Blood System - Overview

The Blood System
Blood serves as the body's primary transport system, maintaining a constant environment for all living tissues. It carries essential nutrients, oxygen, and hormones to cells whilst removing waste products like carbon dioxide and nitrogenous compounds.
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Blood Composition: A Perfect Balance
45%
Formed Elements
Red blood cells, white blood cells, and platelets
55%
Plasma
Water, proteins, sugar, salts, hormones, and vitamins
Blood consists of cellular components suspended in plasma, a clear, straw-coloured liquid. The formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). This precise balance ensures optimal function of the body's transport and defence systems.
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Blood Plasma: The Liquid Foundation
Plasma constitutes 55% of blood volume and serves as the medium for transporting cells and dissolved substances throughout the body. This straw-coloured liquid contains water, proteins, nutrients, wastes, salts, hormones, and vitamins.
The four major plasma proteins are albumin, globulin, fibrinogen, and prothrombin. Albumin maintains proper water content, globulins contain antibodies for immune defence, whilst fibrinogen and prothrombin are essential for blood clotting.
Albumin
Water balance
Globulin
Antibodies
Fibrinogen
Clotting
Prothrombin
Clotting
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Blood Cell Origins
Most blood cells originate in the bone marrow cavity. Both red blood cells that carry oxygen and white blood cells that fight infection arise from the same immature cells called stem cells or haemocytoblasts. These remarkable cells have the ability to differentiate into various types of blood cells based on the body's needs.
Stem Cells
Immature precursors
Differentiation
Specialisation process
Mature Cells
Functional blood cells
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Erythrocytes: The Oxygen Carriers
Structure and Function
As red blood cells mature, the nucleus is expelled and the cell assumes a distinctive biconcave disc shape—resembling a cough drop with a thin central portion. This unique shape provides a large surface area for efficient gas exchange.
Red blood cells contain haemoglobin, a protein that binds oxygen. The combination of oxygen and haemoglobin (oxyhaemoglobin) produces blood's characteristic bright red colour.
120
Days
Lifespan of erythrocytes in circulation
5M
Cells
Red blood cells in a pinhead-sized drop
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Red Blood Cell Lifecycle
Production
Formed in bone marrow
Circulation
120 days in bloodstream
Destruction
Broken down by macrophages
Recycling
Iron reused or stored
After 120 days, worn-out erythrocytes are destroyed by macrophages in the spleen, liver, and bone marrow. Haemoglobin breaks down into heme and protein portions. The heme releases iron and decomposes into bilirubin, a dark green pigment. Iron is reutilised for new red cells or stored, whilst bilirubin is excreted into bile by the liver.
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Leukocytes: The Body's Defence Force
White blood cells are less numerous than erythrocytes but far more diverse. Whilst there is only one type of mature red blood cell, there are five distinct types of mature leukocytes, each with specialised functions in protecting the body from infection and disease.
Granulocytes
Basophils, neutrophils, eosinophils
Agranulocytes
Monocytes and lymphocytes
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Granulocytic Leukocytes
Basophils
Contain dark-staining cytoplasmic granules that stain with basic (alkaline) dye. The granules contain heparin (an anticlotting substance) and histamine (released in allergic responses).
Eosinophils
Contain granules that stain with red acidic dye called eosin. These granulocytes increase in numbers during allergic responses and are thought to engulf substances that trigger allergies.
Neutrophils
Phagocytes that accumulate at sites of infection, where they ingest and destroy bacteria. They are the most abundant type of white blood cell and form the first line of defence.
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Agranulocytic Leukocytes
Lymphocytes
Mononuclear leukocytes that arise in lymph nodes and circulate in both the bloodstream and lymphatic system. They play a crucial role in the immune response, directly attacking foreign matter and producing antibodies that neutralise and destroy foreign antigens such as bacteria and viruses.
Monocytes
Phagocytic cells that fight disease by moving from the bloodstream into tissues. They dispose of dead and dying cells and other tissue debris through phagocytosis. Once in tissues, monocytes become macrophages.
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Platelets: The Clotting Specialists
01
Formation in Bone Marrow
Platelets form from giant multinucleated cells called megakaryocytes
02
Fragmentation Process
Tiny fragments break off from megakaryocytes to form platelets
03
Circulation
Platelets circulate in bloodstream ready to respond to injury
04
Clotting Function
Main function is to help in blood clotting at injury sites
Platelets, or thrombocytes, are essential for haemostasis—the stopping of bleeding. These small cell fragments circulate in the blood, ready to aggregate at sites of vessel injury and initiate the clotting cascade.
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Blood Groups and Compatibility
Blood typing is crucial for safe transfusions. The ABO system classifies blood into four main groups based on the presence or absence of A and B antigens on red blood cells. The Rh factor is another important antigen that determines compatibility.
