Acid-base regulation

Question 1

Give 2 reasons why the regulation of H+ is more complex and tighter than for other ions.

Answer 1

H+ alters protein activity, especially enzymes

H+ alters binding of other ions e.g. a low H+ increases Ca2+ binding to albumin

Question 2

What are the 3 main mechanisms to minimise changes in pH?

Answer 2

Buffer systems

Respiratory regulation

Renal regulation

Question 3

By what mechanism does respiratory regulation help minimise changes in pH?

Answer 3

Adjustment of CO2 levels

Question 4

What is a buffer?

Answer 4

Any substance that can reversibly bind H+

Question 5

What are the 3 main buffer systems?

Answer 5

Bicarbonate buffer system

Phosphate buffer system

Protein buffer system

Question 6

Where does the bicarbonate buffer system take place?

Answer 6

Extracellular

Question 7

Where does the phosphate buffer system take place?

Answer 7

Intracellular and in urine

Question 8

Where does the protein buffer system take place?

Answer 8

Mainly intracellular

Question 9

What is the chemical reaction of the bicarbonate buffer system?

Answer 9

H+ + HCO3- = H2CO3 (carbonic acid) = CO2 + H2O

Question 10

What is the chemical reaction of the phosphate buffer system?

Answer 10

HPO42- + H+ = H2PO4-

Question 11

What is the chemical reaction of the protein buffer system?

Answer 11

Pr- + H+ = HPr

Question 12

Is urine usually acidic or alkaline? Why?

Answer 12

Acidic – due to excretion of H+ from non-volatile acid production

Question 13

What are the 2 main processes by which the kidneys regulate extracellular fluid pH?

Answer 13

Reabsorption of filtered HCO3-

Excretion of H+

Question 14

How many mmol of HCO3- to the kidneys filter each day?

Answer 14

4500 mmol

Question 15

Describe how HCO3- is reabsorbed in the PCT.

Answer 15

It cannot be absorbed directly. H+ is secreted into the tubule lumen by a Na+/H+ antiporter. The H+ combines with bicarbonate to form carbonic acid (H2CO3). Carbonic anhydrase then converts carbonic acid into CO2 and H2O. The CO2 diffuses across the cell membrane into the PCT cell, where it combines with H2O to form carbonic acid, which then dissociates into H+ and HCO3-. The HCO3- is transported across the basolateral membrane into the bloodstream via Na+/HCO3- co transporter. The H+ is secreted back into the tubule lumen, continuing the cycle

Question 16

The secretion of H+ in the late distal tubule and collecting ducts occurs through the action of which cells?

Answer 16

Type A intercalated cells

Question 17

Describe how H+ is secreted into the late distal tubule and collecting tubules.

Answer 17

H+ ATPase and H+/K+ATPase to pump H+ into the tubule lumen against its concentration gradient

Question 18

What are the 2 main urinary buffers?

Answer 18

Phosphate

Ammonia

Question 19

In urine, phosphate exists in which two forms?

Answer 19

Monohydrogen phosphate (HPO42-)

Dihydrogen phosphate (H2PO4-)

Question 20

In the urinary phosphate buffer system, H+ is secreted with phosphate and what else?

Answer 20

Sodium

Question 21

Draw the formula for the urinary ammonia buffer.

Answer 21

NH3 + H+ = NH4+

Question 22

Ammonium is synthesised in the PCT from what?

Answer 22

Glutamine

Question 23

How does a decrease in pH affect the urinary ammonia buffer system?

Answer 23

It causes the kidneys to metabolise more glutamine, resulting in higher levels of ammonia, which can combine with H+ to be excreted, therefore reducing the acidity of the blood

Question 24

What 4 things stimulate H+ secretion by the kidneys?

Answer 24

Increase in pCO2 of the extracellular fluid

A decreased pH of the extracellular fluid

Increased aldosterone levels

Hypokalaemia

Question 25

What is respiratory acidosis?

Answer 25

Low pH due to increased CO2

Question 26

Over what general time frame and how is respiratory acidosis compensated for?

Answer 26

Slowly (days) by the kidneys to increase the production of bicarbonate

Question 27

What is metabolic acidosis?

Answer 27

Low pH due to decreased HCO3-

Question 28

What are the 3 main mechanisms which cause metabolic acidosis?

Answer 28

Addition of exogenous acid (e.g. methanol)

Addition of endogenous acid (e.g. lactic or keto acids)

Failure of H+ excretion or loss of HCO3- (e.g. severe prolonged diarrhoea)

Question 29

Over what general time frame and how is metabolic acidosis compensated for?

Answer 29

Rapidly by the lungs to increase ventilation and thus decrease CO2