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The RBCs carry haemoglobin (Hb).

Morphology and dimensions:

  1. Shape : 
    Biconcave, non-nucleated, very elastic and highly flexible when going through capillaries
  2. Diameter :
    6.5 to 8.8 micron (average = 7.5 micron)
  3. Thickness :
    at the center : 1 micron
    at the periphery : 2.2 microns
    average thickness : 2 microns.
  4. Surface area :
    135 to 140 square microns; the surface area is greatly increased by the biconcave shape (The surface area is much greater than if its volume were contained in a sphere).
    Volume : 90 cubic microns.
  5. Advantage of biconcave shape
    It is more resistant to fragility
    1. it does not get damaged as it passes through capillaries (it assumes a sausage or parachute shape while going through capillaries)
    2. its surface area increases; it helps in more efficient gas exchange.

  1. Average life span in the circulation : 120 days; half-life : 60 days.
    Male : 5.4 millions/cumm of blood
    Female : 4.8 millions/cumm of blood
    (note : 1 cumm = 1 microlitre)
    Each RBC has 20 pg of Hb.
    In adult man has 3 x 1013 RBCs and about 900 gm of Hb.
  2. Energy supply
    Energy to the RBC is provided by :
    -  glycolysis ( 80 %)
    -  pentose phosphate pathway ( 20%)
    Energy is required to maintain the ionic gradient across its membrane and to keep the iron in the ferrous (Fe2+) state.
  3. Permeability of the RBC membraneThe viability of RBC depends on the integrity of its membrane. It is freely permeable to water, sodium, potassium and chloride. But a Na-K pump keeps the intracellular sodium low and potassium high.  The energy for the pump is provided by the membrane ATPase which requires magnesium, sodium and potassium for full activation. ATP is formed during glycolysis and its hydrolysis provides the energy for the sodium pump. When RBC metabolism ceases (as in cold-stored blood), the ions move between plasma and cells according to their concentration gradients.
  4. Production of RBC
    Site: Foetus
    First trimester
    In the early embryo, blood formation takes place first in the mesoderm of the yolk sac (the area vasculosa) and later in the body of the foetus. It is called as the mesoblastic stage of erythropoiesis.
    In the mesoblastic stage, the erythropoiesis takes place intravascularly; the endothelial cells themselves get converted into nucleated RBC and get released in the circulation; in the circulation, they lose their nuclei.
  5. Note :
    Mesoblastic stage is the only stage where RBC is formed intravascularly; later, it is extravascular.
  6. Second trimester
    This stage of erythropoiesis is called the hepatic stage. In this stage, the sites of production are spleen and liver (especially liver). Nucleated RBCs develop from the mesenchyme between the blood vessels and the tissue cells.
  7. Third trimester
    About the middle of foetal life, the bone marrow begins to act as a blood-forming organ. After this, the function of the bone marrow (in RBC production) increases and that of the liver decreases.
  8. Adult
    The only site of production is the bone marrow, however, if bone marrow is destroyed, then extramedullary erythropoiesis takes place in the liver and spleen. Before it enters the circulation from the bone marrow, it loses its nucleus. Thus, the peripheral blood has non-nucleated RBCs.
    At birth, all the bones are filled throughout their length with red marrow. With increasing age, the marrow becomes more fatty (i.e. red marrow becomes yellow marrow). This process first starts in the distal bones of limbs (tarsus and carpus); then in the intermediate bones (tibia, fibula, radius and ulna); finally, in the proximal bones (femur and humerus).
    At age 20 years, all marrow in the long bones is yellow except in the upper end of the femur and humerus.
    In adults, red marrow persists mainly in the vertebrae, sternum, ribs, skull and pelvis bones.
    Weight for weight, children have more red marrow than adults. If one wishes to study extension of haemopoiesis, one can study the shaft of long bone.
  9. Stages in the development of RBCs or erythrocytes
    1. From the pleuripotent hematopoietic stem cell (HSC) (refer) → committed stem cells are formed. The committed stem cells in the RBC series are of 2 types :
      1. BFU-E (or burst forming unit –erythrocyte)
      2. CFU-E (or colony forming unit –erythrocyte)
    2. BFU-E gives rise to CFU-E cells
    3. CFU-E cells give rise to proerythroblast (or pronormoblast)
    4. The stage from proerythroblast to RBC is shown in the following table
Stage Name Size of cell (µm) Cytoplasmic staining Mitosis Nucleus No. per 100 nucleated cells in the bone marrow
I Pro-normoblast or proerythroblast 15 to 20 Deep violet blue (Basophilic) Only during stress Nucleus is 12 µm; it has many nucleoli; chromatin is fine and stippled; No Hb 1 to 3
II Early normoblast Somewhat smaller (10 to 17) Basophilic Active No nucleoliHb appears, chromatin is fine and shows a few nodes of condensation (Hb is formed from stage II to stage IV) 1 to 3
III Intermediate normoblast Still smaller (10 to 14 µm) Polychromatophil Active The resting nucleus shows further condensation of chromatin. Hb increases; its eosinophilic staining gives the cytoplasm a polychramtic appearance 4 to 8
IV Late normoblast
(orthochromatic normoblasts)
7 to 10 µm Eosinophilic No Nucleus is small; the condensed chromatin shows ‘cart-wheel’ appearance; finally it becomes uniformly deeply condensed and stained (this state of the nucleus is called pyknosis; pyknosis = thickened and shrunken nucleus). Pyknosis is a stage in the degeneration of the nucleus. The nucleus finally breaks up and is extruded out 8 to 16
