GASEOUS EXCHANGE IN BONY FISH (E.G. TILAPIA)
- Gaseous exchange in fish takes place between the gills and the surrounding water.
- The gills are located in an opercular cavity covered by a flap of skin called the operculum.
- Each _gill consists of a number of thin leaf-like lamellae projecting from a skeletal base branchial arch (gill bar) situated in the wall of the pharynx.
- There are four gills within the opercular cavity on each side of the head.
- Each gill is made up of a bony gill arch which has a concave surface facing the mouth cavity (anterior) and a convex posterior surface.
- Gill rakers are bony projections on the concave side that trap food and other solid particles which are swallowed instead of going over and damaging the gill filaments.
- Two rows of gill filaments subtend from the convex surface.
Adaptation of Gills for Gaseous Exchange
- Gill filaments are thin walled.
- Gill filaments are very many (about seventy pairs on each gill), to increase surface area.
- Each gill filament has very many gill lamellae that further increase surface area.
- The gill filaments are served by a dense network of blood vessels that ensure efficient transport of gases.
- It also ensures that a favourable diffusion gradient is maintained.
- The direction of flow of blood in the gill lamellae is in the opposite direction to that of the water (counter current flow) to ensure maximum diffusion of gases.
- As the fish opens the mouth, the floor of the mouth is lowered.
- This increases the volume of the buccal cavity.
- Pressure inside the mouth is lowered causing water to be drawn into the buccal cavity.
- Meanwhile, the operculum is closed, preventing water from entering or leaving through the opening.
- As the mouth closes and the floor of the mouth is raised, the volume of buccal cavity decreases while pressure in the opercular cavity increases due to contraction of opercular muscles.
- The operculum is forced to open and water escapes.
- As water passes over the gills, oxygen is absorbed and carbon dioxide from the gills dissolves in the water.
- As the water flows over the gill filaments oxygen in the water is at a higher concentration than that in the blood flowing, in the gill.
- Oxygen diffuses through the thin walls of gill filaments/lamellae into the blood.
- Carbon (IV) oxide is at a higher concentration in the blood than in the water.
- It diffuses out of blood through walls of gill filaments into the water.
Counter Current Flow
- In the bony fish direction of flow of water over the gills is opposite that of blood flow through the gill filaments .
- This adaptation ensures that maximum amount of oxygen diffuses from the water into the blood in the gill filament.
- This ensures efficient uptake of oxygen from the water.
- Where the flow is along the same direction (parallel flow) less oxygen is extracted from the water.
Observation of Gills of a Bony Fish (Tilapia)
- Gills of a fresh fish are removed and placed in a petri-dish with enough water to cover them.
- A hand lens is used to view the gills.
- Gill bar, gill rakers and two rows of gill filaments are observed.