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.