Iron metabolism
- Iron deficiency is commonest cause of anaemia
- Body has limited ability to absorb iron
- Excess loss of iron through bleeding is common
Body iron distribution
- 65% of body iron is in haemoglobin
- 30% is in ferritin and haemosiderin
- 3% is in myoglobin
- Erythropoesis occurs in bone marrow
- Iron is transported to bone marrow by plasma transferrin
- Transferrin obtains iron mainly from reticulo-endothelial system
(RES)
- Only small proportion comes from dietary iron
- Some iron is stored in RES as ferritin and haemosiderin
Ferritin
- Ferritin is a water-soluble protein-iron complex
- Made up of apoferritin and iron-phosphate-hydroxide core
- 20% of its weight is iron
- Synthesis is stimulated by the presence of iron
- Iron is in the ferric (3+) form
Haemosiderin
- Haemosiderin is an insoluble protein-iron complex
- 40% of its weight is iron
- Formed by lysosomal digestion of ferritin
Dietary iron
- Iron is present in food as ferric hydroxide and ferric-protein
complexes
- Meat and liver is good source of dietary iron
- Average western diet contains 10-15mg of iron
- 5-10% is absorbed in duodenum and jejunum
- Absorption is increased in pregnancy and iron-deficiency states
- Daily iron requirements are 1-2 mg per day
Iron absorption
- Absorption is favoured by acid and reducing agents
- Better absorbed in ferrous (2+) form
- The amount of iron absorbed is controlled in the epithelial cells
- Excess iron forms ferritin and is shed with the cells into the gut
lumen
- Iron enter the plasma in the ferric form
Iron transport
- Most iron is transported to the bone marrow
- Used in mainly for erythropoesis
- Binds to transferrin in the portal blood
- About 6g of haemoglobin are produced each day
- Requires about 20mg of iron
- Total plasma iron turns over about seven times per day
Transferrin
- Transferrin is a beta-globulin
- Synthesised in the liver
- Half-life of 8-10 days
- Each molecule binds two iron atoms
- Normally only about 30% saturated
- Erythroblasts have transferrin receptors
Iron deficiency
- Usually due to chronic blood loss
- Occurs from GI tract or associated with menorrhagia
- Rarely due to dietary deficiency or malabsorption
- Asymptomatic until RES stores of iron are depleted
Laboratory features
- MCV, MCH and MCHC are all reduced
- Cells appear hypochromic and microcytic
- Target cells and pencil-shaped poikilocytes are seen
- Reticulocyte count is low relative to the degree of anaemia
- Platelet count maybe raised
- Serum iron is low
- Total iron binding capacity is increased
- Serum ferritin is low
- If bone marrow examination is performed, iron is absent from
macrophages
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