The Role of Ventilation in Acid Base Balance

Question 1

What is pH? How do you calculate the pH?

Answer 1

• pH is a measure of the hydorgen ion concentration of a solution.
• pH = - log₁₀ [H⁺]

Question 2

Why is pH homeostasis important?

Answer 2

• Protein function (e.g. enzyme / receptor binding sites) depends on a specific secondary, tertiary and quaternary structure, achieved via inter- and intra-bonding.
• Various factors (including pH) can denature proteins by disrupting their bonds and structure, thus impairing function.

Question 3

How is arterial pH tightly regulated?

Answer 3

The regulation of arterial pH is achieved by the presence of buffering systems (which resist changes in pH), and by regulating the level of molecules associated with acid and base production.

Question 4

How is pH homeostasis maintained when acid production increases?

Answer 4

• The presence of buffers [a weak acid (H₂CO₃) and a conjugate base (HCO₃^-)] e.g. bicarbonate buffering system, ‘mops up’ the excess H+.
• Any H+ formed by the addition of a strong acid reacts with a conjugate base to form a weaker acid, hence the pH remains relatively stable.

Question 5

Which type of buffers are used to resist sharp changes in pH?

Answer 5

Buffers consisting of a weak acid + conjugate base are used to resist sharp changes in pH.

Question 6

Give examples of other buffering systems (intracellular and extracellular).

Answer 6

1. INTRACELLULAR: -
   
   • Phosphate buffering system.
   • Amino acids/ proteins.
   • Haemoglobin (in RBCs).
2. EXTRACELLULAR: -
   
   1. Bicarbonate buffer system.
   2. Plasma proteins (eg. albumin).

Question 7

How is the transport of CO₂ important in acid-base balance?

Answer 7

• CO₂ is important in the relationship between PCO₂ and [H₂CO₃].

ABG test meaning - It measures the acidity (pH) and the levels of oxygen and carbon dioxide in the blood from an artery. This test is used to check how well your lungs are able to move oxygen into the blood and remove carbon dioxide from the blood.

Question 8

What is the Henderson-Hasselbalch equation?

Answer 8

• The Henderson–Hasselbalch equation can be used to estimate the pH of a buffer solution containing given concentrations of an acid and its conjugate base (or a base and its conjugate acid).
• The numerical value of the acid dissociation constant (K_a) of the acid must also be known.
• pK_a = - log10 [K_a].

Question 9

What is blood pH directly proportional to? (this process is done by the lungs and kidneys)

Answer 9

[In reference to the attached picture]

• ↑ PaCO2 = ↓ pH
• ↓ PaCO2 = ↑ pH

(Above is true unless [HCO₃^-] changes proportionally, in the opposite direction)

• ↑ [HCO3-] = ↑ pH
• ↓ [HCO3-] = ↓ pH

(Above is true unless unless PaCO₂ changes proportionally, in the opposite direction)

Question 10

What is to blame if the CO2 is too high/low, or if the HCO3- is too high/low?

Answer 10

Question 11

For acidosis, list its arterial pH, some causes, effects and compensatory mechanisms.

Answer 11

• pH: - < 7.35
• CAUSES: -
  
  1. Hypercapnia (hypoventilation).
  2. Increased lactic acid (sepsis).
  3. Increased ketone bodies (diabetes).
  4. Decreased kidney acid excretion (renal failure).
  5. Decreased HCO₃^- reabsorption (renal acidosis).
  6. Diarrhoea (loss of HCO3- from the gut).
• EFFECTS: -
  
  1. Tachypnoea (rapid breathing).
  2. Muscular weakness.
  3. Headaches.
  4. Confusion, coma.
  5. Cardiac arrhythmia.
  6. Hyperkalaemia (an increase in blood postassium levels).
• COMPENSATORY MECHANISMS: -
  
  1. Hyperventilation (decreased PaCO₂, respiratory compensation).
  2. Decreased HCO₃^- excretion (renal compensation).

Question 12

For alkalosis, list its arterial pH, some causes, effects and compensatory mechanisms.

Answer 12

• pH: > 7.45
• CAUSES: -
  
  1. Hypocapnia (hyperventilation).
  2. Vomiting (loss of H⁺ in HCl).
  3. Increased kidney acid excretion (diuretics).
  4. Increased alkalotic agent consumption (antacids, NaHCO₃)
• EFFECTS: -
  
  1. Bradypnoea (slow breathing).
  2. Muscular weakness, cramps, tetany (muscular spasms)
  3. Headaches, nausea.
  4. Light-headedness, confusion, coma.
  5. Cardiac arrhythmia.
  6. Hypokalaemia.
• COMPENSATORY MECHANISMS: -
  
  1. Hypoventilation (increased PaCO2, respiratory compensation).
  2. Increased HCO₃^- excretion (renal compensation).

Question 13

Describe how acidosis can induce hyperkalaemia.

Answer 13

• Acidosis increases the concentration of extracellular H+, which decreases the H+ concentration gradient.
• This means that there is less H+ excretion via the H⁺ - Na⁺ exchange.
• The decreased levels of intracellular Na+ affect Na⁺ - K⁺ exchange, decreasing the K+ absorption into the cell.
• The K+ accumulation in the serum is known as hyperkalaemia, and leads to cardiac arrhythmias and muscle weakness.

Question 14

Describe how alkalosis can induce cerebral vasoconstriction.

Answer 14

• CO2 (via H+) acts as a vasodilator in blood vessles (the cerebral arteries are particularly sensitive).
• However, in alkalosis (with decreased CO2 and H+ levels), there is vasoconstriciton in the cerebral arteries.
• This decrease in cerebral flow leads to headaches, lightheadedness, confusion and seizures.