S5 Acid-base Balance

Question 1

What is the effect of alkalaemia on cells?

Answer 1

pH > 7.45
Alkalaemia lowers free calcium by causing Ca2+ ions to come out of solution, this increases neuronal excitability, leads to paraesthesia and tetany.

Question 2

What is the effect of acidaemia on cells?

Answer 2

pH < 7.35
Increases plasma K+ concentration which effects excitability, can cause arrhythmia.
Increasing [H+] affects many enzymes as it denatures proteins

Question 3

How do the kidneys and lungs control plasma pH?

Answer 3

Kidneys – variable recovery of HCO3- and active secretion of H+
Lungs – alveolar ventilation allows diffusion of O2 into blood and CO2 out of blood. Rate of ventilation controlled by chemoreceptors

Question 4

Describe renal recovery of HCO3-

Answer 4

HCO3- filtered at the glomerulus, mostly recovered in PT.
H+ excretion linked to Na+ entry in PT
H+ reacts with HCO3- in the lumen to form CO2 which enters cell
and is converted back to HCO3- which enters ECF

Question 5

Describe creation of HCO3- in proximal tubule

Answer 5

Glutamine –> α-ketoglutarate + ammonium (NH4+)
Produces HCO3- which enters ECF
NH4+ enters lumen in the form of NH3 to be excreted as urine

Question 6

How does the distal tubule control HCO3-?

Answer 6

H+ ions from the reaction of CO2 with water are actively secreted into lumen by H+ ATPase.
H+ buffered by ammonia and phosphate, to produce NH4+ and H2PO4- which are excreted.
No CO2 is formed to re-enter the cell, allows HCO3- to enter plasma

Question 7

How does acidosis cause hyperkalaemia?

Answer 7

K+ move out of cells in response to the high H+ moving in.

Decreased potassium excretion in distal nephron

Question 8

How does alkalosis cause hypokalaemia?

Answer 8

K+ move into cells as H+ moves out

Enhanced excretion of potassium in distal nephron

Question 9

Summarise the different types of acid-base disruption

Answer 9

Respiratory acidosis - CO2 high
Respiratory alkalosis - CO2 low

Metabolic acidosis - HCO3- low
Metabolic alkalosis - HCO3- high

Question 10

What is compensation?

Answer 10

Compensation aims to restore the ratio and pH by affecting PCO2 or HCO3-.
Partial compensation affects the ratio also but does not fully restore the pH (e.g pH 7.31).

Question 11

What is the anion gap?

Answer 11

Difference between measured cations (+) and anions (-).

This gap is increased if HCO3- is replaced by other anions, which may not be measured