Renal Physiology Lecture 3: Acid-Base Balance Renal Contributions

Question 1

Renal contribution to pH balance

Answer 1

Kidneys modulate hydrogen/bicarbonate excretion & reabsorption

Question 2

What are the two mechanisms of renal compensation?

Answer 2

• Direct excretion or reabsorption of H+
• Indirectly alter pH by modifying the rate of HCO3- reabsorption or excretion

Question 3

Renal activity during acidosis

Answer 3

• Kidney secretes H+ into tubule lumen
• ammonia from amino acids, HPO42- are buffers in kidneys
• H+ trapped as NH4+ and H2PO4-
• Buffers increase renal excretion of H+
• New HCO3- formed during this process
  
  • Reabsorbed into blood to act as a buffer

Question 4

Renal processes in alkalosis

Answer 4

• HCO3- excreted, H+ reabsorbed to help restore pH

Question 5

Time for renal changes to occur with acidosis or alkalosis

Answer 5

Renal response to either acidosis or alkalosis takes time

Changes apparent in 24-48 hours

Question 6

What are the renal transporters in acid-base balance?

Answer 6

• Apical Na+-H+ exchanger (NHE)
• Basolateral Na+- HCO3- symporter
• H+-ATPase (proton-pump)
• H+-K+- ATPase
• Na+- NH4+ antiporter

Question 7

Function of Apical Na+-H+ exchanger (NHE)

Answer 7

Sodium into the cell, H+ moved into lumen against concentration gradient

Question 8

Function of Basolateral Na+- HCO3- symporter

Answer 8

Na+ & HCO3- moved out of the cell into the interstitium

Question 9

Function of H+-ATPase (proton-pump)

Answer 9

H+ pumped into lumen of distal nephron (distal tubule + collecting duct) against its concentration gradient

Question 10

Function of H+-K+- ATPase

Answer 10

• H+ pumped into urine in exchange for K+
  
  • Contributes to potassium imbalances accompanying acid-base disturbance

Question 11

Function of Na+- NH4+ antiporter

Answer 11

NH4+ into the lumen in exchange for Na+

Question 12

Mechanism for secretion of acid and reabsorption of buffer

Answer 12

Essentially reabsorbs most HCO3- indirectly

1. NHE secretes H+
2. H+ in filtrate combines with filtered HCO3- to form CO2
3. CO2 diffuses into cell
4. CO2 combines with water to form H+ and HCO3-
5. H+ is secreted again
6. HCO3- is reabsorbed with Na+
7. Glutamine is metabolized to ammonium ion and HCO3-
8. NH4+ is secreted and excreted

Question 13

What are the intercalated cells?

Answer 13

epithelial cells traditionally associated with the regulation of acid-base homeostasis in distal segments of the kidney tubule

• i.e. collecting duct

Question 14

Type A intercalated cells

Answer 14

• Type A: function in acidosis
  
  • H+ is excreted; HCO3- and K+ are reabsorbed

Question 15

Type B intercalated cells

Answer 15

• Type B: function in alkalosis:
  
  • HCO3- and K+ are excreted, H+ is reabsorbed