Erythrocytes

Erythrocytes


ERYTHROCYTES (RED BLOOD CELLS)

RBC – 

  • Non-nucleated biconcave disc.
  • Mean diameter – 7.5 μm.
  • Thickness – 2 μm at periphery.
ERYTHROPOIESIS:
  • Process of RBC formation.

In early fetal life (up to 10 weeks) – 

  • Occurs in yolk sac.

By third month of intrauterine life – 

  • Moves to liver (major) & spleen.
  • Chief site for erythropoiesis until 5 months of fetal life.
  • Later in bone marrow.
  • In children, 
  • Blood formation occurs in most of bone marrow cavities.
  • Gradual decrease of erythropoiesis in marrow of long bones.
  • Gradual increase in marrow of flat bones.

At 20 years – 

  • Only in flat bones of axial skeleton.
  • Eg., Skull, vertebrae, ribs, sternum & pelvis.

Among long bones – 

  • Only in proximal end of femur & humerus.

RBC LIFE SPAN:

  • In adult – 120 days.
  • Neonatal – 60-90 days.
  • Fetal – 80 days (2/3rd of adult RBC lifespan)

ERYTHROCYTE DIFFERENTIATION:

  • RBC on maturation undergoes series of differentiation.

Stages of development within bone marrow (sequentially) –

  • Pluripotent hematopoietic stem cell.
  • Multipotent stem cell.
  • Unipotent stem cell.
  • Pronormoblast.
  • Early normoblast.
  • Intermediate normoblast
  • Late normoblast.
  • Reticulocyte.
  • After these stages, cell from bone marrow is released.
  • Ultimately becomes an “Erythrocyte”/”Mature” RBC.
  • Circulates in peripheral blood.

CHANGES IN RBC ON MATURATION:

  • Reduction in cell size.
  • Cytoplasm changes from basophilic to acidophilic.
  • Nucleus disappears at reticulocyte stage.
  • Because of extrusion in late normoblast stage.
  • Hence, during excessive erythropoiesis (eg: erythroid hyperplasia of bone marrow), 
  • Nucleated cells (immature cells) released into circulation.
  • Hemoglobin appears at intermediate normoblast stage.
  • Mitosis is most active & occurs up to intermediate normoblast stage.
  • Late normoblast is not capable of mitotic division.

HEMOGLOBIN:

  • Most important red cell constituent.

Hb molecule – 

  • Consists of four globular protein.
  • Each subunit has protein (polypeptide) part.
  • I.e., Globin & nonprotein part (heme)
  • Hence, each Hb molecule contains four heme units
  • 2 pairs of globin 
  • Ie., 4 polypeptide chains.
  • Heme part of globular protein is same in all types of hemoglobin.

Protein part vary in different hemoglobin – 

  • Adult hemoglobin (Hb A) – 
  • Consists of 2 identical α-chains & 2 identical β-chains.
  • Fetal hemoglobin (Hb F) – 
  • 2 Identical α-chains & 2 identical γ-chains.
  • Minor hemoglobin (HbA2) – 
  • 2 identical α-chains & 2 identical δ-chains.

Adult blood – 

  • About 96% HbA (αβ2), 3-3.5% HbA2 (αδ2), & small amount of fetal Hb HbF (α2 γ2).
  • Fetal Hb binds to 2-3 DPG less avidly than adult Hb.
  • Hence, more oxygen affinity.
  • Fetal Hb – Resistant to alkali denaturation.
  • Alkali denaturation test (ADT) – Done to identify fetal Hb.

RBC DESTRUCTION:

  • About 1% of RBC’s -Older & abnormal ones.
  • Removed daily from circulation by macrophages.
  • Mainly from spleen (major), liver & bone marrow.
  • Inside macrophage, heme of hemoglobin is degraded to bilirubin.
  • After iron removal.
  • First heme is converted to biliverdin by heme oxygenase.
  • Biliverdin is converted to bilirubin by biliverdin reductase.
  • Certain metalloproteins (like zinc-or tin-porphyrins) – Used in jaundice.
  • Helps decrease bilirubin production.
  • By inhibiting enzyme heme oxygenase.
Exam Question
 

ERYTHROCYTES (RED BLOOD CELLS)

ERYTHROPOIESIS: 

  • In early fetal life (up to 10 weeks) – Occurs in yolk sac.
  • By third month of intrauterine life – Moves to liver (major) & spleen.
  • Later in bone marrow. 

At 20 years –

  • Only in flat bones of axial skeleton (Eg., Skull, vertebrae, ribs, sternum & pelvis).

RBC LIFE SPAN:

  • Neonatal – 60-90 days.
  • Fetal – 80 days (2/3rd of adult RBC lifespan).

CHANGES IN RBC ON MATURATION:

  • Reduction in cell size.
  • Hemoglobin appears at intermediate normoblast stage.
  • Mitosis is most active & occurs up to intermediate normoblast stage.

HEMOGLOBIN: 

  • Hb molecule – An assembly of four globular protein.
  • Also has nonprotein part (heme). 
  • Each Hb molecule contains four heme units.
  • 2 pairs of globin.
  • Ie., 4 polypeptide chains.

Minor hemoglobin (HbA2) – 

  • 2 identical α-chains.
  • 2 identical δ-chains. 

Adult blood – 

  • About 96% HbA (α2 β2).
  • 3-3.5% HbA2 (α2 δ2).
  • Small amount of fetal Hb HbF (α2 γ2).
  • Fetal Hb binds to 2-3 DPG less avidly than adult Hb.
  • Fetal Hb – Resistant to alkali denaturation.
  •  Alkali denaturation test (ADT) – Done to identify fetal Hb.

RBC DESTRUCTION:

  • RBC removed daily from circulation by macrophages.
  • Mainly from spleen (major), liver & bone marrow.
  • Inside macrophage, heme of hemoglobin is degraded to bilirubin.

Certain metalloproteins (like zinc-or tin-porphyrins) – 

  • Used in jaundice.
  • Helps decrease bilirubin production by inhibiting enzyme heme oxygenase.
Don’t Forget to Solve all the previous Year Question asked on Erythrocytes

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