Free Ventilation and Gas Exchange in Gills revision notes for OCR A Level Biology – covering specification point 3.1.1(f).
Gills: Gaseous exchange in bony fish
Gas exchange systems in bony fish maximise the rate of diffusion of oxygen into the bloodstream and carbon dioxide out into the water.
A single gill (of which there are many on each side) consists of a bony or cartilaginous gill arch, from which extend many gill filaments: long, thin, (horizontal) tubular projections.
The table below outlines the structure and function of the gill’s components:
| Component | Structural Description | Function |
|---|---|---|
| Gill Arch | A bony or cartilaginous structure that supports a gill. | Provides rigid support for the gill filaments and lamellae, keeping them well-positioned for gas exchange. |
| Gill Filaments | Long, thin horizontal projections extending from the gill arches. | Increases the surface area available for gas exchange, maximising oxygen uptake and carbon dioxide removal. |
| Lamellae | Thin, plate-like structures lined up along each gill filament; they have a rich capillary network. | Provides a large surface area and thin diffusion distance, maximising oxygen uptake and carbon dioxide removal. |
| Capillary Network | A dense network of capillaries within each lamella. |
– Blood flows opposite to the water flow (counter-current system). – The capillary network maintains a steep oxygen concentration gradient across the entire gill, maximising gas exchange. |
| Operculum | A bony flap covering and protecting the gills. |
– Protects delicate gill structures. – Pumps water over the gill surfaces when the fish is stationary to ensure a constant oxygen supply. |
Ventilation in fish follows the three stages outlined in the table below:
| Stage | Action |
|---|---|
| Inhalation |
– Mouth opens – Floor of buccal cavity lowers → volume increases – Water enters |
| Buccal cavity rises |
– Mouth closes – Buccal cavity contracts → pressure increases – Water forced over the gills |
| Exhalation |
– Operculum (bony flap) opens – Water exits via the opercular cavity |
