Carriage of Gases 2 (Modification to O₂ transport and Carriage of CO₂)

Question 1

What is Methaemoglobin (MetHb)?

Answer 1

• O₂ oxidising haem from Fe²⁺ to Fe³⁺ produces MetHb

- Oxidised Fe³⁺ of MetHb cannot bind oxygen

Question 2

How can MetHb be formed?

Answer 2

• Spontaneously or

- Side effects of drugs such as nitrates or sulphonamides

Question 3

How are levels of MetHb controlled?

Answer 3

• Enzyme MetHb reductase
• Keeps levels at around 1.5% of total Hb
• Reductase deficiency can allow MetHb levels to rise to 25%

Question 4

What is glycosylated Hb (HbA₁C)?

Answer 4

• When high plasma glucose leads to glucose forming covalent bonds with Hb beta chains

Question 5

How does Hb glycosylation affect oxygen carriage?

Answer 5

• No direct effect

Question 6

Define cyanosis

Answer 6

• Inadequate oxygenation of the blood

- Characterised by high levels of deoxygenated-Hb which are blue/purple

Question 7

What two forms can cyanosis exist as?

Answer 7

• Peripheral cyanosis

- Central cyanosis

Question 8

What is cyanosis?

Answer 8

• Poor tissue perfusion leading to increased oxygen extraction from available blood

Question 9

What is central cyanosis?

Answer 9

Poor oxygenation of blood in the lungs

Question 10

What is Raynaud’s disease?

Answer 10

• Excessive vasoconstriction in the fingers- oxygen extraction will decrease
• Blood flow decreases

Question 11

Describe central cyanosis

Answer 11

• Affects mouth where core body temperature is
• -> Not thermoregulatory in the same way, not peripheral, easily accessible
• -> Oxygenation gone wrong

Question 12

When are you likely to peripherally cyanotic?

Answer 12

When you are centrally cyanotic, but not necessarily vice versa.

Question 13

How does polycythaemia link to cyanosis?

Answer 13

• Increased haemoglobin concentration
• Decreased plasma or increased erythrocytes
• Large amount of deoxygenated-Hb
• Unlikely to be hypoxic but probably cyanotic

Question 14

Does anaemia lead to cyanosis?

Answer 14

• Decreased Hb concentration
• Decreased haemtocrit
• Already low Hb, not enough deoxy-Hb
• Lack of cyanosis in anaemic patient does not imply adequate tissue oxygenation
• Those who are cyanotic are likely to be hypoxic
• Not necessarily vice versa

Question 15

What is the effect of carbon monoxide on haemoglobin?

Answer 15

• Haem group can bind to CO
• Affinity for CO is 240x greater than oxygen
• Hb cannot bind both, so binds to CO preferentially
• Cherry red colour
• Shows same saturation

Question 16

How is carbon dioxide removed from tissue?

Answer 16

• Released from mitochondria
• Down partial pressure gradient
• Interstitium to capillary

Question 17

How is dissolved carbon dioxide carried in the blood?

Answer 17

• Accounts for around 10% of total carriage
• Carbon dioxide is 20x more soluble than oxygen
• Proportional to PaCO₂

Question 18

How is Carbamino CO₂ carried?

Answer 18

• Accounts for around 10% of total carriage
• Formed from reactions of CO₂ with protein amine groups
• Most abundant type- carbaminohaemoglobin- occurs in deoxy-Hb
• CO₂ carried by Hb but bound to globin chains not haem

Question 19

How is the bicarbonate ion (HCO₃⁻) carried?

Answer 19

• Accounts for 80%
• Formed from reaction
  CO₂ + H₂O –> H+ + HCO₃⁻)
• Reaction occurs slowly in plasma
• Catalysed by carbonic anhydrase in erythrocytes
• Dissociation of carbonic acid is spontaneous

Question 20

What is the Haldane effect?

Answer 20

• Oxygenated Hb is a stronger acid than deoxy-Hb
• During blood passage through lungs, O₂ taken up, increasing [H+]
• Promotes CO₂ excretion
• Bohr effect in tissue, Haldane in lungs

Question 21

Describe the shape of the CO₂ dissociation curve

Answer 21

Linear rather than sigmoidal

Question 22

How does increasing [H+] promote CO₂ excretion?

Answer 22

HCO₃⁻ + H+ CO₂ + H₂O

Question 23

What is myoglobin?

Answer 23

• Similar to haemoglobin
• Found in striated muscle
• One globin chain and a haem group- however not same globin chain in Hb
• Higher O₂ affinity than Hb

Question 24

What does myoglobin do?

Answer 24

• Facilitates diffusion of oxygen from plasma membrane to mitochondria
• Acts a store to some extent, but does not hold a large amount
• 1-2 heart beats, occasional interruption in heart myoglobin provides small storage for continuous oxygen flow.

Question 25

Describe nitrogen in body fluids

Answer 25

• Oxygen is twice as soluble as nitrogen
• Atmospheric PN₂ is 4x than that of PO₂
• Therefore, body contains 2x as much N than O, not an issue as N is inert

Question 26

Describe the role of nitrogen in Caisson Disease

Answer 26

• AKA ‘Bends’
• Common amongst divers
• At higher pressures, N solubility increases
• Lower pressures cannot maintain N in solution- escapes
• Rapid ascent does not allow equilibrium- excessive N₂ release- caisson disease
• Avoided by slow descent

Question 27

What is the treatment for Caisson disease?

Answer 27

• Re-pressurise in order to re-dissolve gas