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.
Ventilation
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.