Session 6 - Renal control of Acid and Base

Question 1

Give the normal range of plasma pH

Answer 1

7.38 - 7.42

Question 2

How is HCO3 reabsorbed in the tubules?

Answer 2

Luminal side:

Na-H antiporter

Apical side:

Na-K-ATPase

In tubule cell:

a) In lumen HCO3 and H+ form H2O and CO2 and enter cell
b) CO2 and H2O form H+ and HCO3 in cell. Get exported into ECF

Question 3

Describe how H+ is excreted in the distal tubule

Answer 3

1. Metabolism of tubular cells produces CO2 which reacts with h2o to form H+ and HCO3.
2. HCO3 enters ECF. H+ is exported actively into lumen where it reacts with phosphate (taken from bones).
3. H+ also exported by the K+-H+ antiporter. Potassium enters ECF. K+ gradient used to take in sodium from the lumen in the distal tubule.

Question 4

Describe how and why buffering of urine is achieved and required

Answer 4

The minimum pH of urine is 4.5, so acidic. If urine too acidic it would harm the cells of the kidney and the ureters, so some H+ needs to be buffered by phosphate and ammonia (becomes ammonium).

Question 5

What is a titratable acid?

Answer 5

Titratable acid – an acid that can lose protons in an acid-base reaction, e.g. phosphoric acid.

Question 6

Give 4 common causes of metabolic alkalosis

Answer 6

• Loss of hydrogen ions – caused by vomiting.
• Retention of bicarbonate
• Antacids administered in excess
• Shift of hydrogen ions into ICF due to hypokalaemia. Low extracellular potassium leads to K+ exiting cells and H+ entering cells to maintain electroneutrality, raising blood pH

Question 7

Describe the 2 main classes of metabolic acidosis

Answer 7

1) Normal anion gap
2) Increased anion gap

Question 8

What is the anion gap?

Answer 8

• The anion gap is calculated as being the difference between ([Na]+[K]) and ([Cl]+[HCO3]), ie unaccounted anions such as lactate
• It is increased if anions from metabolic acid has replaced HCO3.
• Sometimes renal problems can reduce HCO3 without increasing the anion gap, as it is replaced with Cl-