyton and Hall

  23. Acid-Base Imbalances • Can arise from either respiratory dysfunction or metabolic disturbances • Deviations divided into four general categories • Respiratory acidosis • Respiratory alkalosis • Metabolic acidosis • Metabolic alkalosis

  24. Respiratory Acidosis • Result of abnormal CO2 retention arising from hypoventilation • Possible causes • Lung disease • Depression of respiratory center by drugs or disease • Nerve or muscle disorders that reduce respiratory muscle activity • Holding breath

  25. Respiratory Acidosis • Compensations • Chemical buffers immediately take up additional H+ • Kidneys are most important in compensating for respiratory acidosis

  26. Respiratory Alkalosis • Primarily due to excessive loss of CO2 from body as result of hyperventilation • Possible causes • Fever • Anxiety • Aspirin poisoning • Physiologic mechanisms at high altitude

  27. Respiratory Alkalosis • Compensations • Chemical buffer systems liberate H+ • If situation continues a few days, kidneys compensate by conserving H+ and excreting more HCO3-

  28. Metabolic Acidosis • Includes all types of acidosis other than those caused by excess CO2 in body fluids • Causes • Severe diarrhea • Diabetes mellitus • Strenuous exercise • Uremic acidosis

  29. Metabolic Acidosis • Compensations • Buffers take up extra H+ • Lungs blow off additional H+ generating CO2 • Kidneys excrete more H+ and conserve more HCO3-

  30. Metabolic Alkalosis • Reduction in plasma pH caused by relative deficiency of noncarbonic acids • Causes • Vomiting • Ingestion of alkaline drugs

  31. Metabolic Alkalosis • Compensations • Chemical buffer systems immediately liberate H+ • Ventilation is reduced • If condition persists for several days, kidneys conserve H+ and excrete excess HCO3- in the urine

  32. Analysis of Simple Acid-Base Disorders Figure 30-10; Guyton and Hall

  33. Summary of acid base abnormalities

  34. References • Human physiology by Lauralee Sherwood, seventh edition • Text book physiology by Guyton &Hall,11th edition • Text book of physiology by Linda .s contanzo,third edition