Lecture 5 Acid Base balance PDF

Suggested Reading: Vander 15 edition Chapter 4 Ion Balance in Body Homeostasis in Body Fluids pH Review pH = - log [H+] H+ is really a proton Range is from 0 – 14 (can be 14!!) If [H+] is high, the solution is acidic; pH < 7 If [H+] is low, the solution is basic or alkaline ; pH...

Suggested Reading: Vander 15 edition Chapter 4 Ion Balance in Body Homeostasis in Body Fluids pH Review pH = - log [H+] H+ is really a proton Range is from 0 – 14 (can be 14!!) If [H+] is high, the solution is acidic; pH < 7 If [H+] is low, the solution is basic or alkaline ; pH > 7 © 2018 Pearson Education, Inc. pH Considerations in Physiology Small changes in pH can produce major disturbances Most enzymes function only with narrow pH ranges Acid-base balance can also affect electrolytes (Na +, K+, Cl-) Can also affect hormones The body produces more acids than bases Acids taken in with foods Acids produced by metabolism of lipids and proteins Cellular metabolism produces CO2. CO2 + H20 H2CO3 H+ + HCO3- Buffer Systems – How they work HCl Addition of 3 HCl to Cl– unbuffered H+ solution Cl– Na+ Na+ Unbuffered NaCl solution 3 free H+ present (a) Addition of HCl to an unbuffered solution HCl Addition of 3 HCl to Cl – buffered H2CO3 solution – – HCO3 HCO3 H+ H2CO3 Na+ Na+ Solution containing 1 free H+ present – H2CO3: HCO3 buffer (b) Addition of HCl to a buffered solution Control of Acids by buffers Take up H+ or release H+ as conditions change Buffer pairs – weak acid and a base Exchange a strong acid or base for a weak one Results in a much smaller pH change Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3) Maintain a 20:1 ratio : HCO3- : H2CO3 HCl + NaHCO3 H2CO3 + NaCl NaOH + H2CO3 NaHCO3 + H2O Phosphate buffer - Major intracellular buffer H+ + HPO42- H2PO4- OH- + H2PO4- H2O + H2PO42-  Henderson–Hasselbalch equation -deriving pH in biological systems Strong acid: High Ka, Low pKa. Weak acid: Low Ka,High pKa. pKa of carbonic acid = 6.1 pCO2 = 5% CO2 Atmosphere = 5% of 760 mmHg (atmospheric pressure at sea level) = 38 mmHg Also take solubility of CO 2 (Only dissolved gas exerts partial pressure) Solubility product for CO 2 = 0.03 Henderson–Hasselbalch equation and Arterial Blood Gases (ABGs) pKa of carbonic acid = 6.1 Arterial blood: pH ?? (normal, pH 7.4) Solubility product for CO2 = 0.03 PCO2 40 mm Hg Venous blood: HCO3- 24 mEq/L pH = 6.1 + log (24/(0.03 X 40)) pH = 6.1 + log (20) Remember ratio HCO3-:H2CO3 pH = 6.1 + 1.301 pH = 7.4 Buffer Systems in the Body HCO3- should also be here 1. Protein Buffers Includes hemoglobin, work in blood and ICF Carboxyl group gives up H+ (-COOH->-COO- + H+) Amino Group accepts H+ Side chains that can buffer H+ are present on amino acids. Buffers function almost instantaneously 2. Respiratory mechanisms Exhalation of carbon dioxide Powerful, but only works with volatile acids Doesn’t affect fixed acids like lactic acid CO2 + H20 H2CO3 H+ + HCO3- Body pH can be adjusted by changing rate and depth of breathing Respiratory mechanisms take several minutes to hours  The Basic Relationship between PCO2 and Blood pH. PCO2 pH 35– 45 HOMEOSTASIS 7.35 – 7.45 mm Hg If PCO2 increases If PCO2 decreases H2O + CO2 H2CO3 H+ + HCO3– H+ + HCO3– H2CO3 H2O + CO2 When blood carbon dioxide increases, more carbonic acid forms, When blood carbon dioxide decreases, carbonic acid dissociates additional hydrogen ions and bicarbonate ions are released, and into carbon dioxide and water. This removes hydrogen ions from the pH decreases. solution and increases the pH. 3. Kidney excretion Can eliminate large amounts of acid Can also excrete base Can conserve and produce bicarbonate ions Most effective regulator of pH If kidneys fail, pH balance fails Renal mechanisms may take several hours to days Acid-Base Imbalances pH< 7.35 acidosis pH > 7.45 alkalosis The body response to acid-base imbalance is called compensation May be complete if brought back within normal limits Partial compensation if range is still outside norms. If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation. If problem is respiratory, renal mechanisms can bring about metabolic compensation. Renal Control of Acid-Base Balance The kidneys control acid-base balance by excreting either acidic or basic urine Excreting acidic urine reduces the amount of acid in extracellular fluid Excreting basic urine removes base from the extracellular fluid The kidneys regulate extracellular fluid H+ concentration through three fundamental mechanisms: 1. secretion of H+ 2. reabsorption of filtered HCO3- 3. production of new HCO3- In Acidosis – Control of the rate of kidney tubular H+ secretion and HCO3- reabsorption The Kidneys Adjust Their Rates of Excretion The Kidneys Adjust Their Rate of H+ Excretion By varying the extent of H + secretion Mechanism of renal H+ secretion in the proximal tubule H+ ATPase pumps and Na+-H+ antiporters Mechanism of renal H+ secretion in the distal and collecting tubules Type A and B intercalated cells The Kidneys Conserve or Excrete HCO 3- Depending on the plasma [H+] Coupling of HCO3- reabsorption with H+ secretion Renal handling of H+ during acidosis and alkalosis Renal handling of HCO3- during acidosis and alkalosis Renal Correction of Acidosis -Increased Excretion of H+ and Addition of HCO3- to the ECF Acidosis decreases the ratio of HCO3-/H+ ion in Renal Tubular Fluid As a result, there is excess H+ in the renal tubules, causing complete reabsorption of HCO3- and still leaving additional H+ available to combine with the urinary buffers (phosphate and ammonia) Thus, in acidosis, the kidneys reabsorb all the filtered HCO3- and contribute new HCO3- through the formation of ammonium ions and titratable acid Renal Correction of Alkalosis In metabolic alkalosis, there is an increase in plasma pH and decrease in H+ concentration. The cause of metabolic alkalosis is a rise in the extracellular fluid HCO3- Partly compensated by a reduction in the respiration rate, which increases PCO2 and helps return the extracellular fluid pH toward normal Decreased tubular secretion of H+ and increased excretion of HCO3- Alkalosis increases the ratio of HCO3-/H+ in renal tubular fluid, excess HCO3- in the tubular fluid fails to be reabsorbed because there is no H+ to react with and it is excreted in the urine In metabolic alkalosis, the primary compensations are decreased ventilation, which raises PCO2, and increased renal excretion of HCO3- which helps to compensate for the initial rise in extracellular fluid HCO3- increase in HCO3-concentration in the extracellular fluid leads to an increase in the filtered load of HCO3- which in turn causes an excess of HCO3- over H+ secreted in the renal tubular fluid Alkalosis - HCO3- secretion coupled with H + reabsorption Responses to Metabolic Acidosis Regulation of Respiratory Acidosis or Alkalosis