Blood Cells.
Introduction:
Blood is a fluid connective tissue made of two components Plasma and Blood Cells.
Blood performs many important functions in the body like, transportation, regulation and protection.
Blood is thick in appearance and has a temperature slightly higher than body temp. and pH is 7.4.
Blood Cells:
Cellular components of blood includes the blood cells,
Red Blood Cells (Erythrocytes).
Platelets (Thrombocytes).
White Blood Cells (Leucocytes).
The blood cells are formed in “red bone marrow” present in bones.
In foetus blood cells are formed in the liver and later in bones.
In children the red bone marrow is present in all bones while in adults it is present only in Skull, Ribs, Ending of long bones, Hip Bone, Vertebrae and Sternum.
The formation of blood cells is called “hematopoiesis”.
Red Blood Cells:
Also called “Erythrocytes”.
Bi-concave discs in shape with a diameter of 7-8 µm.
They are flexible hence can pass through capillaries by squeezing through.
The plasma membrane is strong and contains specific “Antigens (glycolipids)” which are responsible for blood grouping.
Each RBC contains around 280 million molecules of haemoglobin (Hb).
Mature RBCs lack nucleus and contain a maximum amount of Hb.
Mature RBCs also lack mitochondrias and synthesize ATPs required by anaerobic way.
A healthy adult male has about 5.4 million rbcs/ microliter (µL) of blood.
A healthy adult female has about 4.8 million rbcs/ microliter (µL) of blood.
The main function of RBCs is gaseous transportation which is due to the presence of Hb.
Hemoglobin:
Haemoglobin is a large, complex protein containing a globular protein (globin) and a pigmented iron containing a complex called haem.
Each haemoglobin molecule contains four globin chains and four haem units, each with one atom of iron.
Hence a single haemoglobin molecule can carry up to four molecules of oxygen.
An average red blood cell carries about 280 million haemoglobin molecules.
Red Blood Cell Formation:
Also called Erythropoiesis.
Erythropoietin (EPO) is a hormone produced by the kidney that promotes the formation of red blood cells by the bone marrow.
During foetal stage the blood cell formation takes place in “liver” while before three monthe to birth blood cell formation starts at “Bone marrow”.
The red bone marrow is the major site of blood cell formation.
During childhood almost all the bones contain “red bone marrow”, after attaining adult stage most of red bone marrow is replaced by “yellow bone marrow” which contains adipocytes and lacks the ability to produce the blood cells.
In adults red bone marrow is specifically found in: Skull, Vertebrae, Sternum, Ribs, Pelvic Girdle, Pectoral Girdle and endings of long bones.
Red bone marrow contains “Pluripotent cells (Stem Cells)” having capacity to get converted into many types of cells.
Pluripotent cells divide and form “progenitor cells” which are specialized cells that get converted into specific cells.
Progenitor cells divide to develop “Proerythroblasts” which contain nucleus and many ribosomes which are essential for the formation of “hemoglobin”.
Proerythroblast starts producing hemoglobin and is then called “Normoblasts”.
Normoblasts are the nucleated RBCs.
Normoblasts that contain approximately 33% hemoglobin nucleus is expelled out and are then called “reticulocytes”.
Reticulocytes get pushed into systemic circulation.
Reticulocytes are premature RBCs which get matured after 1-2 days.
RBC Life Cycle:
Erythropoietin (EPO) is a hormone produced by the kidney that promotes the formation of red blood cells by the bone marrow.
RBCs has a definite lifespan of 120 days.
The old RBCs are destroyed in various organs like Spleen, Liver and even in bone marrow.
The spleen is the major organ for RBC destruction.
The fixed macrophages in the spleen cause breakdown of RBC; the products formed during the process are: Globin and Heme.
Globin is a protein fragment.
Heme is a pigment broken down to iron and bile pigments.
Bile pigments are sent to the liver and processed and then secreted in bile duct as an element of bile.
Iron is recycled to produce new RBCs.
White Blood Cells:
These are the only blood cells which contain “Nucleus” and other cell organelles.
They are termed as “White” as they don't contain “hemoglobin”.
They are the largest blood cells but are only 1% of total blood cells.
NORMAL COUNT: 4500-11000/mm3 of blood.
They are classified as follows,
Classification of WBCs:
Granulocytes:
Neutrophiles.
Eosinophiles.
Basophiles.
Agranulocytes:
Monocytes.
Lymphocytes.
Granulocytes:
The process of formation of granulocytes is called “granulopoiesis”.
All granulocytes contain multilobed nuclei.
Their name represents the dye they take up during the staining process,e.g.
Eosinophiles take up “Eosin” the acidic stain and appear “Reddish Orange” in color.
Basophiles take up the basic stain “Methylene Blue” and appear bluish.
Neutrophiles take both stains and appear “purple” in color.
Neutrophils:
They are the fastest of all WBCs and reach first at the site of the infection.
They get attracted to the site of infection by chemicals released from damaged tissue parts “Chemotaxis”.
They kill microbes at the site of action by phagocytosis.
Their life span is 6-9 hours.
NORMAL COUNT: 50-70%.
Eosinophils:
These are less active in phagocytosis as compared to the neutrophils.
They are specialized in killing bigger parasites that can not be phagocytosed.
They contain chemicals that can produce inflammation and are often found involved in complications of many allergic conditions like asthma.
Their life span in circulation is 8-10 hrs while in tissues is 8-12 days.
NORMAL COUNT: 1-3%.
Basophils:
They are the largest type of granulocyte.
They can be phagocytic but their main function is in mediation of allergic responses.
They contain chemicals like histamine, heparin , serotonin and are involved in allergic responses viz. Asthma, hay fever and hypersensitivity.
Normally their concentration is very less in circulation but increases significantly in allergic responses.
Their lifespan is 3-10 days.
NORMAL COUNT: 0.5-1%.
Agranulocytes:
They have a large nucleus and no cytoplasmic granules.
They are of two types,
Monocytes.
Lymphocytes
They make up 25-50% of total leucocytes count.
Monocytes:
Largest of WBCs.
Some circulate in blood while some migrate to tissues to become “Macrophages”.
Actively motile and phagocytic.
Macrophages have important functions in inflammation and immunity.
The half-life of blood monocytes is about 1 day, whereas the life span of tissue macrophages is several months or years.
They make up 2-6% of total leucocytes.
Lymphocytes:
Lymphocytes are smaller than monocytes and have large nuclei.
They circulate in the blood and are present in great numbers in lymphatic tissue such as lymph nodes and the spleen.
Although they are formed in bone marrow they get activated in lymphoid tissue.
Lymphocytes are of three types
T-Lymphocytes.
B-Lymphocytes.
Natural Killer Cells.
Platelets (Thrombocytes):
These are very small non nucleated cells of blood.
These are formed in the red bone marrow in response to “Thrombopoietin” a substance produced by the kidney in response to low thrombocyte blood count.
They contain many chemicals responsible for blood clotting (Hemostasis).
Lifespan is 8-11 days.
About 1/3 rd of platelets are stored in the spleen for emergency conditions.
Normal count of platelets is: 150000-450000/ microliter of blood.
Commonly Asked Questions.
Write a note on Blood Cells.
Write a note on RBC formation / Erythropoiesis.
Write a note on the life cycle of RBCs.
Write in short about White Blood Cells.
Give normal counts of,
RBCs
Eosinophils.
Basophils.
Neutrophils.
Thrombocytes.
Write in short about Thrombocytes (Platelets).