Respiratory Physiology - Blood gas transport Flashcards
FO2 in air
Fractional concentration of oxygen in air
0.21
Barometric pressure
760 mmHg
PO2 in atmospheric air
Barometric pressure = 760 mmHg
Water vapour pressure at 37 degrees celcius = 47 mmHg
Dry atmospheric pressure = 760 - 47
= 713 mmHg
PO2 = 0.21 x 713
= 150 mmHg
Approximate conversion of mmHg to kPa
mmHg divided by 7.5 roughly equals kPa
What changes water vapour pressure in air
Temperature
Partial pressure of oxygen in physical solution
Partial pressure equilibrates between air and water
Therefore when PO2 in air = 150 mmHg, PO2 in water / solution = 150 mmHg
Concentration of oxygen in physical solution
Whilst partial pressure equilibrates, the concentration of O2 is much lower in water / solution as solubility of O2 in water is much less (0.003 ml/dl/mmHg)
Eg.
[O2] in air = 21
[O2] in water = PO2 x solubility
= 150 mmHg x 0.003
= 0.45 ml/dl
Adult haemoglobin structure
Globin molecule:
2 beta polypeptide chains
2 alpha polypeptide chains
4 Haem / Iron porphyrin compound (one per polypeptide chain)
Types of haemoglobin
Haemoglobin A (adult Hb)
Haemoglobin F (foetal Hb)
Sickle cell haemoglobin
Why is haemoglobin dissociation cure āSā shaped?
Difficult for first haem molecule to bind O2, then configuration of Hb structure changes making it easier for 2nd and 3rd haem molecules to bind O2
Haemoglobin dissociation cure
Mixed venous blood Saturations around 75% with PO2 5.3 kPa
P50 where saturations of 50% correspond with PO2 3.5 kPa
Where does mixed venous saturations correspond to
Venous blood in pulmonary artery going to lung
Saturations around 75% with PO2 around 5.3 kPa
Clinically where would we measure mixed venous saturations
Central line to get blood as close to right atrium as possible as different tissues extract different amounts of O2 (eg brain extracts a lot)
Clinical usefulness of mixed venous saturations
Can be useful with mixed shock states which include cardiogenic as if more O2 being extracted (ie mixed venous sats lower than 75%) suggests pump failure as cause for deterioration as not supplying sufficient oxygenated blood so tissues extracting more from blood
Total oxygen content in blood equation
Total [O2] = Hb bound O2 + O2 dissolved in plasma
= (1.39 x [Hb] x % saturations / 100) + (0.003 x PaO2)
Delivery of oxygen equation
DO2 = CO x CaO2
Bohr effect
Shift of Haemoglobin dissociation curve to the right, thus reducing oxygen affinity of haemoglobin
Factors which shift haemoglobin dissociation curve to the right
Increase in:
- Temperature
- [H+]
- PaCO2
- 2,3 DPG
Can remember as factors in exercising muscle - as it is advantageous for exercising muscle for Hb to unload O2
What is 2,3 DPG
Product of red cell metabolism
Carbon monoxide haemoglobin dissociation curve
CO-Hb (carboxyhaemoglobin) has extremely high affinity for O2
Therefore rapidly saturates and does not unload O2
Effect of Carboxyhaemoglobin on Haemoglobin dissociation curve
If one third of Hb is CO-Hb then the Hb dissociation curve is shifted to the left so O2 is less likely to unload O2 to peripheral tissue
Three forms that carbon dioxide is carried in the blood
1) Dissolved
2) as Bicarbonate
3) as Carbamino compounds
Solubility of CO2 in blood
0.067 ml/dL/mmHg
Carriage of CO2 in blood percentages in each form arterial vs venous blood
Formation of bicarbonate equation
C.A = carbonic anhydrase
H2CO3 = Carbonic acid
HCO3- = Bicarbonate ion
Location of carbonic anhydrase enzyme
RBC
Carbamino compounds
CO2 binds with terminal amine group of proteins in the blood
Most relevant of these proteins are haemoglobin where CO2 binds to Hb to form Carbamino haemoglobin
Formation of carbamino haemoglobin
CO2 binds to terminal amine group of Hb
Uptake of CO2 in systemic capillaries
HCO3- diffuses out of RBC, but H+ ions cannot diffuse out of RBC as membrane is impermeable
Therefore Cl- ions move back into RBC (chloride shift) to maintain electrical equilibrium (Gibbs-Donnan effect)
Haldane effect
Oxygenated Hb has lower affinity for CO2
Previous diagram - reduced Hb better for H+ ion and bicarbonate ion production
In presence of O2, CO2 dissociates from Hb and vice versa, in presence of CO2, O2 dissociates from Hb
Therefore in pulmonary blood CO2 dissociates, and in exercising muscle O2 dissociates
Comparison of O2 and CO2 dissociation curves
CO2 curve much steeper, but difference between arterial and mixed venous PCO2 much lower than the difference between arterial and mixed venous PO2
