gas exchange in fish

why are fish gas system specialised

• water is moved in one direction over an exchange surface- gills

• this is much more energy efficient than pumping viscous and dense water in and out of lung like structures

two classes of fish

• cartilage skeleton = sharks

• bone skeletons= fish

operculum

a flap of tissues that covers the gills in bony fish

gill strucutre

• Series of gills on each side of the head

• Each gill arch is attached to two stacks of filaments (primary lamellae)

• On the surface of each filament, there are rows of lamellae

• The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries

• filaments have a rich blood supply with capillaries close to the lamellae surface

what does the gill raker do

• involved in collecting food particles to stop them going into gills

how are the gills adapted for efficient gas exchange

• tips of adjacent gills overlap to increase resistance and slow the flow of water over the gill surface- increasing the time for gas exchange

• water moving over the gills and blood in gill filaments flow in different directions-counter current system. this means a steep gradient is maintained so more gaseous exchange can take place

• afferent blood vessel brings blood into the system

• many lamellae

• short distance between blood and water

• blood maintains diffusion graident

what is a counter current system

• blood in gills is moving in the opposite direction to the water flowing over the gills

• this means blood is continually meeting fresh water with a higher percentage saturation of oxygen= diffusion gradient is maintained along entire surface

why would a parallel or concurrent flow result in less efficient gas exchange

• diffusion gradient is only maintained for half of distance across lamellae

• only 50% of oxygen from the water diffuses into the blood

how can fish keep water flowing over gills

• by opening their mouth and operculum while swimming

1st stage of ventilation in bony fish

1. mouth open (operculum is closed)

2. buccal cavity is lowered (mouth)

3. this increases the volume and decreases the pressure of the buccal cavity compared to outside

4. water rushes into the mouth down a pressure gradient

2nd stage of fish ventilation

• mouth closes

• Water flows from the buccal cavity (high pressure) into the gill cavity (low pressure)

• As water enters pressure begins to build up in the gill cavity and causes the operculum (a flap of tissue covering the gills) to be forced open and water to exit the fish

• water is pushed over gills

• The operculum is pulled shut when the floor of the buccal cavity is lowered at the start of the next cycle