Renal Acid-Base Regulation (7.6)

Question 1

Outline HCO₃^- reclamation

Draw ion movement

Answer 1

Question 2

Outline HCO₃^- restocking in the CD

Draw ion movement

Answer 2

No change in pH - equal H⁺ excreted and HCO₃^- reabsorbed

Question 3

Outline the net gain of HCO₃^- from ammoniagenesis

Draw ion movement

Answer 3

Question 4

Outline the basis of ammonia/diffusion trapping

Answer 4

• The ascending limb of the Loop of Henle is permeable to ammonia as NH₄⁺ uses the potassium channel of the NKCC (hence, hyperkalcaemia causes reduced secretion of ammonia, leading to acidosis due to excess H⁺)
• This allows ammonia to travel to the intersistial space where it acts to contribute to the medullary concentration, through dissociation into NH₃ and H⁺. This aids the process of concentration of urine
• NH₃ then diffuses into the CD lumen and forms NH₄⁺ for excretion in urine (membrane is impermeable to this molecule)
• H⁺ may also be excreted via the very effective active ATPase pumps on the apical membrane

Question 5

Outline the link between ammoniagensis and net gain of HCO₃^-

Draw the related ion movements

Answer 5

‘Ammonia trapping’

• Ammonia is generated within the PCT and is excreted into the tubule via a Na⁺/NH₄⁺ antiporter
• NH₄⁺ is reabsorbed into the interstitial space at the ascending limb of the Loop of Henle, where it contributes to the medullary concentration (for urine dilution)
• NH₄⁺ dissociates to NH₃ and H⁺
• It is then reabsorbed at the collecting duct
• Here it acts as a urinary buffer through the binding and subsequent excretion of H⁺ generated via cellular metabolism.
• Related to this reaction, NEW HCO₃^- is generated.

Question 6

Ion channels - key points

Answer 6

• HCO₃^- reabsorption uses a Na⁺/HCO₃^- symporter in the PCT and a Cl^-/HCO₃^- antiporter in the CD
• Urinary buffers allow for the excretion of H⁺ along the length of the nephron. As the maximum acidity of the nephron is 4.5, which would not allow for adequate H⁺ removal, they are crucial (also prevents acidotic damage of the tubule)
• H⁺ is excreted via a Na⁺/H⁺ antiporter and H⁺ ATPase in the PCT but uses a K⁺/H⁺ symporter and H⁺ ATPase in the CD
• A basolateral Na⁺/K⁺ ATPase is seen in the PCT but not the CD

Question 7

To explain the two basic metabolic acid-base disturbances and describe for each the primary defect, the changes in arterial blood chemistry (pH, PaCO2 and plasma HCO3- ), common causes and the ensuing compensatory mechanisms.

Question 8

To describe the role of the kidneys in maintaining acid-base balance and explain the consequence of kidney injury on pH balance

Question 9

To explain the cellular processes by which H+ is excreted, HCO3- reabsorbed and replaced in the renal tubules, and influential factors on these processes

Question 10

Outline the main physiology processes by which pH is regulated

Answer 10

1. Buffer systems: Bicarbonate, phosphate, carbonate
2. Respiration: Peripheral chemoreceptors are the first to detect changes in CO2
3. Renal: Excretion of hydrogen and bicarbonate, use of urinary buffers (phosphate)