Acid-Base Balance

Question 1

What effect does alkalaemia have on Ca and how does this affect the body

Answer 1

Alkalaemia reduces Ca solubility so it comes out of solution

This decreases free Ca which causes increased neuronal excitability leading to paraesthesia and tetany

Question 2

Describe the relationship of alkalaemia with K

Answer 2

Alkalaemia causes hypokalaemia

Alkalaemia causes H+ to move out the cell causing K to move into the cell in a reciprocal cation shift

There is also increased K excretion in the distal nephron leading to hypokalaemia

Hypokalaemia also causes alkalaemia by making the intracellular pH of tubular cells more acidic so more H+ is excreted into the tubule and more bicarbonate is produced

Question 3

What effect does acidosis have on [K] in the plasma

Answer 3

Acidosis causes hyperkalaemia by a reciprocal cation shift - H+ move into cells forcing K out of the cells

Acidosis also causes decreased K excretion in distal nephron to cause hyperkalaemia

Question 4

What effect does hyperkalaemia have on plasma pH

Answer 4

Hyperkalaemia makes the intracellular pH of tubular cells more alkaline -> H+ moves out of the cells into the blood while increasing bicarbonate excretion

This results in metabolic acidosis

Question 5

How is the plasma [bicarbonate] not depleted when it reacts with acid produced from metabolism

Answer 5

Kidneys recover all filtered bicarbonate

PCT cells form bicarbonate from amino acids

DCT cells form bicarbonate from CO2 and water

Question 6

How is bicarbonate reabsorbed in the PCT

Answer 6

H+ are excreted into the lumen of the tubule and react with bicarbonate to form CO2 and water - catalysed by carbonic anhydrase

CO2 diffuses into the cell and reacts with water to form bicarbonate again

Bicarbonate is transported into the ECF while the H+ is excreted back into the lumen of the tubule

Question 7

How is bicarbonate formed in PCT cells

Answer 7

Glutamine is broken down to alpha-ketoglutarate and ammonium

Alpha-ketoglutarate is broken down into two bicarbonates which are pumped into the ECF

Ammonium dissocaites into H+ and ammonia

Ammonia diffuses into lumen and then reacts with H+ to form ammonium -> buffers H+

Question 8

How do the PCT and CD control pH

Answer 8

They both secrete H+ into the lumen from the reaction of CO2 and water

Question 9

How are the H+ in the lumen of the tubule buffered

Answer 9

Ammonia and phosphate buffer the large amounts of H+ by forming:

Ammonium

Dihydrogen phosphate

Question 10

What are the characteristics of respiratory acidosis

Answer 10

High pCO2

Normal bicarbonate

Low pH

Question 11

What are the characteristics of respiratory alkalosis

Answer 11

Low pCO2

Normal bicarbonate

Raised pH

Question 12

What are the characteristics of compensated respiratory acidosis

Answer 12

High pCO2

Raised bicarbonate

Relatively normal pH

Question 13

What are the characteristics of compensated respiratory alkalosis

Answer 13

Low pCO2

Lowered bicarbonate

Relatively normal pH

Question 14

What is the anion gap and what is it used for

Answer 14

Anion gap - difference between measured cations and anions

It is used to determine the cause of acidosis - is it due to increased acid production or a renal cause

= ([Na] + [K]) - ([Cl] + [bicarbonate])

Question 15

How is the anion gap used to differentiate between causes of metabolic acidosis

Answer 15

Gap increases if bicarbonate is replaced by another anion - metabolic acid reacts with bicarbonate and the anion of the acid replaces bicarbonate so the gap increases

Gap stays the same if it is a renal cause as the bicarbonate is replaced by Cl -> no change in the anion gap

Question 16

What are the characteristics of metabolic acidosis

Answer 16

Normal pCO2

Low bicarbonate

Low pH

Either increased or normal anion gap

Question 17

What are the characteristics of compensated metabolic acidosis

Answer 17

Low bicarbonate

Lowered pCO2

Relatively normal pH

Question 18

What are the characteristics of metabolic alkalosis

Answer 18

Normal pCO2

Raised bicarbonate

Raised pH

Question 19

How is metabolic alkalosis compensated

Answer 19

Metabolic alkalosis is compensated for by increased bicarbonate secretion

There is some reduction in breathing but not much as much maintain the pO2

Question 20

Name some conditions that lead to respiratory acidosis

Answer 20

Type 2 respiratory failure:

Severe COPD

Severe asthma

Drug overdose

Neuromuscular disease

Question 21

Name some conditions that lead to respiratory alkalosis

Answer 21

Hyperventilation

Type 1 respiratory failure (hyperventilation in response to hypoxia) - e.g. pneumonia, anaemia, pulmonary oedema, COPD, heart conditions

Question 22

Name some conditions that lead to metabolic acidosis and what the anion gap will be

Answer 22

Keto-acidosis (diabetes) - increased anion gap

Lactic acidosis - increased anion gap

Ureamia acidosis - increased anion gap

Renal tubular acidosis - normal anion gap

Severe persistant diarrhoea - normal anion gap (Cl replaces bicarbonate)

Question 23

Name some conditions that lead to metabolic alkalosis

Answer 23

Prolonged vomiting or mechanical drainage of the stomach - bicarbonate not secreted into stomach as little acid in stomach so bicarbonate enters blood

Question 24

Why is an increased bicarbonate difficult to correct if the volume is also depleted

Answer 24

If volume depleted, ability to loose bicarbonate is decreased because of the high rate of Na recovery

Recovery of Na to maintain volume favours H+ excretion and bicarbonate recovery -> causes alkalosis