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1. Fish breathe by absorbing the oxygen present in the water. Therefore, they need to be continuously swallowing water from rivers or the sea, which goes to a respiratory organ, the gills.
2. When it reaches the gills (also known as the gills), the water first passes through small cilia on the organ, which serve to filter impurities such as food remains, sand or debris
3. Then, the filtered water passes through the gills, which have tiny structures formed by filaments and lamellae — very thin and malleable blades
4. It is in the filaments and lamellae that gas exchange takes place — which in the human body takes place in the pulmonary alveoli. Both have very thin vessels, called capillaries, through which blood circulates in the opposite direction to water, which increases the exchange efficiency.
5. As water concentrates more oxygen (O2) and blood more carbon dioxide (CO2), there is a tendency for these concentrations to balance — a process called diffusion. Then, the oxygen in the water passes through the permeable walls of the lamellae and filaments and enters the bloodstream, while the carbon dioxide flows in the opposite direction.
6. While the blood carries oxygen throughout the body of the fish, the water carrying carbon dioxide crosses the gills and leaves the body through the gill slits, lateral openings close to the animal’s head.