Theme 2: Lecture 1 - Erythrocytes (Red Blood Cells) Flashcards
Why is Hb needed to carry O2?
It ‘s poorly soluble in plasma
How much moreO2 is carried on Hb than dissolved in the plamsa in normal arterial blood?
70X
How is it possible for arterial PO2 to be normal but hypoxia to occur?
If there is no Hb to carry the O2
Why do you need O2?
Oxidative phosphorylation produces more energy than anaerobic glycolysis
Cooperativity
- Biochemical phenomenon displayed by proteins with multiple subunits, which depend on each other
- Enzymes or receptors that have multiple binding sites have an increased or decreased affinity to a ligand upon binding a ligand at a different binding site
- I.e. O2 is more likely to bind to a Hb that already has one O2 bound to it than a Hb that doesn’t
- O2 binding leads to more O2 binding and O2 release leads to more O2 release
What does Hb do?
Picks up O2 in the lungs and releases it in the tissues
How much of a RBC is Hb
95% of dry weight
Describe Hb
- Each Hb has 4 subunits
- Each subunit has a small haem group (616 Da) and a large globin group (17 000 Da)
- Has allosteric properties: cooperativity
Describe haem
- A porphyrin ring
- Rigid
- 2D
- Highly coloured due to presence of iron
- Conjugated to one Ferrous Fe2+ (not Ferric Fe3+)
- The site of O2 binding
HbA
- Made up of 2 alpha subunits and 2 beta subunits
- AKA maternal Hb
HbF
- Made up of 2 alpha subunits and 2 gamma subunits
- Binds O2 more strongly than HbA
What Hb do healthy adults have?
Mostly HbA and a small percentage of HbF
The Bohr effect
- Because of the carbonic anhydrase reaction: A higher blood CO2 level leads to a lower blood pH
- A higher blood CO2 level and lower pH leads to a lower affinity of Hb to O2
- CO2 and H+ bind Hb but at a different site from O2
How is CO2 transported in the blood?
- 10% dissolved in plasms
- 22% as carbamino Hb
- 68% as HCO3-
Carbamino Hb
CO2 combines reversibly with Hb-NH2 to form Hb-N-H(COOH) to transported in the blood
How is CO2 carried as HCO3- in the blood?
- CO2 diffuses into RBC
- CO2 combines with H2O in the carbonic anhydrase reaction to form H+ and HCO3-
- Hb- combines with H+ to form HHb (Hb essentially acts as a buffer)
- A band 3 protein (chloride bicarbonate exchanger) transports HCO3- out of the cell and Cl- into the cell
Chloride shift
More Cl- inside RBCs in venous blood than in arterial blood due to transport of CO2
Myoglobin
Similar to haemoglobin but found in the muscle and doesn’t have multiple subunits
Oxygen binding curve of myoglobin shape
Hyperbolic due to it having a high affinity to oxygen
Oxygen binding curve of haemoglobin shape
Sigmoidal (S) shape due to cooperativity of haemoglobin
What causes Hb to have a lower affinity for O2
- CO2
- H+
- Cl-
- 2,3-DPG
Therefore muscle activity encourages Hb to release O2
What is 2,3-DPG
2,3 diphosphoglycerate OR 2,3 bisphosphoglycerate OR 2,3 BPG
What does 2,3-DPG do
Binds to Hb and lowers the affinity for O2
At what concentration is 2,3-DPG found in erythrocytes
5mM (quite high)
Relationship between foetal Hb and 2,3-DPG
- Foetal Hb has a lower affinity for 2,3-DPG than adult Hb
- This means foetal Hb has a higher binding affinity for O2 than HbA
What is the main driver to increase respiratory rate?
