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ACID BASE BALANCE Dr.Mohammed Sharique Ahmed Quadri Assistant professor physiology Al Amaarefa College 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. 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 pH 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 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 The body produces more acids than bases Sources of H+ in the body • Acids taken with foods. • Carbonic acid formation CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3• Inorganic acids produced during breakdown of nutrients( sulfuric acid & phosphoric acid ) • Organic acids resulting from intermediary metabolism( lactic acid) 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. 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. Buffer Systems in the Body Bicarbonate: most important ECF buffer H2O + CO2 H2CO3 H+ + HCO3Phosphate: 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 Respiratory System Second line of defense again changes in pH Acts at a moderate speed Regulates pH by controlling rate of CO2 removal 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 • Ammonia secretion H+ secretion In luminal membrane H+ ATPase pump Na – H + Antiporter Mechanism of HCO3- Reabsorption and Na+ - H+ Exchange Downloaded from: StudentConsult (on 14 March 2009 12:20 PM) © 2005 Elsevier H+ secretion in Distal & Collecting Tubule H+ secretion and excretion couples with addition of HCO3 to plasma 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 Control of rate of tubular secretion & H+ reabsorption 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+ NH3 + H+ H2PO4-2 NH4+ Buffering of Secreted H+ by Filtered phosphate (NaHPO4-) and Generation of “New” HCO3- “New” HCO3- Production and Secretion of NH4+ and HCO3- by Proximal, Thick Loop of Henle, and Distal Tubules H++NH3 “New” HCO3- 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 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 Respiratory Acidosis Compensations Chemical buffers immediately take up additional H+ Kidneys are most important in compensating for respiratory acidosis 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 Respiratory Alkalosis Compensations Chemical buffer systems liberate H+ If situation continues a few days, kidneys compensate by conserving H+ and excreting more HCO3- 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 Metabolic Acidosis Compensations Buffers take up extra H+ Lungs blow off additional H+ generating CO2 Kidneys excrete more H+ and conserve more HCO3- Metabolic Alkalosis Reduction in plasma pH caused by relative deficiency of noncarbonic acids Causes • Vomiting • Ingestion of alkaline drugs 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 Summary of acid base abnormalities 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