Acid-Base Balance

 Blood - normal pH of 7.2 – 7.45  7.45 = alkalosis  3 buffer systems to maintain normal blood pH 1. Buffers 2. Removal of CO 2 by lungs 3. Removal of H + ions by kidneys

The Negative logarithmic relationship between [H + ] and pH

 Acids are H + donors.  Bases are H + acceptors, or give up OH - in solution.  Acids and bases can be: Strong – dissociate completely in solution  HCl, NaOH Weak – dissociate only partially in solution  Lactic acid, carbonic acid

The Body and pH  Homeostasis of pH is tightly controlled  Extracellular fluid = 7.4  Blood = 7.35 – 7.45  8.0 death occurs  Acidosis (acidemia) below 7.35  Alkalosis (alkalemia) above 7.45

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 take in with foods  Acids produced by metabolism of lipids and proteins  Cellular metabolism produces CO 2.  CO 2 + H 2 0 ↔ H 2 CO 3 ↔ H + + HCO 3 -

Control of Acids 1. Buffer systems 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

Bicarbonate buffer  Sodium Bicarbonate (NaHCO 3 ) and carbonic acid (H 2 CO 3 )  Maintain a 20:1 ratio : HCO 3 - : H 2 CO 3 HCl + NaHCO 3 ↔ H 2 CO 3 + NaCl NaOH + H 2 CO 3 ↔ NaHCO 3 + H 2 O

Phosphate buffer  Major intracellular buffer  H + + HPO 4 2- ↔ H 2 PO4 -  OH - + H 2 PO 4 - ↔ H 2 O + H 2 PO 4 2-

Protein Buffers  Includes hemoglobin, work in blood and ISF  Carboxyl group gives up H +  Amino Group accepts H +  Side chains that can buffer H + are present on 27 amino acids.

2. Respiratory mechanisms  Exhalation of carbon dioxide  Powerful, but only works with volatile acids  Doesn’t affect fixed acids like lactic acid  CO 2 + H 2 0 ↔ H 2 CO 3 ↔ H + + HCO 3 -  Body pH can be adjusted by changing rate and depth of breathing

3. Kidney excretion  Can eliminate large amounts of acid  Can also excrete base  Can conserve and produce bicarb ions  Most effective regulator of pH  If kidneys fail, pH balance fails

Hydrogen Ion Excretion in Kidney

Buffering of hydrogen ions in urine

Decrease H ion secretion and HCO3 ion reabsorption Increase in H ion secretion and HCO3 ion reabsorption PCO2 H, HCO3 ECF volume Angiotensin II Aldosterone HyperkalemiaHypokalemia Factors that increase or decrease H secretion and HCO3 Reabsorption by renal tubules :

Rates of correction  Buffers function almost instantaneously  Respiratory mechanisms take several minutes to hours  Renal mechanisms may take several hours to days

Buffers  Protein Buffer Systems  Amino Acid buffers  Hemoglobin buffers  Plasma Protein buffers  Phosphate Buffer Systems  Carbonic Acid – Bicarbonate Buffer System

 Buffer systems are used to keep the body in pH balance (homeostasis) It consists of a weak acid (H + )and its dissociation products (an anion) 3 major buffer systems in human  Protein buffer system (includes hemoglobin buffer system) Regulates ICF & ECF (both plasma & interstitial fluid) Most important in ICF & hemoglobin  Hemoglobin buffer system = carbonic anhydrase in RBC * it absorbs CO2 from ECF & get immediate effect Amino acids have carboxyl group (gives up H+) and Amino acids have amino group(can accept H+)

 Carbonic acid-bicarbonate buffer system Important in ECF Lots of carbon dioxide from metabolic acids It mixes with water & get carbonic acid which dissociates into H + & HCO3 - Metabolic acids have H+ ; Our body has “bicarbonate reserve”  Bicarbonate reserve = ample supply of bicarb in ECF  These combine to form CO 2 + H 2 O  CO 2 excreted via lungs Think of CO 2 as an acid since it readily combines with water to become carbonic acid

Maintenance of Acid-Base Balance  Respiratory System: removal of CO 2 by lungs – stabilizes the ECF, has direct effect on Carbonic Acid – Bicarbonate Buffer System  Urinary System: removal of H + ions by kidneys

Regulation of blood pH by the respiratory system

Kidney excretion of H +  Metabolic reactions produce nonvolatile acids  One way to eliminate this huge load is to excrete H + in urine  In the proximal convoluted tubule, Na + /H + antiporters secrete H + as they reabsorb Na +  Intercalated cells of collecting duct include proton pumps that secrete H + into tubule fluid; reabsorb K + and HCO 3 -  Urine can be up to 1000 times more acidic than blood  2 other buffers can combine with H + in collecting duct HPO 4 2- and NH 3

Secretion of H + by intercalated cells in the collecting duct