18. Blood Gas Transport

Question 1

Gas exchange occurs through diffusion and is dependent upon what?

Answer 1

• Diffusion surface area (large, moist)
• Diffusion distance for gases (short)
• Concentration gradient between alveolar air and blood (differences in partial pressures)
• The gases are lipid soluble
• Solubility of gases
• Coordinated blood flow and airflow = more efficient

Question 2

Describe Henry’s Law.

Answer 2

“amount of gas that dissolves in water is determined by its solubility in water and its partial pressure in air”

• at equilibrium, the amount of dissolved gas in solution is proportional to the partial pressure of that gas
  ↑ Pgas = ↑ # of gas molecules in solution
  e.g. ↑PO2 = ↑ amount of O2 in solution

Question 3

Describe the structure of haemoglobin.

Answer 3

• 4 globular protein subunits
  ○ (2α + 2β)
• Each subunit:
  ○ protein (globin)
  ○ + non-protein group (haem)
  Haem: Fe2+ in a porphyrin ring structure

Question 4

How is Oxygen transported in the blood?

Answer 4

• Approx. 97% of O2 transported in blood in combination with Hb
• Remainder in plasma
• Each haem portion of Hb can carry 4 molecules of O2
  ○ after binding with O2 Hb changes shape to facilitate further uptake – positive feedback

Question 5

What is the Oxygen - Haemoglobin dissociation (saturation) curve?

Answer 5

• Relates the saturation of Hb to the PO2

- How much oxygen binds to the haemoglobin is affected by the partial pressure of oxygen in the blood

Question 6

How does pH affect Oxygen Unloading in Tissues?

Answer 6

↑tissue activity = ↑ CO2 production = ↑ H+ ion production = ↓pH

↓pH & ↑CO2
○ Curve shifts to right
○ better unloading
○ ‘Bohr shift’

Normal blood range: 7.35-7.45 pH

Question 7

How does Temperature affect Oxygen Unloading in Tissues?

Answer 7

↑ Temperature
○ high in active tissues

↑ 2,3-diphosphoglycerate (2,3-DPG)
○ anaerobic conditions
Binds more strongly to reduced form of haemoglobin (deoxyhaemoglobin) than to oxyhaemoglobin. E.g. in hypoxia = shifting curve to the right

Question 8

What are the other haemoglobins?

Answer 8

• Carboxyhemoglobin
• Methaemoglobin
• Fetal haemoglobin

Question 9

Describe Carboxyhemoglobin

Answer 9

• CO binds tighter than O2 (200x greater)

- dramatically reduce ability of O2 to bind to Hb

Question 10

Describe Methaemoglobin

Answer 10

• Fe2+ oxidised to Fe3+ by drugs etc.
• Unable to carry O2
• Slowly converted back

Question 11

Describe Fetal haemoglobin

Answer 11

• 2α + 2γ (Gamma)
• higher affinity for O2
• Important in transferring O2 across the placenta

Question 12

How is CO2 Transported in blood stream?

Answer 12

• 70% converted to carbonic acid formation, H2CO3- and transported in plasma as bicarbonate ion (HCO3-) = chloride shift
• Bound reversibly to haemoglobin: carbaminohaemoglobin (23%)
• Dissolved in plasma (7%)

Question 13

Why is a buffer required in RBC?

Answer 13

1. CO2 is converted to Carbonic Acid by carbonic anhydrase.
2. Carbonic Acid (H2CO3) dissociated into H+ and HCO3- (bicarbonate ions).
3. HCO3- moves out of RBC for Cl- (chloride shift)
4. H+ increased acidity so is removed by buffers.

Question 14

What buffers are used in RBCs? and why?

Answer 14

Imidazole groups of histidine residues in haemoglobin.

• Can accept the H+ ions and minimise the pH changes
• Deoxygenated haemoglobin has the strongest affinity for H+

Question 15

Describe the steps of Haldane effect in the lungs.

Answer 15

1. Oxygenation of Hb
2. Hb undergoes conformational changes
3. = Lower affinity for H+ ions
4. = Decreased buffering power
5. = Release of H+

This aids unloading of CO2 in the lungs

Question 16

Describe the steps of Haldane effect in the tissues.

Answer 16

1. Deoxygenation of Hb
2. Hb undergoes conformational changes
3. = Higher affinity for H+ ions
4. = Increased buffering power
5. = Uptake of H+

CO2 transport from tissues.