Haematology and Immunology - RBC & Platelets

Question 1

What effect does acute anaemia have on the body?

Answer 1

Less Hb means less O2 carrying capacity, even if SpO2/PaO2 remain the same.

Equation for oxygen carrying capacity:

CaO2 = (Hb x Saturation x Huffner's Constant) + PaO2 x 0.23ml/L
(150g/L x 97% x 1.34ml) + (13.3kPa x 0.23ml/L) = 198.029ml/L

Hufner’s constant is the amount of oxygen each gram of Hb can carry (1.34ml)

Question 2

How does the body respond to acute anaemia?

Answer 2

Acute
Increase CO (HR and SV)
Splanchnic vasoconstriction
Arteriolar vasodilation (due to tissue hypoxia).
Renin-aldosterone system retains sodium and water.
Improved O₂ extraction (increased 2,3 DPG and right shift of dissociation curve)
Anaemia stimulates increased MV & release of NO to improve VQ matching
Redistribution of blood to essential organs

Chronic:
Erythropoietin production increases
Left ventricular hypertrophy
Atherosclerosis & artery remodelling
Cardiac morbidity

Question 3

How is iron metabolised by the body?

Answer 3

Absorption:
< 10% dietary iron absorbed as either Free iron or haem

Free iron
Ferric Fe³⁺ reduced to Ferrous (Fe²⁺) by acid and vitamin C in stomach
Ferrous iron soluble up to pH 7.5 in duodenum, but ferric precipitates above pH 3

Haem
Pinocytosis via haem receptors - haemoxygenase releases Fe²⁺ from haem complex.

Metabolism:
Haemoprotein formation (Cytochromes, oxidases, haemoglobin, myoglobin)
Stored as ferritin
Transported in plasma bound in ferric state to transferrin, requires pinocytosis by target cell

Excretion:
No active physiological excretion mechanism
Largely via blood loss (menstruation, GI tract, venesection).
Pharmacologically via chelation with desferrioxamine

Question 4

Describe platelets and their function

Answer 4

Cellular fragments derived from megakaryocytes, anucleic with few organelles.

2 μm diameter
α granules contain clotting factors (eg VWF & fibrinogen)
Surgace receptors to trigger activation
Life span 1 week (33% sequestered by spleen)

Function:
Primary haemostasis - activated by exposure to subendothelial collagen, causing adhesion, activation of other platelets, aggregation.
Release mediators to induce vasospasm

Secondary haemostasis - Negatively charged platelet plug sticks to fibrin clot - release of fibrinogen & VWF from α granules

Antifibrinolytic - Secretes plasminogen activator inhibitor-1 (inhibiting tPA)

Wound healing - PDGF secretion

Secrete multiple substances:
Thromboxane A2 (Stimulates G-protein receptor, cAMP production to induce vasoconstriciton & platelet activation)
α granules (Clotting factors and vwf, PDGF)
Dense granules - ADP activates platelets, Serotonin causes vasospasm and platelet activation

t-PA (Tissue plasminogen activator)
PDGF (Platelet derived growth factor)

Question 5

Describe red blood cells

Answer 5

Highly specialised, anuclear, biconcave discs, lifespan of 120 days. Approx 45% of blood volume.

Primary role to carry & deliver oxygen

Crucial role in CO2 clearance using carbonic anhydrase, employing the Bohr effect, Chloride shift, and Haldane effect to optimise oxygenation & CO2 clearance depending on tissue’s individual need.

2μm size allows it to flex & pass through narrow capillaries, facilitating diffusion.

Rely on glycolysis & anaerobic respiration (lack mitochondria)

Produced in bone marrow, stimulated by renal erythropoeitin