(f) the mechanisms of ventilation and gas exchange in bony fish and insects

The idea

Bony fish ventilate gills by a buccal-opercular pump: the mouth (buccal cavity) opens and the operculum closes while the floor of the mouth lowers, drawing water in; then the mouth closes and the operculum opens while the buccal floor rises, forcing water across the gill filaments. Gas exchange occurs across gill lamellae, which have a large surface area, thin epithelium and a rich blood supply. A counter-current flow of water and blood across the lamellae maintains a concentration gradient along the whole length, maximising oxygen diffusion. Insects ventilate tracheae through spiracles along the abdomen and thorax; air moves in and out by diffusion and by abdominal pumping movements that change pressure. Tracheae branch into tracheoles with thin walls and fluid-filled tips that deliver oxygen directly to respiring tissues, so insects lack a circulatory carrier for oxygen.

What earns the marks

• Bony fish use a buccal-opercular pump: lowering the buccal floor with the operculum closed draws water in, then raising it with the operculum open forces water over the gills.
• Gill lamellae provide a large surface area, thin epithelium and rich blood supply for efficient gas exchange.
• Counter-current flow of water and blood across lamellae maintains a diffusion gradient along the whole gill, maximising oxygen uptake.
• Insects ventilate via spiracles and tracheae; abdominal pumping movements and diffusion move air through the tracheal system.
• Tracheoles have thin walls and fluid-filled tips delivering oxygen directly to tissues, so no respiratory pigment is needed for transport.
• Spiracles can close to reduce water loss, and the tracheal system delivers oxygen directly without a circulatory carrier.

Common mistakes and how to put them right

• Describing fish gills as having parallel (concurrent) flow of water and blood.

  Put it right: The flow is counter-current, which maintains the gradient along the whole lamella.

• Stating that insects use blood to transport oxygen.

  Put it right: Oxygen travels through tracheae and tracheoles directly to tissues.

• Saying water is forced over the gills by the operculum alone.

  Put it right: Both the buccal cavity floor and the operculum act together in the buccal-opercular pump.

Exam tips

• For fish, describe the sequence of mouth and operculum movements in order, naming the buccal-opercular pump.
• For counter-current exchange, explain why a gradient is maintained along the whole lamella rather than only at one end.
• For insects, link each tracheal feature (spiracles, tracheae, tracheoles) to its function in ventilation or gas exchange.