Acid-Base Balance

Question 1

Describe the clinical effects of acidaemia and alkalaemia

Answer 1

Acidaemia - increased potassium –> increased excitability, arrhythmias, dentures proteins
Alkalaemia - increased calcium –> increased neuronal excitability, paraesthesia, tetany

Question 2

Describe the carbon dioxide/hydrogen carbonate buffer system and the factors influencing pCO2 and [HCO3-]

Answer 2

CO2 + H2O H+ + HCO3-
PCO2 influenced by respiration
[HCO3-] determined by kidneys

Question 3

Describe, and be able to identify from values, respiratory acidaemia (acidosis) and alkalaemia (alkalosis) and metabolic acidosis and alkalosis

Answer 3

Respiratory acidosis - high pCO2, low pH, same HCO3-
Respiratory alkalosis - low pCO2, high pH, same HCO3-
Metabolic acidosis - same pCO2, low pH, low HCO3-
Metabolic alkalosis - same pCO2, high pH, high HCO3-

Question 4

Describe the respiratory mechanisms controlling pCO2

Answer 4

Ventilation rate

Hyperventilation due to anxiety, type 1 respiratory failure - low pCO2

Question 5

Explain the respiratory response to changes in plasma pH

Answer 5

Low pH - compensate by hyperventilation

Question 6

Explain the cellular mechanisms of reabsorption of HCO3- on the proximal tubule

Answer 6

80% reabsorbed in proximal tubule
Apical side - Na/H exchanger
Basolateral side - Na/K ATPase, HCO3-/Na symporter

Question 7

Explain the cellular mechanisms of reabsorption of H+ excretion in the distal tubule

Answer 7

H+ actively secreted in a-intercalated cells of DCT

Question 8

Describe the mechanism of buffering of H+ in urine, and explain the concept of titratable acid, and the role of NH4+

Answer 8

H+ buffered by ammonia and phosphate –> NH4+ and H2PO4- excreted
In PCT:
Glutamine –> NH4+ a-ketoglutarate
NH4 –> NH3 + H+, NH3 can move freely and combine with H+ in lumen
In DCT:
Actively excreted H+ combines with phosphate –> H2PO4-

Question 9

Describe the interactions between acid base status and plasma [K+]

Answer 9

Acidosis - increased potassium

Alkalosis - decreased potassium

Question 10

Describe the interaction between renal control of acid base balance and control of plasma volume

Answer 10

Renal compensation - HCO3- can increase or decrease to adjust to normal pH
Takes 2-3 days

Question 11

Describe the common causes of metabolic alkalosis, in particular the effects of persistent vomiting

Answer 11

Severe prolonged vomiting, potassium depletion, diuretics e.g. loop
Vomiting - loss of acid and chloride –> hyperactivity of parietal cells –> alkaline tide (bicarbonate ions in blood) –> metabolic alkalosis, hypokalaemia
Kidney compensating for loss of acid –> low potassium

Question 12

Describe the main causes of metabolic acidosis

Answer 12

Diabetes

Exercise to exhaustion, renal failure, severe persistent diarrhoea, renal tubular acidosis

Question 13

Describe the role of anion gap measurements in distinguishing between causes of metabolic acidosis

Answer 13

Increased anion gap - HCO3- replaced by other anions e.g. lactate, ketones, urate
Normal anion gap - HCO3- replaced by chloride

Question 14

State the normal range of plasma pH

Answer 14

7.35-7.45