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Learn4BI1Paper 2B · Paper 2 (all content)Structure and functions in living organismsBlood vessels and blood
Structure and functions in living organisms · Subtopic 17 of 21

Blood Vessels and Blood

Arteries carry blood at high pressure away from the heart: thick muscular, elastic walls, narrow lumen. Veins carry blood at low pressure back to the heart: thin walls, wide lumen, valves. Capillaries are one cell thick, allowing exchange with tissues. Blood contains red blood cells (haemoglobin carries oxygen), white blood cells (phagocytes engulf pathogens; lymphocytes make antibodies), platelets (clotting) and plasma (carries CO₂, glucose, urea, hormones, heat).

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Relate the structure of arteries, veins and capillaries to their functions.
  • State the functions of blood components.
  • Describe how blood clots.
Pause and think

When you cut yourself, blood spurts from an artery but oozes from a vein. Why?

Section 2 of 14

Core idea

Start hereRed river

Five litres of blood circulate through your body about once a minute, carrying oxygen, food, waste, heat and an army of defensive cells.

The big idea

Vessel structure matches blood pressure. Each blood component has a specific role: transport, defence or clotting.

1. Vessels

Three types of vessel

Structure matches pressure and function.

2. Blood components

Worked exampleMatching cells to jobs

A blood smear shows many red cells, a few large white cells with lobed nuclei and tiny fragments.

Goal
Name each and its function.
Strategy
Use structure clues.
  1. red cells: haemoglobin\text{red cells: haemoglobin}
    Carry oxygen.
  2. lobed nucleus: phagocyte\text{lobed nucleus: phagocyte}
    Engulfs pathogens.
  3. fragments: platelets\text{fragments: platelets}
    Clot blood.
AnswerRed cells, phagocytes, platelets.
✓ CheckLymphocytes have a large round nucleus ✓.
Key takeawayClotting Damaged tissue and platelets trigger soluble fibrinogen to form insoluble fibrin threads, which trap red cells to form a clot.
Pause and think

Why do people who stand still for hours sometimes faint?

Summary

  • Arteries thick, veins valves, capillaries exchange.
  • Red cells, white cells, platelets, plasma.
  • Fibrinogen → fibrin.
Key words
Lumen
The space inside a blood vessel.
Haemoglobin
Red pigment that carries oxygen.
Plasma
Liquid part of blood.
HistoryBlood groups. In 1901 Karl Landsteiner discovered the ABO blood groups, explaining why early transfusions often killed patients, and making safe transfusions possible.
Fun fact100,000 km of vessels. Laid end to end, all your blood vessels, mostly capillaries, would stretch around the Earth more than twice.
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