Role of Kidneys in Acid/base Balance

Question 1

What kind of acid is excreted by the lungs?

What kind of acid is excreted by the kidneys?

Answer 1

• Lungs excrete volatile acids (CO₂)
• Kidney’s excrete nonvolatile acids
  
  • Proteins -> Sulfuric Acid (H₂SO₄)
  • Nucleic acids -> Phosphaturic acid (H₃PO₄)

Question 2

Bicarbonate Buffering System (BBS)

1. What does it do?
2. What is the chemical equation for it?
3. What are the conditions for optimal function of the BBS?

Answer 2

1. Buffers H⁺ and eliminates it from the body (other buffers like albumin just bind protons but do not eliminate it from the body)
2. H⁺+ HCO₃^- => H₂CO₃ => CO₂ + H₂O (removed by lungs via expiration)
3. A low P_CO2
   
   • Drives the eq. to the right
   • Acidemia (low blood pH) stimulates ventilation which lowers P_CO2

Question 3

What 3 functions does the kidney have in maintaining acid balance?

Answer 3

1. Eliminate acid anions
   
   • HSO₄ & H₂PO₄
   • Filtered by glomerulus and excreted
2. Reabsorb all of the filtered bicarbonate
   
   • Freely filtered by the glomerulus
   • Avidly reabsorbed
     
     • Vast majority in proximal tubule
3. Synthesize/generate new bicarbonate
   
   • HCO₃^- is continuously consumed via H⁺ buffering
   • Primarily in the intercalated cells of distal tubule

Question 4

T/F: The process of proximal bicarb reabsorption itself results in a net decrease of H⁺

Answer 4

• FALSE
  
  • Proximal bicarb reabsorption only results in secretion of bicarb into the blood from the lumen of the proximal tubule, there is no net change in H⁺

Question 5

What would result if proximal bicarb reabsoprtion is impaired?

Answer 5

• An acidosis would occur

Question 6

For every H⁺ that is secreted/excreted, how many HCO₃^- are generated/reabsorbed?

Answer 6

• 1 H⁺:1 HCO₃^-

Question 7

What are the net results of distal tubule bicarb synthesis?

Answer 7

• Excretion of H⁺
• Reabsorption of HCO₃^-

*Notice there is a net excretion of H⁺ in bicarb synthesis vs in bicarb rebasorption in the proximal tubule there is no net change in H⁺

Question 8

T/F: Bicarbonate synthesis is taking place constantly, 24/7

Answer 8

• FALSE
  
  • No bicarbonate synthesis can take place until bicarbonate reabsoprtion is complete
    
    • As long as there is HCO₃^- in the distal tubule lumen, bicarb will be reabsorbed, but not be synthesized

Question 9

What are the two ways urine is buffered?

Answer 9

• Titratable acid
  
  • The acid anions that are excreted by the kidney (remember this was one of the main roles) binds back protons in the urine and neutralizes the proton
    
    • E.g. HPO₄^2- => HPO₄^2- + H⁺ => H₂PO₄^- => excretion
• Ammonia trapping
  
  • NH₃ diffuses easily through the apical membrane
  • NH₃ + H⁺ => NH₄⁺
  • NH₄⁺ is impermeable to apical membrane (trapped)

Question 10

Where does ammoniagenesis occur and how?

Answer 10

Ammoniagenesis

• Occurs in proximal tubule cells
• Glutamine metabolized to NH₃ and bicarb
  
  • NH₃ is secreted into lumen, binds to H⁺, and is excreted as NH₄⁺

Question 11

What changes are made to your net acid excretion in chronic metabolic acidosis

Answer 11

Chronic Metabolic acidosis

• NH₄⁺ excretion increases
• Titratable acid excretion unchanged
• Bicarb excretion remains zero

Question 12

What changes are made to your net acid excretion in chronic metabolic alkylosis?

Answer 12

Chronic Metabolic Alkylosis

• Bicarb excretion increased
  
  • Up to 80 mmol/day
  • Decreased reabsoprtion
• NH₄⁺ and titrat acid excretion decrease

Question 13

How does hypokalemia effect acid-base balance?

Answer 13

Hypokalemia

• K⁺ shifts out of the cells into the ECF
• K⁺ Exchanges for H⁺ which shifts into the cell
  
  • more intracellular H⁺ available in renal tubules for secretion (mass effect)
  • Increased ammoniagenesis => more H⁺ trapping and excretion
• Intercalated cells will preferentially reabsorb K⁺ & secrete H⁺
• Hypokalemia predisposes to metabolic alkalosis

Question 14

How does hyperkalemia effect acid-base balance?

Answer 14

Hyperkalemia

• K⁺ shifts into the cells from the ECF
• K⁺ exchanges for H⁺ which shift out of cells
  
  • Less intracellular H⁺available in renal tubules for secreteion
  • Decreased ammoniageneisis => less H⁺ trapping and excretion
• Hyperkalemia predisposes to metabolic acidosis
• BUT, hyperkalemia also stimulates aldosterone
  
  • Aldosterone increases H⁺ secretion and HCO₃^- synthesis
  • Counteractive effect