Acid Base

Question 1

name 3 reasons why HCO3/CO2 is a better buffer system than H2PO4/HPO4

Answer 1

• [HCO3] is higher (24 mEq) than HPO4 and can be adjusted by kidney
• CO2, the acid form of the buffer, is volatile and can be expired by the lungs

Question 2

describe the HCO3/PCO2 ratio

Answer 2

• for pH = 7.4, the HCO3/PCO2 ratio must = 20
  
  • 24/ (.03)(40) = 20

Question 3

describe renal reabsorption of HCO3

Answer 3

• ~80-90% of HCO3 is reabsorbed (H+ secretion) in the prox. tubule
  
  • not making new bicarb, just reabsorbing what was filtered

Question 4

describe NH4 excretion

Answer 4

• prox. tubule
  
  • glutamine metabolism will give 2 NH3 and 1 alpha-ketoglutarate
    
    • AKG -> 2 CO2 + H2O + CA = HCO3 + H⁺
• thick ascending
  
  • NH4+ reabsorbed by cotransport for K+; secreted into CD for excretion
• CD
  
  • secretion of H+ via antiport will form 1 new HCO3-
    
    • H+ joins with NH3 and forms NH4+ which is not lipid soluble and is trapped in the lumen
  • NH3 is lipidophilic and can move freely bt cells

Question 5

describe a titratable acid

Answer 5

• H+ excreted with urinary buffers (phosphate)
• phosphate is most important buffering system in this category bc concentrated in tubules and pK = 6.8
• 85% of HPO4 filtered is reabsorbed leaving 15% to be excreted as titratable acid in H2PO4 form

Question 6

describe K/H interactions

Answer 6

• acidosis
  
  • increase in plasma [H+] therefore H+ moves into the cells
  • intracellular K+ moves into blood = hyperkalemia
• alkalosis
  
  • decrease in plasma K+ (hypokalemia) therefore H+ replaces K+ in cells
  • increased intracellular [H+] leads to H+ secretion into the urine

Question 7

describe alkalosis begets alkalosis

Answer 7

Question 8

describe base excess/deficit

Answer 8

• base deficit = amount of strong base that must be added
• [buffer base] in arterial blood = 48 mEq/L

Question 9

describe salicylate poisoning

Answer 9

• phase 1 (12 hours): hyperventilation resulting from direct resp. center stimulation, leading to resp. alkalosis
  
  • K+ and HCO3 are excreted in the urine
• phase 2 (12-24 hours): paradoxic aciduria - continued resp. alkalosis w/ sufficient K being lost from the kidneys
• phase 3: dehydration, hypokalemia and progressive metabolic acidosis caused by lactic acid production