Respiratory System of a fish

Respiration is the process of taking in oxygen, using it in oxidation of glucose, releasing energy and eliminating waste products like carbon dioxide and water in the presence of respiratory enzymes. Respiration is brought about in two steps namely external respiration and internal respiration

Respiration in fish takes place with the help of gills. Most fish possess gills on either side of their head. Gills are tissues made up of feathery structures called gill filaments providing a large surface area for exchange of gases. A large surface area is crucial for gas exchange in aquatic organisms as water contains very little amount of dissolved oxygen. The filaments in fish gills are organized in rows in the gill arch. Each filament comprises lamellae, which are discs supplied with capillaries. Blood moves in and out of the gills through these small blood vessels. Though gills in fish occupy only a small section of their body, the extensive respiratory surface produced by the filaments renders the whole organism with efficient gas exchange.

Fish take in oxygen-rich water via their mouths and pump it over their gills. When water moves over the gill filaments, the blood within the capillary network takes up the dissolved oxygen. Then, the circulatory system supplies oxygen to all tissues of the body and finally to the cells while taking up carbon dioxide that is eliminated through the gills from the body. It exits the body of the fish once the water moves past the gills through the openings provided in the sides of the throat or through the operculum, a flap, usually found in bony fish, that covers and protects the fish gills.

Several fish, such as lampreys and sharks, have multiple gill openings. Rohu, a bony fish, has a single gill opening on either side.

How Do Some Fishes Respire Without Gills?

Although most fishes in the ecosystem employ gills for respiration, some others adapt other methods to facilitate the same. Below mentioned are the most significant among them:

Cutaneous respiration (gas exchange occurs across the skin of an organism instead of gills or lungs). E.g., Reedfish and mudskippers.

An electric eel can respire through buccal cavities.

Some fishes like Scolopacidae respire through digestive tracts.

Some fishes have accessory breathing organs, like labyrinth organs above gills in labyrinth fish, etc.