GAS EXCHANGE BonyFish

Specialised Exchange Surface Bony fish have a small surface area to volume ratio for gas exchange. They have an impermeable membrane so gases can’t diffuse through their skin. They need a specialised exchange surface. For bony fish their specialised exchange surfaces are their gills.

Features of Gills Large surface area : volume - allows more diffusion permeable membranes - allows gases to diffuse thin (flattened cells) - short diffusion distance good vascular supply - maintains concentration gradients

Lamellae On the filaments of the gills there are lamellae which take part in the exchange of gases. Blood and water flow across the lamellae in a counter current direction. This means that the blood flows in one direction and the water flows in the opposite direction. Diffusion takes place along the whole length of the lamellae because equilibrium between the oxygen in the water and the oxygen in the blood is not reached.

Ventilation in bony fish Ventilation is a mechanism which maintains diffusion gradients. The mode of ventilation in fish is called gill irrigation. The cycle of gill irrigation starts by the fish opening it’s mouth and lowering the floor of it’s buccal cavity. Water flows into the buccal cavity. The fish closes it’s mouth and raises the floor of it’s buccal cavity. This makes the volume of it’s buccal cavity smaller so therefore the pressure increases. This forces the water over the gills. The fish opens it’s mouth again and the sides of the operculum bulge out. This increases the volume of the opercula cavity which decreases the pressure so water is drawn over the gills. The opercula flaps open and the operculum contracts forcing the water out. The flaps then close again. The fish opens it’s mouth and the cycle begins again.