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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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
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.