A diver surfaces too quickly from a deep-sea dive. Paramedics suspect the diver's blood is not delivering enough oxygen to the body's tissues. Explain how the structure of red blood cells and the role of plasma make blood an efficient transport tissue.

OCR A-Level Biology B — Advancing Biology (H422) — 2.5 Cardiovascular system · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Blood is a tissue that contains red blood cells, white blood cells, plasma and platelets. Red blood cells are biconcave in shape and carry oxygen around the body. Plasma transports many dissolved substances, including nutrients, urea and hormones.

Model answer (5 marks)

Red blood cells are biconcave, which increases their surface‑area‑to‑volume ratio and allows rapid diffusion of oxygen into the cell and carbon dioxide out of it. This shape also gives them flexibility to squeeze through capillaries. The cells contain haemoglobin, which binds and carries oxygen from the lungs to tissues.
Plasma is a pale‑yellow liquid that carries dissolved substances such as nutrients, urea, hormones and salts.
Together, the high surface area of the red cells for gas exchange and the plasma’s ability to transport a wide range of dissolved molecules mean that blood can efficiently deliver both oxygen and other essential substances to the body.

Examiner tips

  • Use the word ‘biconcave’ and link it to increased surface area and flexibility. Mention haemoglobin as the oxygen carrier. Give one example of a dissolved substance carried by plasma. Show how the two components work together for efficient transport.

Common mistakes

  • Forgetting to mention haemoglobin or the role of oxygen transport. Describing plasma as a solid or ignoring its dissolved‑substance function. Using vague terms like ‘good shape’ instead of ‘biconcave’ and surface area.

Mark scheme (5 marks)

  1. Red blood cells are biconcave in shape
  2. The biconcave shape increases surface area (for gaseous exchange / oxygen absorption)
  3. Red blood cells carry/transport oxygen (to the body's tissues)
  4. Plasma is a pale yellow liquid that transports dissolved substances such as nutrients, urea, hormones or salts (any one valid example accepted)
  5. Together red blood cells and plasma allow blood to transport both gases and dissolved substances, making it an efficient transport tissue (idea of combined/dual role)

Key terms in this question

red blood cells · plasma · blood

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