  Reticulocyte Slightly larger than the mature RBCs Eosinophilic (also basophilic reticulum is present)   No nucleus; Hb synthesis continues in this stage.  
  Mature RBC 7.2 µm Fully eosinophilic (no reticulum)     x
  1. Duration
    The entire process of erythropoiesis takes about 7 days. Out of this, time taken for conversion from proerythroblast to reticulocyte is 3 days; time taken for reticulocyte to become matured RBC is 4 days. Out of these 4 days, the last one day is spent in the peripheral circulation by the reticulocytes.
  2. Note :
    Maturation of the erythroblasts involves:
    1. a decrease in the size of the cell
    2. increased condensation and finally pyknosis and disappearance of the nucleus
    3. accumulation of Hb
    4. a change in the staining of the cytoplasm from basophil via polychromatophil to eosinophil (initially the cytoplasm is basophilic due to plenty of RNA; later it turns eosinophilic due to accumulation of Hb and also due to decrease in RNA)
  3. Reticulocyte
    This is the name given to the young red cell; it is so called because on vital staining with cresyl blue, it shows a network of basophilic reticulum in the cytoplasm.
    All the nucleated precursors of the reticulocyte (i.e. the normoblasts) also give this staining reaction. 
  4. The reticulum
    This probably consists of remnants of the basophil cytoplasm of the immature cell (chemically, the reticulum is made up of RNA).
    If red cells are stained with eosin and methylene blue, the presence of the reticulum in the young cells (i.e. the reticulocytes) leads to a diffuse mauve staining of the cell; this is called polychramtophilia. (The cytoplasm is stained eosinophilic due to Hb and the reticulum is stained basophilic due to RNA)
  5. Importance
    1. In pathological states, this stained basophil material is sometimes present in clumps which appear as discrete blue particles. This finding known as basophil punctation (or punctate basophilia) is especially obvious in lead poisoning.
    2. As the red cell ages, the reticulum disappears. In the newborn, 2 to 6 % of the red cells in the circulation are reticulated; the number falls during the first week to less than 1%, at which level it remains throughout life. Their number is increased whenever red cells are being rapidly manufactured. In such cases, 25 to 35 %  of the circulating cells can be reticulocytes. An increase in the reticulocyte count (reticulocytosis) is the first blood change noted when pernicious anaemia is treated with vitamin B12.
  6. Spleen as a blood filter
    It removes spherocytes and other abnormal RBCs. Abnormal RBCs are removed if they are not as flexible as the normal RBCs; if they are not flexible, they are not able to go between the endothelial cells that line the splenic sinuses. Thus, they get trapped and are removed.
    Spleen also contains many platelets; it also plays a significant role in immunity.
  7. This can be classified as under : 
    1. Mechanical fragility
      When RBCs are shaken with glass beads for one hour, about 2 to 5 % of the cells get lysed. In some hemolytic anaemia, the % is more.
    2. Autohemolysis
      If normal blood with an anticoagulant is kept at 37 degree centigrade for 24 hours, less than 0.5 % cells get hemolysed.  A higher percentage is seen in some hemolytic anaemia.
    3. Osmotic fragility
      a. If RBCs are suspended in hypertonic solution, they shrink. If suspended in hypotonic solution, they swell, become spherical (from disc-shaped) and eventually break and lose their Hb (haemolysis). The Hb of the hemolysed cells dissolves in the plasma, colouring it red. 
      b. 0.9 % NaCl solution is isotonic with plasma
      c. Values of normal RBC osmotic fragility
      d. Haemolysis begins in 0.5% saline; 50 % lysis occurs in 0.40 to 0.42 % solution and complete haemolysis occurs at 0.3 % solution.
      e. In hereditary spherocytosis (also called congenital haemolytic icterus)
      f. The cells are already spherocytic in the normal plasma and not disk-shaped; thus, their osmotic fragility is more (i.e. they start getting hemolysed at less hypotonic or more hypertonic solutions than the normal cells). Therefore, hereditary spherocytosis is one of the most common causes of hereditary hemolytic anaemia. 
      g. Why are the cells spherocytic in hereditary spherocytosis,
      h. Here, there is defect in the RBC membrane; there is abnormality in the protein network that normally maintains the shape and flexibility of RBC membrane. 
      i. The RBC membrane skeleton is normally made up of the following proteins : 
      spectrin, band 3 protein and ankyrin
      j. Spectrin is anchored to the trans-membrane protein band 3 by ankyrin. 
      (the band 3 protein also functions as an anion exchanger)
      k. Hereditary spherocytosis can occur due to defects in spectrin, band 3, or ankyrin. 
      l. RBC of venous blood are slightly more fragile than those of arterial blood in normal persons. 
      m. Osmotic fragility is related to the shape of the RBC; the more spherical it is, the greater the fragility i.e. the higher the concentration of saline at which hemolysis occurs. 
    4. Drugs and Infections
      Hemolysis of RBC can occur due to drugs and infections. Deficiency of G-6-P-D increases the susceptibility to hemolysis by these agents. Why? G-6-P-D catalyses the first step in the oxidation of glucose via the hexose monophosphate shunt (HMS) pathway. This pathway generates NADPH; NADPH is required for the integrity of the  normal red cell membrane.
      Severe G-6-P-D deficiency also inhibits the killing of bacteria by granulocyte (and so predisposes to severe infections).

