Structure and Function of Normal RBCs Flashcards
Why do RBCs have no nucleus?
makes it more deformable
=> more room for Hb molecules
RBCs have no mitochondria to make energy. TRUE/FALSE?
TRUE
- can only use glycolysis for energy
Why do RBCs have a high surface area to volume ratio?
To allow for gas exchange
Why must RBCs be flexible?
To squeeze through capillaries
What problems can occur due to the RBC structure?
Full of haemoglobin => High oncotic pressure (pulls water in) also free radicals can form from oxygen rich environment
No nucleus => Can’t divide => limited cell lifespan
No mitochondria => glycolysis for energy generation
Flexible => Specialised membrane required can go wrong
Describe the structure of the specialised RBC membrane
Not just a lipid bilayer
- Protein ‘spars’ anchored to membrane by proteins (ankyrin etc)
What ion pump is responsible for keeping water OUT of RBCs?
Na/K Pump
- 3 Na out and 2 K in
- sets up membrane potential
- doesn’t allow water in
Describe the structure of haemoglobin
- tetramer protein
- in Adult = 2 alpha and 2 beta chains
- Heme group in centre composed of Fe2+ in a flat porphyrin ring
What are the main functions of haemoglobin?
- Deliver oxygen to the tissues
- Act as a buffer for H+
- CO2 transport
What is produced by the kidney which regulates RBC production?
Erythropoietin
- this increases RBC production if it senses relative or absolute hypoxia
Where are red cells destructed and how long do they usually live for?
Normally occurs in spleen (and liver) - average red cell lifespan 120 days
HOw are RBCs removed from the circulation?
- Aged red cells engulfed by macrophages
- contents are recycled
=> Globin chains recycled to amino acids
=> Heme group broken down to iron and bilirubin - Bilirubin taken to liver, conjugated and excreted in bile
Why are free radicals dangerous for RBCs?
- Can oxidise Fe2+ to Fe3+ which doesn’t transport oxygen
- Can damage proteins (these cant repair/replace once damaged)
What molecule created during glycolysis acts as a “sacrifice” to stop Fe2+ from oxidising to Fe3+?
NADH
- it gets oxidised instead
How does the RBC use glycolysis to stop damage from free radicals?
- superoxides are turned into hydrogen peroxide (still damaging)
- glutathione is sacrificed to hydrogen peroxide
- NADPH is sacrificed to replenish glutathione stores
=> Hexose Monophosphate shunt
WHat is the rate limiting step in the hexose monophosphate shunt and why is this clinically relevant?
Rate limiting enzyme = glucose-6-phosphate dehydrogenase (G6PD)
- this enzyme is coded on the X chromosome, therefore if patients have a genetic deficiency of this enzyme they will have early blood breakdown
How is CO2 transported in the blood?
10% is dissolved in solution
30% is bound directly to Hb (carbamino-Hb)
60% = bicarbonate
How many Oxygen molecules are found in one haemoglobin molecule, and how many of these are bound to the Fe2+ of the heme group?
4 O2 molecules per Hb
BUT only 1 bound to Fe2+
Hb in patients who are not adults (e.g. in a foetus) have different subunits of Hb for different oxygen carrying capacities. What subunits are found in foetal Hb (HbF)?
HbF
- two alpha, two gamma
- picks up less O2 as baby does not yet need to use lungs to breath in utero
Why is the oxygen dissociation curve sigmoidal?
Want high O2 carrying in high partial pressures O2 e.g. lungs
And low O2 carrying capacity in tissues where O2 needs to be released
=> As O2 binds to subunit, Hb shape changes
=> makes it easier for next O2 to bind
=> Cooperative binding
Why does foetal Hb (a2g2) saturate more at the same pO2 as in an adult?
Takes O2 from the maternal circulation
How can the introduction of other small molecules influence the dissociation curve?
Small molecules burrow into Hb and change the shape
=> make it harder for Hb to pickup oxygen
=> more free in tissues
shifts curve to the right
What small molecules can shift the dissociation curve to the right?
2,3-BPG (increases in chronic anaemia)
H+ ions
CO2
