7. Mechanics of Acid-Base balance

Question 1

What is the normal plasma pH range?

Answer 1

7.35-7.45

Question 2

Outside what range of plasma pH is considered incompatible with life?

Answer 2

<6.8

>7.8

Question 3

What is the urine pH range?

Answer 3

5 - 9

Question 4

What is the normal extracellular concentration of H+?

Answer 4

40 nmol/L

Question 5

What can cause metabolic acidosis?

Answer 5

Gain of H+

Loss of HCO3-

Question 6

What can cause metabolic alkalosis?

Answer 6

Loss of H+

Gain of HCO3-

Question 7

What controls the PCO2?

Answer 7

Alveolar ventilation

Question 8

What controls plasma HCO3- concentration?

Answer 8

Renal excretion of H+ and reabsorption of HCO3-

Question 9

Where does bicarbonate reabsorption take place and which areas reabsorb more bicarbonate?

Answer 9

PCT: 80%
AL of LOH: 10%
DCT: 6%
CD: 4%

Question 10

Henderson hasselback pH equation including bicarbonate and CO2 and typical values

Answer 10

pH = pK + log [HCO3-]/[CO2]
pK= 6.1
HCO3-= 24 mmol/L
CO2= 1.2 mmol/L

Question 11

Describe the davenport diagram

Answer 11

Metabolic alkalosis: HCO3- increases, pH increases
Metabolic acidosis: HCO3- decreases, pH decreases
Respiratory split into chronic and acute
Chronic disorders cross into normal pH as adaptations bring pH back towards normal

Question 12

Calculate BE if plasma bicarbonate is 34

Answer 12

BE= +10

as normal bicarbonate is 24

Question 13

Describe the process of excretion of H+ and retention of HCO3-.

Answer 13

H+ reacts with HCO3- in filtrate to form H2CO3
Carbonic anhydrase converts H2CO3 to H2O + CO2
H2O + CO2 are absorbed into the tubular cell and react to form H2CO3
Carbonic anhydrase in the cell converts H2CO3 to H+ + HCO3-
H+ pumped into filtrate via H+ ATPase
Na+ gradient into cells is used to antiport HCO3- into the blood

Question 14

Name the 2 transporters on the basolateral membrane that allow HCO3- reabsorption

Answer 14

Cl-/ HCO3- exchanger

Na+/ HCO3- cotransporter

Question 15

What 2 identities can intercalated cells of the CD take?

Answer 15

Alpha: Acid secreting
Beta: Bicarbonate secreting

Question 16

Describe the mechanisms occurring in an alpha cell of the CD

Answer 16

HCO3- and H+ in filtrate combine
Carbonic anhydrase catalyses formation of water and CO2
CO2 moves into alpha cell, combines with water
Carbonic anhydrase catalyses formation of H+ and HCO3-
Transporters on apical surface drive H+ into filtrate
Cl-/HCO3- exchanger on basolateral membrane allow HCO3- reabsorption

Question 17

Which 3 transporters on the apical membrane of alpha cells drive H+ back into the filtrate?

Answer 17

Na+/K+ antiporter
H+/K+ ATPase
H+ ATPase

Question 18

How do the mechanisms in the beta cell differ to that of the alpha cell in the CD?

Answer 18

Mechanisms are the same, just flipped upside down

Bring H+ into body and push HCO3- out

Question 19

Describe bicarbonate production from AAs

Answer 19

In cuboidal epithelial cells of PCT
AAs e.g. Glutamine split into constituents
The ammonium produced is excreted
HCO3- produced is reabsorbed

Question 20

What happens to hydrogen phosphate in the filtrate?

Answer 20

When secreting H+, combines with HPO4,2- to form H2PO4-

This is analysable from urine

Question 21

What movement occurs at the chloride bicarbonate exchanger?

Answer 21

Cl- moves into cell
HCO3- moves out to be reabsorbed
Cl- moves out through channels

Question 22

Describe the compensatory mechanisms for respiratory acidosis (acute and chronic)

Answer 22

Acute: Intracellular buffering
Chronic: HCO3- generation and increased ammonium excretion

Question 23

Describe the compensatory mechanisms for respiratory alkalosis (acute and chronic)

Answer 23

Acute: Intracellular buffering
Chronic: decreased HCO3- reabsorption and decreased ammonium excretion

Question 24

Describe the compensatory mechanisms for metabolic alkalosis

Answer 24

Hypoventilation to decrease CO2 excretion

Question 25

Describe the compensatory mechanisms for metabolic acidosis

Answer 25

Hyperventilation to increase CO2 excretion

Question 26

Can Metabolic acidosis and metabolic alkalosis coexist?

Answer 26

Yes

HCl can be vomited and lactic acid accumulated

Question 27

Describe phosphate filtration/ reabsorption

Answer 27

Freely filtered
90% reabsorbed
Unabsorbed may bind to H+, making it unavailable for reabsorption