1 - Renal Regulation of [H+]

Question 1

Objectives: Explain renal excretion of bicarbonate

Answer 1

• Excretion during alkalosis
• Cl^- enters cell in exchange for bicarb; H+ is added to blood; bicarb is excreted in urine

Question 2

Objectives: Explain how the kidneys add new bicarbonate to the blood

Answer 2

• Secretion of H+ which combine with non-bicarbonate buffers (phosphate)
• Catabolism of Glutamine (+ excretion of ammonium in urine)

Question 3

Objectives: Explain homeostatic control of renal acid-base compensation

Answer 3

• Glutamine Metabolism and NH₄+ excretion are increased during acidosis and decreased during alkalosis
• Tubular Hydrogen Ion Secretion:
  
  • Increased by increased P_CO2 of respiratory acidosis, and decreased by decreased P_CO2 of respiratory alkalosis
  • Increased by local effects of low extracellular pH on the tubules; and vise versa

Question 4

What are the four sources of Hydrogen Ion Gain?

Answer 4

• Generation of H+ from CO2 in tissue
• Production of nonvolatile acids from metabo;ism of protein
• Gain of H+ from loss of bicarbonate in diarrhea or other nongastric GI fluids
• Gain of H+ from loss of bicarbonate in urine

Question 5

What are four sources for hydrogen loss?

Answer 5

• Recombination of H+ and bicarbonate in pulmonary capillaries
• Utilization of H+ in metabolism of organic ions
• Loss of H+ in vomit (stomach ACID … HCl)
• Loss of H+ in urine

Question 6

How does the body generate H+ from CO₂?

Answer 6

CA

CO₂ + H₂O ↔ H₂CO₃ ↔ HCO₃^- + H⁺

• CO2 combines with H20 to yield bicarbonate and hydrogen ions
• H+ generation occurs by oxidative metabolism during passage of blood through tissues

Question 7

Hypoventilation vs Hyperventilation

Answer 7

• Hypoventilation - Net gain of H+
• Hyperventilation - Net loss of H+

Question 8

How does the body produce nonvolatile (Fixed) Acids?

Answer 8

• Oxidation of sulfhydryl groups (Cystine, Methionine) to Sulfuric Acid
• Hydrolysis of phosphoesters (phosphoric acid)
• Incomplete breakdown of neutral carbohydrates, fats, proteins

Question 9

How is H+ affected by gastrointestinal secretions?

Vomit

Diarrhea

Answer 9

• Vomit: Loss of H+ (Stomach Acid = H-Cl)
• Diarrhea: Loss of Bicarbonate (Gain of H+)

“I’m shitting out bicarbonate”

Question 10

Explain the difference between extracellular and intracellular buffers

Answer 10

• Intracellular: Phosphates / Proteins
• Extracellular: CO₂ / HCO₃^- System
  
  • PCO2 Regulated by Lungs
  • HCO3 Regulated by Kidneys
    
    • ​Can remove OR add bicarb

Question 11

How do the kidneys respond to alkalosis and acidosis?

Answer 11

• Alkalosis: Excrete Bicarbonate, raising plasma H+ (lowering pH)
• Acidosis: Add Bicarbonate to blood, reducing H+ (increase pH)

Question 12

How is Bicarbonate regulated?

Conservation

Excretion

Replenishment

Answer 12

• Conservation: Freely filtered and mostly reabsorbed indirectly
  
  • CA is both embedded in membrane, and found in cells; combines with H+ to enter cells as CO2 and H2O
  • Diuretics can inhibit CA here
• Excretion
• Replenishment: Acid Excretion - Two Mechanisms
  
  • Secretion of H+ combine with non-bicarb buffers from filtration (phosphate)
  • Catabolism of Glutamine with excretion of ammonium in urine

Question 13

Explain Quantitation of Net Acid Excretion (NAE)

Answer 13

• NAE = Net Bicarbonate gain or loss from body
• NAE = (urinary ammonium+titratable acid-urinary bicarbonate)
  
  • Titratable Acid = H-Phosphate Buffer
  • Negative NAE = Loss of Bicarb (alkalosis)
  • Positive NAE = Gain of Bicarb (normal western diet, acidosis)