These blood cells are arise from a single type of cell called a pluripotcnt stem cells. Pluripotential stem cells are the earliest stem cells found within the bone marrow.
These cells divided into multipotential stem cells by differentiation, which are of two types – lymphoid multipotential stem cells and myeloid multipotential stem cells.
Blood cells are of three types – Erythrocytes, Leucocyte and Thrombocytes (Platelets).
Erythrocytes or the red blood corpuscles (RBC) are the most abundant of the three types. They have a count of about 5-5.5 millions per cubic mm of blood in adult male, and 4.5-5 million/mm3 in female.
Red blood cells are circular, biconcave and enucleated in mammals (except in camel and Llama where they are oval and nucleated).
It is biconcave as such a shape has increase surface area (for 02 transfers) and allows easy squeezability of the RBC’s through the blood vessels. Process of RBC formation is known as erythropoiesis. Iron, vitamin B12 and folate are essential in the production of RBCs.
During formation the RBC eventually loses its nucleus and leaves the bone marrow as reticulocyte. At this point, the reticulocyte contains some remnants of organelles. Eventually there organelles leave the cell & a mature erythrocyte is formed. Erythropoiesis is completed in 72 hours. Erythropoeitic organ in foetus is liver, lymph nodes and spleen whereas after birth erythropoeitic tissue is bone marrow.
Average life of RBC is 120 days after which they are broken down in spleen or liver. Product of breakdown of haemoglobin is a pigment (yellow colour), called bilirubin which is normally disposed off through bile whereas haem transferred to red bone marrow. Retention of bilirubin leads to jaundice.
Each RBC is bounded by a dynamic enzyme containing plasma membrane. It serves as an elastic and semipermeable barrier.
They contain a reddish respiratory pigment, haemoglobin. Haemoglobin is a protein complex and occupies almost the entire volume of red blood cells. Essential to its structure and function is the mineral iron.
It is a tetramer, consisting of two pairs of polypeptide chains (2a and 2(3). To each of the four chains is attached a highly coloured prosthetic group haem, a complex of iron and protoporphyrin. The protein portion of the molecule is globin. (Foetal haemoglobin has 2a and 2y chains).
Optimum amount of haemoglobin in male is about 16 g/100 ml blood, and 13-14 g/100 ml in female. When RBC count is very high, the condition is called polycythemia. Low RBC count and lower content of haemoglobin is called anaemia.
If clotting is prevented red blood cells sink. ESR is a rate of setting down of RBC when anticoagulant blood is held in a vertical column. It is measured by wintrobe tube. Its value in case of male is 3-5 mm/hr. and in female it is 7-12 mm/hr.
Higher ESR is indicative of infections particularly of tuberculosis. ESR decreases in anaemia. Westergrens tube is also used to measure but its values are different.
Leucocytes or the white blood corpuscles (WBC) are without haemoglobin and therefore, colourless and considerably larger than RBC. The normal WBC count is 5000-6000/cubic mm of blood.
Lower count due to the deficiency of folic acid or any other reason is called leukopenia. High WBC count is termed as leukemia or leucocytosis or blood cancer. The life span of WBC in man is about 10-13 day.
Leucocytes are of 2 main types – granulocytes and agranulocytes.
These are leucocytes with granules in cytoplasm. They include eosinophils, basophils and neutrophils and all have lobed nucleus. They are produced in red bone marrow.
(i) Eosinophils or Acidophils (about 2%):
Nucleus bilobed. They are motile phagocytic cells. Their number increases during allergy or worm infections.
(ii) Basophils (about 1%):
Nucleus with 2 or 3 lobes. Secrete histamines, serotonin and heparin. They are probably like mast cells of connective tissue.
(iii) Neutrophils or Heterophils or Polymorphs:
They are most abundant and active type of WBCs. It comprises .40-60% of all leucocytes in the blood. Nucleus with 2-5 lobes. In some neutrophils of women a barr-body like drumstick body is present which is attached to the nucleus.
The primary function of neutrophils is the phagocytosis and destruction of bacteria. Bacteria can be phagocytosed after opsonization.
These are not found in the cytoplasm. Agranulocytes are formed in spleen and lymph nodes. They include lymphocytes and monocytes.
(i) Lymphocytes (about 30%):
They are smaller with large rounded nucleus. They are non-motile and nonphagocytic. They produce antibodies as they are the key cells of immune system.
Lymphocytes exist in two major forms in circulation – B lymphocyte (life span 3-7 days) and T lymphocytes (long life span of few years)
(ii) Monocytes (about 4%):
Largest amongst all types of leucocytes with a rounded nucleus. They are motile and phagocytic in nature. Since they are the direct percursors of macrophages so after entering into the tissue fluid, these transform into macrophages for phagocytising invading microbes.
III. Blood Platelets:
Occur only in mammals, non-nucleated, colourless, round or oval, biconvex disc like bodies and bud off from megakaryocyte cells of red bone marrow.
They are much smaller than RBC. It helps in blood coagulation. Life span is 8 days. Blood platelets are source of thromboplastin necessary for blood clotting.