Gas transport to tissues

Question 1

What is the chloride shift?

Answer 1

In the periphery, carbonic anhydrase produces H+ and HCO3-. The HCO3- diffuses out of the cell, but the cell membrane is relatively impermeable to H+, and so to maintain electrical neutrality, Cl- diffuses into the cell.

Question 2

What is the Haldane effect?

Answer 2

This refers to the fact that deoxyhaemoglobin binds H+ more avidly than oxyhaemoglobin, and also forms carbamino compounds more readily. Deoxygenated blood has greater capacity to remove CO2 from tissues.

Question 3

How is oxygen carried in the blood?

Answer 3

Mostly bound to haemoglobin.

Dissolved : 0.3mL/100mL blood (assuming PaO₂ 100mmHg)

Total 20.8mL/100mL blood (assuming Hb 15g/dL)

Question 4

Draw and label the oxygen dissociation curve.

Answer 4

X-axis is PaO2

Y-axis is SpO2

PaO2 27mmHg = SpO2 50%

PaO2 40mmHg = SpO2 75%

PaO2 100mmHg = SpO2 97%

Question 5

What are the implications of the curved shape of the oxygen-haemoglobin dissociation curve?

Answer 5

In peripheral tissues with low PaO2, there is a higher degree of oxygen offloading from Hb. In tissues with high PaO2, there is a higher degree of oxygen binding to Hb. Even if the lungs are diseased and the alveolar PO2 drops from 100mmHg to 80mmHg, there is very little reduction in SpO2.