Type A
A antigens, anti-B antibodies
Type B
B antigens, anti-A antibodies
Type AB
Both antigens, no antibodies
Type O
No antigens, both antibodies
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Key Blood Terminology
Agglutination
Clumping of recipient's blood cells when incompatible bloods are mixed
Antibodies
Protein substances formed by lymphocytes to neutralise antigens
Antigens
Foreign materials that stimulate antibody production
Coagulation
The process of blood clotting
Differentiation
Change in structure and function as cells mature and specialise
Erythropoietin
Hormone secreted by kidneys to stimulate red blood cell production
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Clotting Factors and Proteins
Fibrin
Protein threads forming the basis of blood clots
Fibrinogen
Plasma protein converted to fibrin during clotting
Thrombin
Enzyme that converts fibrinogen to fibrin
Thromboplastin
Clotting factor promoting fibrin clot formation
The clotting process involves a complex cascade of proteins and factors working together to form a stable clot. Prothrombin is converted to thrombin, which then converts fibrinogen into fibrin threads that form the clot structure.
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Anaemia: Red Blood Cell Deficiency
Anaemia is characterised by a deficiency of erythrocytes or haemoglobin, resulting in reduced oxygen-carrying capacity of the blood. There are several types of anaemia, each with distinct causes and characteristics.
Aplastic Anaemia
Bone marrow failure
Haemolytic Anaemia
Excessive destruction
Pernicious Anaemia
B12 absorption failure
Sickle Cell Anaemia
Abnormal cell shape
Thalassaemia
Haemoglobin defect
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Aplastic Anaemia
Aplastic anaemia results from failure of blood cell production due to aplasia (absence of development) of bone marrow cells. The cause is often unknown (idiopathic), but some cases have been linked to benzene exposure and certain antibiotics such as chloramphenicol.
Pancytopenia occurs as stem cells fail to produce leukocytes, platelets, and erythrocytes. This condition affects all blood cell lines, making it particularly serious and requiring prompt medical intervention.
3
Cell Types
All affected in pancytopenia
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Haemolytic and Pernicious Anaemia
Haemolytic Anaemia
Reduction in red cells due to excessive destruction. Congenital spherocytic anaemia (hereditary spherocytosis) is one example where erythrocytes are spheroidal instead of biconcave. This abnormal shape makes them fragile and easily destroyed through haemolysis.
Pernicious Anaemia
Lack of mature erythrocytes owing to inability to absorb vitamin B12. This vitamin is necessary for proper red blood cell development. Patients lack intrinsic factor in gastric juice, which is essential for B12 absorption from the digestive tract into the bloodstream.
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Sickle Cell Anaemia
Sickle cell anaemia is a hereditary condition characterised by abnormal crescent or sickle-shaped erythrocytes and haemolysis. The distorted shape is caused by abnormal haemoglobin S in the red cells.
Abnormal Shape
Crescent or sickle-shaped erythrocytes due to haemoglobin S
Poor Oxygenation
Distorted, fragile cells are poorly oxygenated
Vascular Blockage
Cells clump together, blocking blood vessels
Tissue Damage
Leads to thrombosis and infarction (dead tissue)
Symptoms
Arthralgia, abdominal pain, extremity ulcerations
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Thalassaemia and Haemochromatosis
Thalassaemia
An inherited defect in haemoglobin production, usually seen in persons of Mediterranean background (thalassa is Greek for "sea"). This condition consists of various forms and degrees of severity, with the more severe form called Cooley's anaemia.
Thalassaemia usually leads to hypochromic anaemia, characterised by diminished haemoglobin content in red cells.
Haemochromatosis
A condition characterised by excessive deposits of iron throughout the body. This iron overload can damage organs including the liver, heart, and pancreas if left untreated.
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Blood Cell Disorders
Polycythaemia Vera
General increase in red blood cells (erythremia), leading to increased blood viscosity and potential complications such as thrombosis.
Haemophilia
Excessive bleeding caused by congenital (hereditary) lack of protein factor VIII necessary for blood clotting. Requires careful management and factor replacement therapy.
Purpura
Multiple pinpoint haemorrhages and accumulation of blood under the skin, appearing as purple spots or patches on the skin surface.
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Leukaemia: Types and Classification
Leukaemia is characterised by excessive increase in cancerous white blood cells. This disease of the bone marrow results in malignant leukocytes filling the marrow and bloodstream, crowding out normal blood cell production.
