Carbon Dioxide Transport

Question 1

What are the 3 ways CO₂ is transported in the blood?

Answer 1

• Dissolved (approx 10%)
• Carbamino compounds (approx 21%)
• Bicarbonate (approx 69%)

Question 2

What are the approxiamte partial pressures of CO₂ in:

Venous blood

Alveoli

Arterial blood

Answer 2

Venous blood: 6.5kPa

Alveoli: 5.3 kPa

Arterial blood: 5.3kPa

Question 3

Explain how CO₂ is transported dissolved in the blood

Answer 3

Reacts with water to form carbonic acid (H₂CO₃) which then dissociates to form HCO₃^- and H⁺. This happens slowly in plasma due to the existing high concentration of HCO₃^-.

This is a negligible amount

Question 4

What are weak and strong acids?

Answer 4

Strong acids:

• Completely dissociate in water, releasing large amounts of H⁺
• E.g. HCl → Cl^- + H⁺

Weak acids:

• Incompletely dissociate in water, forming a buffer pair with their conjugate base until they reach equilibrium. Reversibly bind H⁺ in response to H⁺ concentrations.
• E.g. Carbonic acid H₂CO₃ → H⁺ + HCO₃^-

Question 5

What is normal blood pH?

Answer 5

7.36-7.44

Question 6

What are volatile and non-volatile acids?

Answer 6

Volatile acids:

• More easily vapourised
• Excreted by lungs
• 1400 mmol H⁺ produced per day by aerobic metabolism and CO₂ production by tissues.

Non-volatile acids:

• Not easily vapourised
• 70-100mmol H⁺ produced per day by other metabolic processes
• Excreted by kidneys

Question 7

How is CO₂ transported in the blood as carbamino compounds?

Answer 7

Formed when CO₂ reacts with protein amino groups

CO₂ + protein NH₂ → protein-NHCOOH

Question 8

What determines the amount of H⁺ (and therefore pH) in the blood?

Answer 8

Amount of bicarbonate (reduces amount of H⁺)

Amount of CO₂ (increases amount of H⁺)

Question 9

Explain the Henderson-Hasselbach equation

Answer 9

pH = pK + log₁₀ [HCO₃^-] / CO₂ x 0.23

Allows measurement of pH using CO₂ and HCO₃^- concentrations.

pK= constant for the reaction

Question 10

What are the physiological buffer systems?

Answer 10

Respiratory (rapid):

• If the body produces too much acid, H⁺ reacts with HCO₃^- ions to produce CO₂
• CO₂ excreted by the lungs

Renal (slow):

• If pCO₂ is too high, the kidneys excrete less HCO₃^-
• HCO₃^- in plasma is raised, restoring pH

CO₂ + H₂O ⇔ HCO₃^- + H⁺

Question 11

How is CO₂ transported as bicarbonate?

Answer 11

In red blood cells, CO₂ and water are converted to HCO₃^- and H⁺, catalysed by carbonic anhydrase.

This reaction is further promoted as the products of the reaction are removed:

• H⁺ is buffered by histidine residues on Hb → HHb
• Excess HCO₃^- leaves the red blood cell in exchange for Cl^- via the Cl^-/ HCO₃^- exchanger (chloride/hamburger shift)

The end result of these reactions is that the majority of CO₂ is transported in the blood as HCO₃^-

Question 12

Describe the buffering ability of haemoglobin

Answer 12

The ability of haemoglobin to buffer H⁺ is enhanced by deoxygenation, therefore venous blood can carry more H⁺ than arterial.

Question 13

What happens if O₂ is given up by the Hb without taking up CO₂?

Answer 13

The pH of the cell will rise as excess H⁺ will be buffered by deoxygenated Hb.

The cell will become alkaline

Question 14

What is the effect of Hb taking up CO₂ on O₂ avidity?

What is the effect of Hb giving up O₂ on CO₂ avidity?

Answer 14

Hb avidity for oxygen reduces with its uptake of CO₂ by modifying making Hb more tense (Bohr shift)

Hb avidity for CO₂ increases as it gives up O₂ by making Hb more relaxed (Haldane effect)

Question 15

In which direction does the oxygen dissociation curve shift to the left and right?

What does this mean for O₂ avidity?

Answer 15

Shift to the left:

• Alkaline conditions
• For any given pO₂, Hb binds more O₂

Shift to the right:

• Acidic conditions
• For any given pO₂, Hb binds less O₂