RBC Indices

Adult men
Adult women
Children 1 year
Direct measures
Red cell count (RCC) (in millions per cumm)
Hb (gm/dL)
Mean corpuscular volume (MCV)(in fL)
Packed cell volume (PCV) or hematocrit (%)
Derived measures
MCH (in pg)
MCHC (g/dL)


1 microlitre = 1 cu mm
1 fl = 10-15 litre
1 pg = 10 –12 gram
MCH = mean corpuscular Hb; it is the average amount of Hb in one RBC
MCH = ------


1 Formula for calculating MCH:
                                        Hb (g/dL)
            MCH =      ---------------------- --------------------------- x 10
                                RBC (as so many millions) per cumm
MCV = mean corpuscular volume; it is the average volume of one RBC
MCV = --------
2. Formula for calculating MCV:
               PCV (as percentage)
 MCV =  ---------------------- --------------------------- x 10
                 RBC (as so many millions) per cumm
3. MCHC = mean corpuscular hemoglobin concentration
This gives the amount of hemoglobin in one RBC as per its volume
                        MCHC = -------------
                                Since MCH = Hb/RBC and MCV = PCV/RBC,
MCHC can be expressed as =    -------------------------------------
                                        MCV x RBC

4. Formula for calculating MCHC:
                                        Hb (in g/dL)
                        MCHC = ----------------------- x 100
                                           PCV (%)
      > 95 fl = macrocytes
      < 80 fl = microcytes

      < 25 pg = hypochromic

5. ‘D’ Blood Groups
The membranes of human red cells contain a variety of blood group antigens, which are also called agglutinogens. The most important are ABO system
ABO System
  1. The ABO locus is located on chromosome 9
  2. The H antigen is an essential precursor to the ABO blood group antigens. The H locus is located on chromosome 19.
  3. The A allele encodes a glycosyltransferase that bonds a-N-Acetylgalactosamine to D-galactose end of H antigen, producing the A antigen.
  4. The B allele encodes a glycosyltransferase that joins a-D-galactose bonded to D-galactose end of H antigen, creating the B antigen.
  5. They are IgM antibodies. Anti-A and anti-B antibodies are not present in the newborn, appear in the first years of life.
  6. Acquired via cross reaction to food or bacterial antigen.
  7. Most Common blood group in India is B
  8. Persons with type AB blood are "universal recipients" because they have no circulating agglutinins and can be given blood of any type without developing a transfusion reaction due to ABO incompatibility.
  9. Type O individuals are "universal donors"
  10. In individuals who are “secretors”, a soluble form of the ABO blood group antigens is found in saliva and in all bodily fluids (Semen, Sweat, Saliva) except for the cerebrospinal fluid.
The RH Group
  1. The Rh factor (named for the rhesus monkey) is a system composed primarily of the C, D, and E antigens.
  2. The system has not been detected in tissues other than red cells. D is by far the most antigenic component.
  3. Rh-positive means that the individual has antigen D. The Rh-negative individual has no D antigen and forms the anti-D agglutinin only when exposed with D-positive cells (Exception to Landsteiners Law i.e is antigen is absent antibody against the antigen are present in serum).
  4. Eighty-five percent of Caucasians are D-positive and 15% are D-negative; over 99% of Indians are D-positive.
  5. When Rh-negative mother carries an Rh-positive fetus it can result in hemolytic disease of the newborn (erythroblastosis fetalis)

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