Fishes breath through their gills which is a small flap like structure which has millions of blood capillaries which absorb oxygen from the influx of water and give carbon dioxide in same water.

Breathing Organ in Fish: Gills

Gills are the primary breathing organs in fish. These specialized structures are responsible for extracting oxygen from water and eliminating carbon dioxide. Gills are located on either side of the fish's head, protected by a bony structure called the operculum.

Structure of Fish Gills:
Fish gills consist of numerous thin, feathery structures called gill filaments. These filaments are packed with tiny blood vessels called capillaries, which are responsible for gas exchange. Each filament is covered with tiny projections called gill lamellae, which increase the surface area available for oxygen absorption.

Breathing Process in Fish:
1. Inhalation: To breathe, fish open their mouths, allowing water to flow in. As water enters the fish's mouth, it passes over the gills.

2. Gill Filtration: Inside the fish's gills, water flows over the gill filaments. Oxygen present in the water diffuses across the thin walls of the filaments and into the fish's bloodstream, while carbon dioxide moves from the bloodstream into the water.

3. Countercurrent Exchange: The efficiency of gas exchange in fish is enhanced by a mechanism called countercurrent exchange. Blood flows through the gill filaments in the opposite direction to the water flow. This allows for a concentration gradient that ensures maximum oxygen absorption from the water.

4. Exhalation: After oxygen has been absorbed and carbon dioxide has been released, the water is expelled through the fish's gill slits. The operculum, a protective covering, helps regulate the flow of water over the gills.

Adaptations for Efficient Respiration:
- Fish have a highly efficient respiratory system due to their gills' large surface area and the countercurrent exchange mechanism.
- Some fish species, such as lungfish and mudskippers, are capable of breathing air when oxygen levels in the water are low.
- Certain fish, like the betta fish, possess a labyrinth organ that allows them to extract oxygen directly from the air.

In conclusion, gills are the breathing organs in fish. They play a crucial role in extracting oxygen from water and eliminating carbon dioxide. Fish have adapted to efficiently breathe underwater through specialized structures and mechanisms, ensuring their survival in aquatic environments.