Objective: List the four main components of blood and state their functions using the exact Cambridge IGCSE/A‑Level wording (Core 9.4.1). Be able to identify red and white blood cells in photomicrographs/diagrams (Core 9.4.2) and explain the role of platelets and fibrinogen in clotting (Core 9.4.2).
1. Major Components of Blood
Red blood cells (RBCs) – erythrocytes
White blood cells (WBCs) – leukocytes
Platelets – thrombocytes
Plasma – liquid portion of blood
2. Functions and Approximate Volume Contributions
Component
Function (syllabus wording)
Approx. % of blood volume
Red blood cells (RBCs)
Transport of oxygen from the lungs to the tissues and of carbon dioxide from the tissues to the lungs (via the protein haemoglobin).
≈ 45 %
White blood cells (WBCs)
Defence against infection – phagocytosis of invading microorganisms and production of antibodies.
≈ 0.1 %
Platelets
Assist in clot formation so that blood loss is prevented and entry of pathogens is blocked.
≈ 0.01 %
Plasma
Carrier medium for cells, nutrients, ions (Na⁺, K⁺, Ca²⁺, Cl⁻), glucose, amino acids, fatty acids, urea, hormones and carbon dioxide (as bicarbonate).
Maintains blood pressure, pH and body temperature.
Contains proteins:
Albumin – maintains osmotic pressure.
Globulins – transport hormones and act as antibodies.
Fibrinogen – converted to fibrin during clotting.
≈ 55 %
3. Detailed Notes for Each Component
3.1 Red Blood Cells (RBCs)
Contain the iron‑rich protein haemoglobin, which binds O₂ and CO₂.
Shape: biconcave disc – maximises surface area for gas exchange.
Mature cells have no nucleus.
Life span ≈ 120 days; removed by the spleen.
3.2 White Blood Cells (WBCs)
All possess a nucleus and are larger than RBCs.
Two groups relevant to the syllabus:
Phagocytes (e.g., neutrophils, macrophages) – engulf and destroy microorganisms.
Lymphocytes – produce antibodies that neutralise specific pathogens.
Very small proportion of blood volume but essential for immunity.
3.3 Platelets
Cell‑fragment fragments derived from megakaryocytes in bone marrow.
When a vessel is damaged they:
Adhere to exposed collagen (platelet adhesion).
Release chemicals that activate nearby platelets (activation).
Stick together to form a temporary platelet plug (aggregation).
Trigger the coagulation cascade that converts plasma fibrinogen into fibrin, stabilising the clot.
3.4 Plasma
≈ 90 % water; the remaining 10 % consists of dissolved substances.
Transports ions, nutrients, waste products, hormones and CO₂ (as HCO₃⁻).
Plasma proteins and their key roles:
Albumin – maintains osmotic (colloid) pressure.
Globulins – carry hormones and act as antibodies.
Fibrinogen – essential for clot formation; converted to fibrin.
4. Identification Checklist (Core 9.4.2)
Use the following visual cues when examining a stained blood smear or diagram.
Red blood cell (RBC)
Shape: biconcave disc.
Size: ~7 µm diameter.
No nucleus.
Uniform pink/red colour after eosin staining.
White blood cell (WBC)
Larger than RBCs (≈ 12–15 µm).
Visible nucleus.
Colour varies with stain (typically purple).
Neutrophil (phagocyte)
Multi‑lobed nucleus (2–5 lobes).
Fine granules in cytoplasm.
Lymphocyte
Large, round nucleus that occupies most of the cell.
Thin rim of pale cytoplasm.
Platelet
Very small (2–4 µm), irregularly shaped fragments.
Platelet adhesion – platelets stick to exposed collagen.
Platelet activation – release of ADP, thromboxane A₂ and other chemicals.
Platelet aggregation – activated platelets bind to each other, forming a temporary plug.
Coagulation cascade
Two pathways (intrinsic and extrinsic) converge on activation of factor X.
Factor X converts prothrombin to thrombin.
Thrombin converts soluble fibrinogen (plasma protein) into insoluble fibrin strands.
Clot stabilisation: Fibrin strands mesh with the platelet plug, forming a stable clot that seals the breach.
Clot retraction and repair: Actin‑myosin filaments in platelets contract the clot, pulling wound edges together; tissue repair follows.
6. Suggested Classroom Activities
Microscope practice: Compare a stained blood smear with labelled diagrams; use the identification checklist to mark each cell type.
Clotting demonstration: Add a small amount of calcium chloride to fresh blood in a test tube and observe clot formation; discuss each step of the cascade.
Volume‑percentage activity: Use coloured beads to model the relative percentages of RBCs, plasma, WBCs and platelets.
Suggested diagram: Cross‑section of blood showing RBCs, a neutrophil, a lymphocyte, platelets and plasma. Include a labelled photomicrograph of each cell type for identification practice.