11.4 Blood

2026 📋 Syllabus Objectives

By the end of this subtopic, you should be able to:

  1. Identify red blood cells and white blood cells (lymphocytes and phagocytes) as seen under a light microscope, in diagrams, and in photomicrographs
  2. List the four components of blood: red blood cells, white blood cells, platelets, and plasma
  3. State the functions of each component of blood:
    • Red blood cells → oxygen transport
    • White blood cells → antibody production (lymphocytes) and engulfing pathogens (phagocytes)
    • Platelets → blood clotting
    • Plasma → transport of many substances
  4. Describe the transfer of substances between blood in capillaries, tissue fluid, and body cells

📖 Main Content

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1. The Four Components of Blood

Blood is a liquid tissue — this means it is made of living cells suspended in a liquid. It travels around your entire body through blood vessels, delivering what cells need and collecting waste.

Blood has four main components:

ComponentWhat it is
Red blood cellsCells that carry oxygen
White blood cellsCells that defend the body against infection
PlateletsTiny cell fragments that help blood clot
PlasmaThe pale yellow liquid that everything else floats in

2. Red Blood Cells

What do they look like?

Under a light microscope (the type of microscope used in school labs), red blood cells have a very distinctive appearance:

  • They are small, round, and disc-shaped
  • They are biconcave — this means they are squashed in the middle on both sides, like a doughnut that hasn't had its hole punched all the way through
  • They have no nucleus (the control centre of a cell is completely absent in mature red blood cells)
  • They appear pink/red when stained on a prepared slide
  • They are very numerous — far more red blood cells are visible than any other type

🔍 Exam tip: The lack of a nucleus and the biconcave shape are the two most important features to identify red blood cells.

What do they do?

Red blood cells have one main function: transporting oxygen from the lungs to every cell in the body.

They are able to do this because they contain a special protein called haemoglobin (pronounced "hee-mo-glo-bin"). Haemoglobin is a red-coloured protein that combines with oxygen in the lungs to form oxyhaemoglobin. When the red blood cell reaches a part of the body that needs oxygen, the oxyhaemoglobin releases the oxygen back out.

Their biconcave shape is a perfect adaptation for this job because it gives them a large surface area relative to their volume — meaning more haemoglobin is close to the surface, so oxygen can move in and out quickly.

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