Carriage of Gases 2 (Modification to O₂ transport and Carriage of CO₂) Flashcards
What is Methaemoglobin (MetHb)?
- O₂ oxidising haem from Fe²⁺ to Fe³⁺ produces MetHb
- Oxidised Fe³⁺ of MetHb cannot bind oxygen
How can MetHb be formed?
- Spontaneously or
- Side effects of drugs such as nitrates or sulphonamides
How are levels of MetHb controlled?
- Enzyme MetHb reductase
- Keeps levels at around 1.5% of total Hb
- Reductase deficiency can allow MetHb levels to rise to 25%
What is glycosylated Hb (HbA₁C)?
- When high plasma glucose leads to glucose forming covalent bonds with Hb beta chains
How does Hb glycosylation affect oxygen carriage?
- No direct effect
Define cyanosis
- Inadequate oxygenation of the blood
- Characterised by high levels of deoxygenated-Hb which are blue/purple
What two forms can cyanosis exist as?
- Peripheral cyanosis
- Central cyanosis
What is cyanosis?
- Poor tissue perfusion leading to increased oxygen extraction from available blood
What is central cyanosis?
Poor oxygenation of blood in the lungs
What is Raynaud’s disease?
- Excessive vasoconstriction in the fingers- oxygen extraction will decrease
- Blood flow decreases
Describe central cyanosis
- Affects mouth where core body temperature is
- -> Not thermoregulatory in the same way, not peripheral, easily accessible
- -> Oxygenation gone wrong
When are you likely to peripherally cyanotic?
When you are centrally cyanotic, but not necessarily vice versa.
How does polycythaemia link to cyanosis?
- Increased haemoglobin concentration
- Decreased plasma or increased erythrocytes
- Large amount of deoxygenated-Hb
- Unlikely to be hypoxic but probably cyanotic
Does anaemia lead to cyanosis?
- 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
What is the effect of carbon monoxide on haemoglobin?
- 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
How is carbon dioxide removed from tissue?
- Released from mitochondria
- Down partial pressure gradient
- Interstitium to capillary
How is dissolved carbon dioxide carried in the blood?
- Accounts for around 10% of total carriage
- Carbon dioxide is 20x more soluble than oxygen
- Proportional to PaCO₂
How is Carbamino CO₂ carried?
- 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
How is the bicarbonate ion (HCO₃⁻) carried?
- 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
What is the Haldane effect?
- 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
Describe the shape of the CO₂ dissociation curve
Linear rather than sigmoidal
How does increasing [H+] promote CO₂ excretion?
HCO₃⁻ + H+ CO₂ + H₂O
What is myoglobin?
- 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
What does myoglobin do?
- 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.
