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Red blood cells
Claudia Cheng
Updated in
August 2006
Functions
1. Oxygen carriage – RBC help uptake,
transport oxygen from the lungs and release oxygen to the tissues. This is made
possible by the following mechanisms:-
- Haemoglobin
- Hb is synthesize in the RBC
- Hb allows transportation of
much greater amounts of oxygen than if oxygen were to be dissolved in
plasma alone. 1 molecule of Hb can carry 4 molecules O2. At
atmospheric pressure, 98% of O2 in blood is carried by Hb
- Hb-Adair effect – positive
cooperativity whereby reaction of the 4 subunits of Hb and O2 occur
sequentially, each facilitating the next so that the 4th unit
reaction is the fastest despite less binding areas for O2
- Hb has high affinity for oxygen
– modifying factors include Bohr effect (increased in H+, temperature
and pCO2.) and 2,3-DPG will increase release of O2.
in the tissues
- Relationship of pO2
and % saturation of HbO2 is sigmoid-shape and the p50 (usual
is 26 mmHg) can be used to define the position of the curve, moving it
to the left or right on oxygen-dissociation curve: p50 is the pO2
at which the blood is 50% saturated with oxygen
- RBC needed in O2 transport
rather than Hb alone as free Hb can be toxic: free Hb will increase the
plasma osmolality, globin chains can blocked renal tubules and cause
renal failure
- RBC shape
- Biconcave disc that allows a
large surface area relative to its volume. This promotes gaseous
diffusion into cell
- Flexibility – able to squeeze
into the capillaries to allow oxygen delivery to the tissues
- 2,3-diphosphoglycerate (DPG)
synthesis from RBC
- Allows Hb to unload O2
by reducing affinitry of Hb for O2
- 2,3-DPG is synthesize from the
Rapaport-Luebering shunt
2. CO2
removal
- In the tissue capillaries, oxyHb
releases O2 and becomes deoxyHb
- DeoxyHb is a better buffer than
oxyHb as it dissociates more
- DeoxyHb then helps remove the CO2
and H+ that is produced in the tissues with the help of carbonic anhydrase
- H+ is taken up by the deoxyHb and
CO2 can enter venous blood
- As a result, venous blood is only
slightly acidic than arterial blood despite large amounts of CO2
being produced/day. This process is referred to as “isohydric buffering”
- 30% or CO2 is carried in
venous blood as carbamino-Hb compounds in the RBC
- Carbamino compounds are formed by
CO2 and carbonic acid combine with terminal amino groups of the
amino acids of Hb
HbNH2
+ CO2 ® HbNHCOO - + H+
HbNH2
+ H2CO3 ® HbNH3 +
+ HCO3-
- The Haldane effect allows deoxyHb
to carry more CO2 and increase the formation of carbamino
compounds
3. Hb in the RBC acts as a buffer
- This is a powerful buffering
system. It is the primary non-bicarbonate buffer of blood for both
respiratory and metabolic acids
- Hb is present in high concentration
inside RBC and Hb in turn has a high concentration of imidazole side chains
that are responsible for its buffering activity
- Hb in RBC is present as a weak acid
(HHb) and its potassium salt (KHb)
- H+ is buffered by Hb and
HCO3- is produced in the RBC. The bicarbonate in the
RBC diffuse out of RBC into plasma to maintain electrical neutrality
4. Red cell metabolism
- Energy generation
- Generates ATP for energy to
maintain RBC shape and flexibility
- Via anaerobic glycolytic
pathway (Embden-Meyerhof pathway)
- 2 ATP for each molecule glucose
- NADPH and NADH generation
- Iron in Hb must be maintained
in the ferrous state in order to bind O2. Ferric form of Hb =
Met Hb and is unable to bind O2
- NADPH is a major reducing agent
in the RBC and protects against oxidation. NADPH is generated for the
hexose monophosphate shunt and is required by the RBC to maintain
reduced glutathione
- NADH is a cofactor in the metHb
reductase reaction to reduce MetHb to Hb
5. Iron metabolism – role of RBC as a
transporter for iron
- Hb contains 65-70% of total body
iron
- Old RBCs are removed by the
reticuloendothelial system mainly spleen and haem is reutilized by the
marrow to make Hb
6. Other functions
- RBC esterases involved in
metabolism of some drugs
- Binding of nitric oxide to form S-nitrosohemoglobin
(SNO-Hb) with function in linking regulation of vascular tone to tissue
oxygenation
- ABO compatibility – antigen present
on the surface of RBC
Ó Claudia Cheng August 2006
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