Explain how the structure of a gill lamella is adapted to facilitate efficient gas exchange in bony fish.

Gill lamellae are highly specialised structures that maximise the efficiency of gas exchange in bony fish.
1. The lamellae are long, thin sheets that give a very large surface area relative to the volume of the fish, providing more area for diffusion.
2. The epithelial lining of each lamella is only one cell thick, which minimises the diffusion distance for O₂ and CO₂ between the water and the blood.
3. A dense network of capillaries lies immediately beneath the epithelium, keeping the blood in close contact with the water and maintaining a steep concentration gradient.
4. Blood flows through the capillaries in the opposite direction to the flow of water (counter‑current flow), so the gradient is maintained along the whole length of the lamella, allowing continuous uptake of oxygen and release of carbon dioxide.