SM 200a/201a - Acid Base

Question 1

What is the effect of angiotensin II on HCO3- reabsorption and acid excretion?

Answer 1

Angiotensin II stimulates bicarbonate reabsorption and acid excretion

Angiotensin II -> kidney works to increase pH

(Angiotensin II is secreted in response to signals that indicate low volume; low volume -> hypoperfusion -> lactic acidosis -> decreased serum pH; kidney reacts by retaining bicarb and excreting acid)

Question 2

What is citrate?

What is its role in acid-base balance?

Answer 2

Citrate is an organic anion

Citrate reabsorption is equivalent to base retention

• Main urinary base
  
  • Reabsorbed and converted to CO2 and H2O
  • This consumes H+ and generates OH-
• Chelates Ca2+
  
  • Prevents precipitation with phosphate and oxalate ->
• *prevents kidney stones**

Question 3

What is the differential for anion-gap metabolic acidosis?

Answer 3

MUDPILES: Acid add-on state

• Methanol
• Uremia
• Diabetic ketoacidosis (any ketoacidosis)
• Phenformin, paracetamol/acetaminophen, paraldehyde
• Iron, Isoniazid, Inborn errors of metabolism
• Lactic acidosis
• Ethanol/Ethylene glycol
• Saliclates/ASA/Aspirin

Question 4

Where in the kidney tubule is NH4+ reabsorbed?

Answer 4

Loop of Henle

Question 5

Why might does amiloride cause metabolic acidosis?

Answer 5

Amiloride blocks Na+ reabsorption in the collecting duct

• Decreased Na+ reabsorption
• -> Decreased Na+/H+ exchange
• -> Decreased H+ secretion
• -> Metabolic acidosis

Question 6

Describe the transport of NH4+ (ammonia) in the kidney tubule

Answer 6

• Glutamine –> NH₄⁺ in the proximal tubule
  
  • Glutamine into PCT epithelial cell through the basolateral membrane
  • Glutamine –> NH₄⁺ –> NH₃ + H⁺
  • H⁺ is secreted into the tubule via H+/Na+ exchanger
  • NH₃ diffuses into the tubule
  • In the tubule, NH₃ + H⁺ –> NH₄⁺
• NH₄⁺ is reabsorbed in the Loop of Henle
  
  • ​NH₄⁺/K⁺ exchanger
  • Na+/NH₄⁺/Na⁺ exchanger
  • NH₄⁺ channel
• NH₃ diffuses into the medullary collecting duct, binds to H+ and is trapped as NH₄⁺
  
  • ​Acid (H+) secreted by collecting duct epithelial cells binds to NH₃ and becomes trapped as NH₄⁺
  • This is how the body secretes acid while retaining bicarb

Question 7

What is the differential for non-anion gap (hyperchloremic) metabolic acidosis?

Answer 7

USED CARS: Bicarb loss

• Uretrosigmoidostomy
• Saline administration (NaCl)
  
  • In the face of renal dysfunction
• Endocrine (addison’s), Ethanol
• Diarrhea
• Carbonic anhydrase inhibitors
• Ammonium chloride
• Renal tublar acidosis
• Spironolactone

Question 8

What are the major causes of metabolic alkalosis?

Answer 8

• Loss of H+ ions
  
  • GI
    
    • Loss of gastric acid secretions
      
      • Vomiting, NG tube
  • Renal
    
    • Loop or thiazide diuretics
      
      • More Na+ delivery to cortical collecting duct stimulates principal cells to reabsorb more Na+
      • This creates a favorble charge gradient for alpha intercalated cells to secrete H+
    • Mineralcorticoid excess
      
      • Aldosterone stimulates alpha intercalated cells to reabsorb bicarb and secrete H+
    • Bartter and Gitelman syndromes
• Retention of administered bicarbonate
  
  • Milk Alkali syndrome
  • Administration of NaHCO3

Question 9

What are the 4 general mechanisms of metabolic acidosis
(a low HCO3 state)?

Answer 9

• Loss of HCO3- externally
  
  • Diarrhea
  • Proximal RTA
• Failure of the kidneys to excrete acid
  
  • Distal RTA
  • CKD
  • Acid retained in the body uses up bicarb
• Addition of H+ which titrates HCO3-
  
  • Drinking something with H+
• H+ buildup in the circulation
  
  • Lactic acidosis
  • Ketoacidosis

Question 10

How do you treat metabolic alkalosis?

Answer 10

Cl- administration

NaCl usually

KCl if the patient is hypokalemic

Question 11

Based on pCO2 and HCO3- levels, how can we tell if an acid/base process is appropriately compensated?

Metabolic acidosis:

Metabolic alkalosis:

Chronic respiratory acidosis:

Chronic respiratory alkalosis:

Answer 11

Metabolic

• Acidosis: pCO2 and HCO3- decrease about the same amount
  
  • ΔpCO2 = 1.2 * ΔHCO3-
  • Metabolic acidosis = Decreased pH, decreased pCO2
  • Bicarb will be used up to neutralize extra acid?
• Alkalosis: pCO2 inreases about twice as much as HCO3-
  
  • ΔpCO2 = 0.6 * ΔHCO3-
  • Metabolic alkalosis = Increased pH, increased pCO2
  • Bicarb will be produced

Respiratory

• Acidosis: Increase in HCO3- is about twice as much as increase in pCO2
  
  • ΔHCO3- = 0.4 * ΔpCO2
  • Respiratory acidosis = Decreased pH, increased pCO2
  • Kidney will reclaim bicarb to compensate for low pH
• Alkalosis: Decrease in HCO3- is about twice as much as decrease in pCO2
  
  • ΔHCO3- = 0.5 * ΔpCO2
  • Respiratory alkylosis = Increased pH, decreased pCO2
  • Kidney will excrete bicarb to compensate for high pH