List the components of blood: red blood cells, white blood cells, platelets and plasma.

Resources → Subject Notes → Biology → Lesson Plan

Transport in Humans – Blood Components

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:
    1. Adhere to exposed collagen (platelet adhesion).
    2. Release chemicals that activate nearby platelets (activation).
    3. 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.
    • No nucleus.
    • Often appear as dark purple dots.

5. The Clotting Process (Full AO1 Description)

  1. Vascular spasm: Immediate contraction of damaged vessel walls reduces blood loss.
  2. Platelet plug formation
    1. Platelet adhesion – platelets stick to exposed collagen.
    2. Platelet activation – release of ADP, thromboxane A₂ and other chemicals.
    3. Platelet aggregation – activated platelets bind to each other, forming a temporary plug.
  3. 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.
  4. Clot stabilisation: Fibrin strands mesh with the platelet plug, forming a stable clot that seals the breach.
  5. 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.

Create an account or Login to take a Quiz

29 views
0 improvement suggestions

Log in to suggest improvements to this note.