Absorption Of Iron

Absorption Of Iron


ABSORPTION OF IRON

  • Absorbed from upper small intestine mainly duodenum.
  • In diet iron occurs in two forms –
  • Heme iron – Better absorbed than inorganic iron.
  • Inorganic (non-heme) iron – Major fraction in diet.
  • Mostly in ferric form.
  • Reduction to ferrous form required.
  • Because iron is absorbed in ferrous form.
  • After absorption, Iron transported into enterocytes.
  • Occurs via divalent metal transporter – 1 (DMT-1). 
  • Inside enterocytes,Ferrous form → Oxidized to ferric form.
  • Fraction of absorbed iron is rapidly delivered to plasma transferrin.
  • Most iron deposited in enterocytes as “Ferritin”. 
  • Excretion of iron occurs predominately via feces.

TRANSPORT OF IRON:

Transferrin – 

  • Transport glycoprotein for iron in blood.
  • Transported into cells through transferrin attachment to specific membrane-bound receptors.

STORAGE:

  • Stored as ferritin & hemosiderin.
Ferritin:
  • Major storage form.
  • Complex of iron & apoferritin.
  • (Iron + apoferritin = Ferritin).
  • Storage of iron inside ferritin is in ferric form.
  • Most of body iron is contained in hemoglobin and not in ferritin.
  • Mainly stored in reticuloendothelial cells monocytes/macrophages of liver, spleen & bone marrow.
  • Also in Hepatocytes (parenchymal cells of liver).
  • Myocytes of skeletal muscles.

REGULATION OF IRON ABSORPTION:

  • Regulated according to demand –
  • E.g., During iron deficiency → Increased absorption.
  • Regulation mediated by “Hepcidin” – 
  • Iron metabolism regulatory hormone inhibiting iron absorption.
  • On iron deficiency → Reduced hepcidin concentration → Increased iron absorption & Decreased release of iron from storage sites.

FACTORS AFFECTING IRON ABSORPTION:

1. Factors decreasing iron absorption – 

  • By 2 mechanisms:

1a. By complexing (forming chelate) with iron – 

  • Phytate, Phosphate, EDTA, Tetracycline & Milk.

1b. By opposing reduction of ferric to ferrous forms –

  • Antacids, alkalies & pancreatic secretions.

2. Factors increasing iron absorption – 

  • By enhancing reduction of ferric to ferrous form –
  • Ascorbic acid, HCl, gastric secretions & citric acid.

CONSERVATION OF IRON:

  • Mechanisms conserving & preventing iron loss –

1On RBC lysis Hemoglobin enters circulation.

  • Being small molecular weight substance, urinary loss is easy.
  • Prevented by immediate HB binding to haptoglobin Hence, preventing loss.

2. On removal of globin part from Hb → Heme produced & released into circulation.

  • Heme is bound by hemopexin Hence, preventing urinary loss.

IRON RECYCLING MECHANISM:

Exam Question

ABSORPTION OF IRON

  • Iron absorbed from upper small intestine mainly duodenum.
  • Iron is absorbed in ferrous form.
  • Excretion of iron occurs predominately via feces.
  • Transferrin – Transport glycoprotein for iron.
  • Ferritin – Major storage form.
  • Storage of iron inside ferritin is in ferric form.
  • Mainly stored in reticuloendothelial cells monocytes/macrophages of liver, spleen & bone marrow.
  • Factors decreasing iron absorption –

By complexing (forming chelate) with iron – 

  • Phytate, Phosphate, EDTA, Tetracycline & Milk.
  • By opposing reduction of ferric to ferrous forms –
  • Antacids, alkalies & pancreatic secretions.
  • Factors increasing iron absorption – 
  • By enhancing reduction of ferric to ferrous form –
  • Ascorbic acid, HCl, gastric secretions & citric acid.
  • Conservation of iron by Haptoglobin & Hemopexin.
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