A-Level Biology May/June 2024 Q1(c)(ii): Explain how the structure of a capillary is related to its function in smooth muscle.

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  Capillaries function to exchange substances with surrounding tissues, such as smooth muscle. They supply the muscle cells with required substances like oxygen and glucose for respiration, and remove waste products like carbon dioxide.

  This function is supported by several structural adaptations:

  1. The capillary wall is extremely thin, consisting of a single layer of endothelial cells. This creates a very short diffusion pathway, allowing for rapid and efficient exchange of substances between the blood and the muscle cells.

  2. The lumen of the capillary is very narrow (approximately 7-8 µm in diameter). This slows the rate of blood flow, which maximises the time available for diffusion to occur. It also forces red blood cells to pass close to the capillary wall, reducing the diffusion distance for oxygen.

  3. The endothelial cells that form the capillary wall have pores between them, known as fenestrations. These pores increase the permeability of the capillary, allowing for more efficient passage of substances and the formation of tissue fluid around the smooth muscle cells.

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  How the marks are awarded

  • B1 — For correctly naming a substance supplied to smooth muscle cells (oxygen/glucose) and a waste product removed (carbon dioxide).
  • B1 — For stating that the capillary wall is very thin, and linking this structure to the functional advantage of a short diffusion pathway.
  • B1 — For explaining that the narrow lumen slows blood flow to increase the time for diffusion, OR that it reduces the diffusion distance for oxygen from red blood cells.

  Common mistakes

  • Stating that 'the capillary is thin' instead of specifying that the 'capillary wall is thin' or 'one cell thick'. The capillary itself is a tube, not just a thin line.
  • Incorrectly stating that red blood cells or whole blood leave the capillary to form tissue fluid. Only plasma and some white blood cells can pass through the capillary walls.
  • Giving a vague answer like 'it allows for exchange' without explaining how a specific feature (e.g., thin walls, narrow lumen) facilitates this exchange.
  • Confusing pores (fenestrations) with the general permeability of the wall, or failing to mention them as a distinct feature that aids efficient transport.

  Examiner tip: For 'structure relates to function' questions, always state a specific structural feature first, and then explicitly link it to a functional advantage using connecting phrases like 'this allows for...', 'which results in...', or 'creating a short...'

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