Acute Myelogenous Leukaemia (AML)
Rapid progression affecting myeloid cell line
Acute Lymphocytic Leukaemia (ALL)
Rapid progression affecting lymphoid cell line
Chronic Myelogenous Leukaemia (CML)
Slower progression affecting myeloid cells
Chronic Lymphocytic Leukaemia (CLL)
Slower progression affecting lymphoid cells
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White Blood Cell Disorders
Granulocytosis
Abnormal increase in granulocytes in the blood. This elevation often occurs in response to infection, inflammation, or other stress conditions. The body produces more granulocytes to combat the underlying cause.
Multiple Myeloma
Malignant tumour of bone marrow affecting plasma cells. This cancer causes abnormal plasma cells to accumulate in bone marrow, producing abnormal antibodies and causing bone destruction, anaemia, and kidney problems.
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Laboratory Blood Tests
Complete Blood Count (CBC)
Includes RBC count, WBC count with differential, platelet count, haemoglobin, haematocrit, and red cell indices (MCH, MCV, MCHC)
Coagulation Time
Time required for venous blood to clot in test tube; normal is less than 15 minutes
Bleeding Time
Time for small puncture wound to stop bleeding; normal is 8 minutes or less
Erythrocyte Sedimentation Rate
Speed at which RBCs settle out of plasma; altered in infections and inflammation
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Blood Cell Counts and Analysis
Red Blood Cell Count
Number of erythrocytes per cubic millimetre; normal is 4-6 million per cu mm
White Blood Cell Count
Number of leukocytes per cubic millimetre; normal is 5,000-10,000 per cu mm
Platelet Count
Number of platelets per cubic millimetre; normal is 200,000-400,000 per cu mm
Haemoglobin Test
Total amount of haemoglobin in peripheral blood sample
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Haematocrit and Cell Morphology
Haematocrit (Hct)
Percentage of erythrocytes in a volume of blood. A blood sample is spun in a centrifuge so that erythrocytes fall to the bottom. The proportion of cells to plasma is then measured, providing valuable information about blood composition.
Red Blood Cell Morphology
A stained blood smear is examined to determine the shape or form of individual red cells. The presence of anisocytosis (varying cell sizes), poikilocytosis (abnormal shapes), sickle cells, and hypochromia (pale cells) can be noted and aid in diagnosis.
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White Blood Cell Differential
This test determines the numbers of different types of leukocytes, both immature and mature forms. Cells are stained and counted under a microscope by a technician. A minimum of 100 cells is counted, and percentages for neutrophils, monocytes, basophils, and eosinophils are given.
60%
Neutrophils
Normal range in differential count
30%
Lymphocytes
Normal range in differential count
5%
Monocytes
Normal range in differential count
3%
Eosinophils
Normal range in differential count
1%
Basophils
Normal range in differential count
Shift to the Left: This term describes a condition with increased immature neutrophils and decreased mature forms in the blood, often indicating acute infection.
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Specialised Blood Tests
Antiglobulin Test (Coombs Test)
Demonstrates whether patient's erythrocytes are coated with antibody. Useful in determining presence of antibodies in infants of Rh-negative women or in patients with autoimmune haemolytic anaemia.
Prothrombin Time
Tests blood's ability to clot by measuring time elapsed between calcium addition to plasma sample and visible clot appearance. Used to monitor patients taking anticoagulants.
Partial Thromboplastin Time
Measures presence of factors acting at early points in the coagulation pathway. Important for detecting clotting factor deficiencies.
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Apheresis Procedures
Apheresis is the separation of blood into its component parts. It is performed to remove toxic substances or autoantibodies from the blood or to harvest specific blood cells for therapeutic purposes.
Leukapheresis
Separation and collection of white blood cells
Plateletpheresis
Separation and collection of platelets
Plasmapheresis
Separation and removal of plasma
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Blood Transfusion
In this procedure, whole blood or cells are taken from a donor and, after appropriate testing to assure a close match of red cell or platelet type, are infused into a patient. Prior to transfusion, tests are performed to ensure the specimen is free from hepatitis or AIDS virus.
Autologous Transfusion: The collection and later reinfusion of a patient's own blood or blood components. This eliminates risks of disease transmission and transfusion reactions.

1. Donor Selection
Blood type matching and screening
2. Testing
Screening for infectious diseases including hepatitis and AIDS virus
3. Cross-Matching
Ensuring compatibility between donor and recipient blood
4. Transfusion
Careful monitoring during infusion
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Bone Marrow Procedures
Bone Marrow Biopsy
A needle is introduced into the bone marrow cavity, and a small amount of marrow is aspirated. The marrow is then examined under microscope. This procedure is helpful in diagnosis of blood disorders such as anaemia, cytopenias, and leukaemia.
Bone Marrow Transplant
Bone marrow cells from a donor whose tissue and blood cells closely match the recipient are infused into a patient with leukaemia or aplastic anaemia. First, the patient receives total-body irradiation or aggressive chemotherapy to kill diseased cells. The donor's marrow is then intravenously infused.
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