Respiratory Physiology - Blood gas transport

Question 1

FO2 in air

Answer 1

Fractional concentration of oxygen in air

0.21

Question 2

Barometric pressure

Answer 2

760 mmHg

Question 3

PO2 in atmospheric air

Answer 3

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

Question 4

Approximate conversion of mmHg to kPa

Answer 4

mmHg divided by 7.5 roughly equals kPa

Question 5

What changes water vapour pressure in air

Answer 5

Temperature

Question 6

Partial pressure of oxygen in physical solution

Answer 6

Partial pressure equilibrates between air and water
Therefore when PO2 in air = 150 mmHg, PO2 in water / solution = 150 mmHg

Question 7

Concentration of oxygen in physical solution

Answer 7

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

Question 8

Adult haemoglobin structure

Answer 8

Globin molecule:
2 beta polypeptide chains
2 alpha polypeptide chains

4 Haem / Iron porphyrin compound (one per polypeptide chain)

Question 9

Types of haemoglobin

Answer 9

Haemoglobin A (adult Hb)
Haemoglobin F (foetal Hb)
Sickle cell haemoglobin

Question 10

Why is haemoglobin dissociation cure ‘S’ shaped?

Answer 10

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

Question 11

Haemoglobin dissociation cure

Answer 11

Mixed venous blood Saturations around 75% with PO2 5.3 kPa

P50 where saturations of 50% correspond with PO2 3.5 kPa

Question 12

Where does mixed venous saturations correspond to

Answer 12

Venous blood in pulmonary artery going to lung

Saturations around 75% with PO2 around 5.3 kPa

Question 13

Clinically where would we measure mixed venous saturations

Answer 13

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)

Question 14

Clinical usefulness of mixed venous saturations

Answer 14

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

Question 15

Total oxygen content in blood equation

Answer 15

Total [O2] = Hb bound O2 + O2 dissolved in plasma

= (1.39 x [Hb] x % saturations / 100) + (0.003 x PaO2)

Question 16

Delivery of oxygen equation

Answer 16

DO2 = CO x CaO2

Question 17

Bohr effect

Answer 17

Shift of Haemoglobin dissociation curve to the right, thus reducing oxygen affinity of haemoglobin

Question 18

Factors which shift haemoglobin dissociation curve to the right

Answer 18

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

Question 19

What is 2,3 DPG

Answer 19

Product of red cell metabolism

Question 20

Carbon monoxide haemoglobin dissociation curve

Answer 20

CO-Hb (carboxyhaemoglobin) has extremely high affinity for O2

Therefore rapidly saturates and does not unload O2

Question 21

Effect of Carboxyhaemoglobin on Haemoglobin dissociation curve

Answer 21

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

Question 22

Three forms that carbon dioxide is carried in the blood

Answer 22

1) Dissolved
2) as Bicarbonate
3) as Carbamino compounds

Question 23

Solubility of CO2 in blood

Answer 23

0.067 ml/dL/mmHg

Question 24

Carriage of CO2 in blood percentages in each form arterial vs venous blood

Answer 24

Question 25

Formation of bicarbonate equation

Answer 25

C.A = carbonic anhydrase

H2CO3 = Carbonic acid

HCO3- = Bicarbonate ion

Question 26

Location of carbonic anhydrase enzyme

Answer 26

RBC

Question 27

Carbamino compounds

Answer 27

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

Question 28

Formation of carbamino haemoglobin

Answer 28

CO2 binds to terminal amine group of Hb

Question 29

Uptake of CO2 in systemic capillaries

Answer 29

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)

Question 30

Haldane effect

Answer 30

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

Question 31

Comparison of O2 and CO2 dissociation curves

Answer 31

CO2 curve much steeper, but difference between arterial and mixed venous PCO2 much lower than the difference between arterial and mixed venous PO2