Acid-Base Balance

Question 1

What is the normal pH range of plasma?

Answer 1

pH 7.35 - 7.45

Anything greater 7.45 = alkalemia

Anything less 7.35 = acidaemia

Question 2

What is the consequence of alkalemia?

Answer 2

• Alkalemia lowers free calcium as it causes Ca²⁺ ions to come out of solution
• Decreased Ca²⁺ concentration makes neuronal cells more excitable
• Can lead to paraethesia and tetany

Question 3

What are some of the consequence of acidaemia?

Answer 3

1. Acidaemia increases plasma K⁺ concentration
   
   • ​​affects excitability of cardiac cells leading to arrhythmia
2. Acidaemia can denature proteins affecting muscle contractility, glycolysis, hepatic function

Question 4

What is the equation that controls plasma pH?

Answer 4

Question 5

How do the kindneys control plasma pH?

Answer 5

By adjusting the recovery of HCO₃^- and actively secreting H⁺ ions

Question 6

How do the lungs control plasma pH?

Answer 6

Alveolar ventilation determines pCO₂ and pO₂

Rate of ventilation controlled by chemoreceptors

Question 7

In normal circumstances, why does increased acid production from metabolism not deplete HCO₃^-?

Answer 7

• The kidneys recover all filtered HCO₃^-
• Proximal tubules makes HCO₃^- from amino acids, putting NH₄⁺ into urine
• H⁺ ions excreted to lumen but reacts with HCO₃^- to give H₂O + CO₂ which can diffuse into the tubular cell
  
  • HCO₃^- enters ECF

Question 8

Explain how the proximal tubule creates HCO₃^-

Answer 8

1. Inside proximal tubule cells Glutamine → alpha- ketogluturate and NH4 ⁺
2. alpha- ketogluturate breaks down to HCO₃^- which is transported to ECF along with Na⁺ ions
3. NH₄⁺ breaks fown to NH₃ (ammonia) and H⁺ ions
4. Ammonia freely diffused into lumen of tubule and combines with H⁺ → NH₄⁺ acting as an ion buffer

Question 9

What is the function of ammonia throughout the kidney?

Answer 9

Ammonia acts as a buffer by binding with H⁺ ions

Question 10

What is the minimum pH of urine?

Answer 10

pH 4.5

Question 11

How do acidosis and alkalosis affect K⁺ ions in the nephon?

Answer 11

H⁺ and K⁺ are both cations, therefore movement of one will cause the reciprocal movement of th other

Acidosis: causes K⁺ to move out cell causing hyperkalaemia and decreased K⁺ excretion at the distal nephron

Alkalosis: causes K⁺ to move into cells causing hypokalaemia and increased K+ excretion at the distal nephron

Question 12

How do changes in K⁺ ions lead to metabolic acidosis?

Answer 12

Metabolic acidosis caused by hyperkalaemia

Hyperkalemia causes H+ ions to move out of tubular cells which favours HCO₃^- excretion → metabolic acidosis

Question 13

How do changes in K⁺ ions lead to metabolic alkalosis?

Answer 13

Hypokalaemia makes intracellular pH of tubular cells more acidic → causes H⁺ to move into tubular cells

Favour H⁺ excretion → metabolic alkalosis

Question 14

What is respiratory acidosis and what would you expect to see in the blood results to confirm this?

Answer 14

Hypoventilation causing hypercapnia (increased pCO₂) → equilibrium shifts right → fall in plasma pH

Blood results:

• High pCO₂
• Normal HCO₃^-
• Low pH

Question 15

What is respiratory alkalosis and what would you expect to see in the blood results to confirm this?

Answer 15

Hyperventilation → hypocapnia (low CO₂) → equilibrium shifts left → pH rises

Blood results:

• pCO₂ low
• HCO₃^- normal
• pH high

Question 16

In respiratory acidosis and alkalosis, what is compensation? What will you see in the bloods if fully compensated?

Answer 16

To compensate for changes in pH the kidneys can alter [HCO₃^-]

acidosis: kidneys increase [HCO₃^-]

alkalosis: kidneys decrease [HCO₃^-]

If fully compensated pH will be normal

Question 17

How long does it take for the kidneys to compensate from respiratory acid/ alkalosis?

Answer 17

2-3 days

Question 18

What is metabolic acidosis? What will you see in the blood results to confirm this?

Answer 18

Metabolically active tissues produce H⁺ which reacts the HCO₃^- to deplete stores → pH drops

Bloods:

• pCO₂ normal
• low [HCO₃^-]
• pH low

Question 19

What is the anion gap?

Answer 19

The difference between measurable cations and anions, mainly:

([Na⁺] + [K⁺] ) - ([Cl^-] + [HCO₃^-])

Question 20

What is the normal anion gap? In what circumstances would the gap change?

Answer 20

Normally 10-18 mmol.l^-1

Gap increases if HCO₃^- replaced by other anions (not measurable) e.g. when lactic acid reacts with HCO₃^-

Question 21

Why is the anion gap unchanged in renal causes of acidosis?

Answer 21

Although less HCO₃^- is made, it is replaced by Cl^- ions which are measurable

Question 22

How does the body compensate for metabolic acidosis and what will you see in the blood results?

Answer 22

Drop in pH detected by peripheral chemoreceptors → signal to respiratory centre to increase ventilation → decreases pCO₂

Bloods:

• normal pH
• low pCO₂
• low HCO₃^-

Question 23

What happens in metabolic alkalosis?

Answer 23

[HCO3-] increases

Bloods:

• pH high
• pCO₂ normal
• [HCO₃^-] raised

Question 24

Can metabolic alkalosis be compensated?

Answer 24

Yes but only to a small extent as you cannot reduce breathing too much as pO₂ needs to be maintained

Question 25

What conditions can lead to respiratory acidosis and how?

Answer 25

**Type 2 respiratory failure** alveoli are not properly ventilated meaning low pO₂ and **high pCO₂** Includes conditions: COPD, severe asthma, drug overdose and neuromuscular disease

Question 26

What can cause respiratory alkalosis?

Answer 26

1. **Hyperventilation** - in anxiety / panic attacks 2. **Type 1 respiratory failure**

Question 27

What conditions can lead to metabolic acidosis?

Answer 27

* Diabeteic ketoacidosis * Lactic acidosis * Uremic acidosis in advanced renal failure * Severe, persisent diarrhoea

Question 28

What can lead to metabolic alkalosis?

Answer 28

* severe, prolonged vomiting (loss of H⁺) * K⁺ depletion * Certain diuretics

Question 29

How do non-renal causes of metabolic acidosis cause hyperkalaemia?

Answer 29

Acidosis = increased H⁺ ions H⁺ moves into cell and K⁺ moves **out** of cells cause hyperkalaemia

Question 30

How can diabetic ketoacidosis lead to **total body depletion of K⁺?**

Answer 30

K⁺ moves out of cells due to acidosis and osmotic diuresis means the ions are lost in urine

Question 31

How does metabolic acidosis lead to hypokalaemia?

Answer 31

* Less H⁺ excretion in nephron causes more K⁺ to be excreted * K+ also moves into cells * Leads to hypokalaemia