GAS TRANSPORT FROM LUNGS TO TISSUES

Question 1

GAS TRANSPORT FROM LUNGS TO TISSUES

Answer 1

• The blood is transporting the gases (the plasma is predominantly water)
• oxygen has low water solubility
• the water solubility of carbon dioxide is much higher
• To transport oxygen, a carrier - haemoglobin (Hb) - is required. Each Hb molecule can carry four oxygen molecules (it has four binding sites)

Question 2

Saturation

Answer 2

• Saturation is a measure of how many binding sites are occupied: if every Hb molecule is carrying four oxygens, the saturation is 100% - although this level is never reached, the max is 95 - 98%
• Measured with pulse oximeter (SpO2)

Question 3

Arterial O2 content

Answer 3

• Arterial oxygen content indicates how much oxygen the blood is carrying
• content = saturation X [Hb]
• If a person has normal [Hb] + normal saturation O2 transport is normal
• If a person has low [Hb] and normal saturation O2 transport is low: saturation can be normal in severe anaemia
• 100 molecules of Hb, 95% saturated carry 380 O2 molecules
• 10 molecules of Hb, 95% saturated carry 38 O2 molecules

Question 4

r/s b/w O2 pressure + saturation

Answer 4

• When the PO2 is high, saturation is high - haemoglobin collect oxygen
• When the PO2 is low, saturation is low because haemoglobin is releasing oxygen
• This r/s is represented by the Hb dissociation curve

Question 5

r/s of PO2 + saturation w/ the dissociation curve

Answer 5

When the PO2 is high (in the lungs) the dissociation curve is flat
- Small changes in PO2 in the lungs don’t affect saturation

When the PO2 is low (in the tissues) the dissociation curve is steep
- At low PO2, small changes in PO2 cause large changes in saturation

Question 6

SHIFTING THE Hb DISSOCIATION CURVE (to the right)

Answer 6

• If the curve is shifted to the right, the saturation will be lower at a given PO2 ie. the Hb will have given up more O2
• Shifts in the Hb dissociation curve don’t affect haemoglobin saturation in the lungs

What shifts the curve to the right?

• The need for more oxygen in the tissues
  
  • increased PCO2
  • increased [H+] and lower pH
  • increased temp
  • increased 2,3 - DPG - produced in RBCs during glycolysis, hypoxaemia

OPP for shift to left

Question 7

TRANSPORT OF CARBON DIOXIDE

Answer 7

• Most carbon dioxide is carried as bicarbonate ions in the blood

Some CO2 is also carried

• dissolved in the plasma
• as carbamino compounds (reversibly bound to Hb)

Question 8

The carbonic anhydrase reaction

Answer 8

The bicarbonate is produced by the CAR 
= H+ + HCO3  H2CO3  H20 + CO2
In the tissues PCO2 is high:
CAR shifts from right to left
In the lung PCO2 is low:
CAR shifts from left to right