1. Acid-Base Balance

Question 1

What range should plasma pH be within?

Answer 1

7.35-7.45

Question 2

What is the concentration of H+ in the plasma?

Answer 2

Low and tightly controlled 44.5-35.5 nmol.l^-1.

Question 3

What is alkalaemia?

Answer 3

Plasma pH > 7.45

Question 4

What is acidaemia?

Answer 4

Plasma pH < 7.35

Question 5

What is the effect of alkalaemia on Ca2+ and therefore neuronal excitability?

Answer 5

Lowers free calcium by causing Ca2+ to come out of solution. It increases excitability.

Question 6

What are the effects of pH > 7.45?

Answer 6

Paraesthesia and tetany.

Question 7

What is the effect of acidaemia on K+?

Answer 7

Increases plasma potassium ion concentration.

Question 8

What are the effects of increased H+ concentration?

Answer 8

Denatured proteins (affecting enzymes) so muscle contractility, glycolysis, and hepatic function are altered.

Question 9

What is the purpose of the CO2/HCO3- buffer system?

Answer 9

To minimise changes in pH by buffering H+ ions.

Question 10

How do CO2, HCO3-, and H+ interact?

Answer 10

CO2 reacts with water –> HCO2- + H+ - this is a reversible reaction.

Question 11

What is pCO2 determined by?

Answer 11

Respiration - controlled by chomoreceptors.

Question 12

What is HCO3- concentration controlled by?

Answer 12

Kidneys.

Question 13

How does high pCO2 affect pH?

Answer 13

Makes it more acidic, more H+ made.

Question 14

Where is hydrogen carbonate in plasma made?

Answer 14

In red blood cells.

Question 15

How does HCO3- affect plasma pH?

Answer 15

Large concentration nearly stops all CO2 reacting so makes pH alkaline as it uses up the H+.

Question 16

What is the Henderson-Hasselbach equation?

Answer 16

pH = pK + Log ([HCO3-]/(pCO2 x 0.23))

Question 17

How is HCO3- controlled by the kidneys?

Answer 17

It’s filtered at the glomerulus and recovered in PCT. H+ excretion is linked to Na+ entry in PCT and H+ reacts with HCO3- to make CO2. It’s converted back to HCO3- in cells.

Question 18

What is a key renal response to acidaemia?

Answer 18

Excretion of ammonium.

Question 19

How does hyperkalaemia affect pH?

Answer 19

Makes it acidotic as K+ move out of cells and there’s decreased excretion in distal nephron.

Question 20

How does hypokalarmia affect pH?

Answer 20

Makes it alkalotic, K+ moves into cells and enhances excretion in distal nephron.

Question 21

How does ventilation affect acid base balance? (Think at either extreme)

Answer 21

Hypoventilation -> hypercapnia -> fall in plasma pH = respiratory acidosis.
Hyperventilation -> hypocapnia -> increased pH = respiratory alkalosis.

Question 22

What are the characteristics of respiratory acidosis?

Answer 22

High pCO2, normal HCO3-, low pH.

Question 23

What are the characteristics of respiratory alkalosis?

Answer 23

Low pCO2, normal HCO3-, raised pH.

Question 24

How does the kidneys compensate for respiratory acidosis and alkalosis?

Answer 24

Acidosis - increase HCO3-. Alkalosis - decrease HCO3-.

Question 25

How long does it take for the kidneys to compensate for respiratory acidosis or alkalosis?

Answer 25

2-3 days.

Question 26

What are the characteristics of compensated respiratory acidosis?

Answer 26

High pCO2, raised HCO3-, normal pH.

Question 27

What are the characteristics of compensated respiratory alkalosis?

Answer 27

Low pCO2, lowered HCO3-, normal pH.

Question 28

What is the effect on pH of metabolic acid?

Answer 28

Reacts with and removes HCO3- so there's a fall and a fall in pH = metabolic acidosis.

Question 29

What is the anion gap?

Answer 29

Difference between measured cations and anions.

Question 30

How is the anion gap calculated?

Answer 30

([Na+] + [K+]) - ([Cl-] + [HCO3-]).

Question 31

What increases the anion gap?

Answer 31

Increases - if HCO3- is replaced by other anions, e.g. in metabolic acidosis.

Question 32

What are the characteristics of metabolic acidosis?

Answer 32

Normal pCO2, low HCO3-, low pH, increased anion gap is HCO3- is replaced by another organic anion or normal if replaced by Cl-.

Question 33

How is metabolic acidosis compensated for?

Answer 33

Peripheral chemoreceptors detect pH drop -> stimulate ventilation -> decreased CO2.

Question 34

Which conditions cause respiratory acidosis?

Answer 34

Type 2 respiratory failure (alveoli aren't properly ventilated so low pO2, high pCO2) - COPD, severe asthma, drug overdose, neuromuscular disease.

Question 35

Which conditions lead to respiratory alkalosis?

Answer 35

Hyperventilation (anxiety, panic attacks) in acute settings. Hyperventilation in long-term hypoxia - type 1 respiratory failure.

Question 36

Which conditions lead to metabolic acidosis with increased anion gap?

Answer 36

Keto-acidosis, lactic acidosis, uraemic acidosis.

Question 37

Which conditions lead to metabolic acidosis with a normal anion gap?

Answer 37

Renal tubular acidosis, persistent diarrhoea.

Question 38

Which conditions lead to metabolic alkalosis?

Answer 38

Prolonged vomiting or mechanical drainage of stomach, potassium depletion, diuretics.