Session 4 - Blood Gas carriage

Question 1

Draw an oxygen - Hb dissociated curve.

What are the effects of: a) a fall in pH b) a rise in temperature

Answer 1

Question 2

What properties of Hb allow it to transport oxygen in the blood?

Answer 2

• Quaternary structure can change its shape in order to make tight and relaxed conformations. Relaxed binds oxygen, tight does not bind oxygen well.
• Hard to bind first oxygen as it is in a tense state, but binds more easily afterwards. Hence the sigmoid curvature of the dissociation curve
• Highly reversible reaction

Question 3

How would the rate of delivery of oxygen be changed if the ppO2 in the capillaries fell?

Answer 3

If the pO2 in the capillaries of tissues falls, pH falls and temperature rises so that Hb will give up more oxygen. Therefore the saturation of Hb leaving the capillaries will be greatly reduced.

Question 4

What 3 factors influence the diffusion of gases across the alveolar membrane?

Answer 4

• Structure of alveolar membrane
• ECF barrier
• Capillary wall

Question 5

How does Co2 react in the blood?

Answer 5

reacts with h20 to form h+ and hco3- If pco2 rises, more H+ and hco3- formed

Question 6

Where does all the hydrogen carbonate in the blood come from?

Answer 6

Hydrogen Carbonate comes from reactions of CO2 in a red blood cell:

• Dissolved CO2 reacts with water and the H+ binds to Hb
• Therefore equation shifts to the right and lots of CO2 react, forming lots of hydrogen carbonate
• Hydrogen carbonate leaves RBC in exchange for chloride
• Forming the 25 mmol/l of HCO3 in plasma. Kidneys also control HCO3 conc by variable excretion

Question 7

Explain how Hb has a buffering capacity

Answer 7

Buffering of acid by Hb:

• The more oxygen bound, the less CO2 is bound
• Once oxygen is removed the HB has the potential to generate more HCO3
• More CO2 in venous blood, but this binds to Hb and forms more HCO3, preventing pH from falling.
• When Hb reaches lungs and picks up oxygen, H+ given up and reacts with hydrogen carbonate to form CO2 which is breathed out.

Question 8

How do carbamino compounds contribute to the buffering in the blood?

Answer 8

CO2 also binds to proteins and this contributes to CO2 transport but not to the acid base balance. Allows a bit more buffering to occur.

Question 9

State the normal arterial and venous content of Co2 per litre

Answer 9

Arterial - 22 mmol co2 per litre

Venous - 24 mmol co2 per litre

Question 10

Give the proportions of co2 travelling in the blood as hydrogen carbonate, carbamino compounds, and dissolved co2

Answer 10

• Hydrogen carbonate – 80%
• Carbamino compounds – 11%
• Dissolved CO2 – 9%