H+ in the CSF (not H+ in the blood)
How is respiratory rate increased
- CO2 gas can get into CSF (H+ in blood is slow to get to CSF)
- Once CO2 is in the CSF it makes carbonic acid and H+
- Medullary receptors (how the brain controls breathing) sample from the interstitial and CSF
Erythrocytes appearance
- Biconcave disc
- Anucleate, lack organelles
- 7 um diameter, 2 um height
- Contain haemoglobin
- Red when oxygenated
Erythrocytes function
- “Bag” of haemoglobin
- Transport O2 and CO2
- Survive ~120 days
- Very flexible – fold and stack in blood vessels
Haematocrit/Packed cell volume (PCV)
measurement of the proportion of blood that is made up of cells
Mean Cell Haemoglobin (MCH)
Amount of Hb in a single cell
Mean corpuscular volume (MCV)
value that describes the average size of red blood cells
Normal PVC range
Males: 40-52%
Females: 36-48%
Normal MCH
27-34 pg
Normal MCV
80-100 fL
Normal Hb range
Males: 13-17 g/dL
Females: 12-16 g/dL
Erythropoiesis
The development of RBCs
Where does erythropoiesis take place in the body after birth
- The bone marrow only
- After 20, in membranous bone only
Where does erythropoiesis take place in the body in embryogenesis
- Liver
- Spleen
- Lymph node
- Yolk sac
Haematopoiesis
The development of all blood cells
Describe the general process of development from stem cells
Multipotent stem cells => Multipotent progenitor cells => Lineage committed progenitor cells => Mature cells
Erythroblast
Nucleated cell in the stages of development into a RBC
What stages does a haematopoietic stem cell go through to mature into a erythrocyte
- Haematopoietic stem cell
- Burst forming unit-erythroid (BFU-E)
- Colony forming unit-erythroid (CFU-F)
- Erythroblast
- Reticulocyte
- Erythrocyte
Which stages of erythropoiesis are dependant on erythropoietin?
Burst forming unit-erythroid to erythroblast
Which stages of erythropoiesis are iron dependant?
Erythroblasts to erythrocytes
What is erythropoietin (EPO)
A cytokine/hormone that drives erythropoiesis
Where is (EPO) made
In the kidney in response to hypoxia in the kidney
What is EPO used for?
- Medically to stimulate erythropoiesis (there’s a risk of severe off target effects)
- Performance enhancing drug for athletes
What are reticulocytes
-RBC precursor before the nucleus and organelles are expelled
How long do reticulocytes last in the blood
2 days (become RBCs after that)
How is the reticulocyte count used as a diagnostic in anaemias
- Indicator of bone marrow activity
- High in haemolytic anaemias (homeostatic response)
- Low when erythropoiesis is low
- Machine counts cells and detects those with basophilic material (eg DNA) in them
What is methaemoglobinaemia
- Hb cannot transport O2
- Fe in haemoglobin is oxidized (Fe3+) instead of usual ferrous (Fe2+)
What causes methaemoglobinaemia
- Congenital globin mutations (Hb M)
- Hereditary decrease of NADH
- Toxic substances
Carbon Monoxide poisoning
- Hb cannot transport O2
- Due to CO Displacing O2 from Hb as they have the same binding site
- Affinity for CO is 250X stronger
- Low levels of CO can completely displace most O2
- PO2 dissolved in blood remains normal
- Lethal
Treatment for CO poisoning
- 95% O2 and 5% CO2
- The CO2 makes the Hb have a lower affinity for CO, this helps to temporarily knock off the CO molecules so the high levels of o2 can grab onto any vacated spaces
Polycythaemias
- Increased number of RBCs (PCV)
- Causes increased viscosity of blood
- Clogs up blood vessels
Physiological polycythaemia
Increased number of RBCs caused by living at high altitude
Polycythaemia vera
- Is a neoplasm
- Often asymptomatic
- Risk of thrombotic events
- Affects all ages but risk increases with age
- Possibly genetic
Treatment for polycythaemia vera
- No cure
- Treated with venesection
Venesection
Removal of excess blood by blood letting
Why is Vitamin B12 and Folic acid important
- Important for rapidly dividing tissue
- Essential for forming DNA (thymidine)
- Nuclear maturation fails (if deficient)
- Important in RBCs, Skin, Gametogenesis
What does Vitamin B12 and Folic acid deficiency cause?
-Megaloblastic anaemia (macrocytic)
These are large, fragile RBCs cells
What causes Vitamin B12 and Folic acid deficiency?
- Diet
- Malabsorption
- Increased utilisation
Pernicious anaemia
Vitamin B12 deficiency causing anaemia because the stomach fails to produce intrinsic factor which is essential for the absorption of B12 from the digestive tract
What is vitamin B12 and Folic acid deficiency treated by?
- Oral folic acid
- Intramuscular hydroxocobalamin
How is iron stored intracellularly
- Ferritin - An immediately form of stored iron, it’s small and dispersed
- Haemosiderin - A more insoluble storage form made when the apoferritin stores are overwhelmed, forms in large clusters
Where is iron stored in the body
In reticulo endothelial system:
Liver, spleen, erythrocytes, bone marrow, macrophages/monocytes
What does depleting all the iron stores in the body lead to?
Anaemia, specifically microcytic and hypochromic cells
Why is it easy to be iron deficient
- Only a small % of dietary iron absorped
- Lose roughly 1mg of iron a day (more in pregnancy, menstruation, peptic ulcers)
