How Do Fish Breathe Underwater?

How Do Fish Breathe Underwater?

We all know that aquatic animals live under water. But have you ever wondered how do fish ‘breathe’ underwater?

Why Oxygen?

To understand how they breathe, we need to understand the basics. Life on Earth is Carbon based, which uses oxygen to oxidize compounds to produce the energy all living beings consume to carry out their daily lives. In life systems, without oxygen, there can be no oxidation and without oxidation, energy cannot be released chemically. Hence all animals alive and living on the Earth depend upon Oxygen – even if it’s in the form of Water.

What is Water?

h2omoleculeWater is a compound which is a liquid under normal temperatures and pressures, ironically composed by two gas molecules – Oxygen and Hydrogen. Two molecules of Hydrogen and one of Oxygen combine together in a strong covalent bond to form a Water molecule. Quintillions and Cotillions of such molecules make water bodies on Earth. Liquid water has dissolved Oxygen, and this Oxygen is used by aquatic animals to oxidize their food to produce energy.


How is Oxygen ‘extracted’ by Fish?

Oxygen is used by aqueous animals much in the same exchange mechanism as Lungs do for land based animals. A pair of Gills act as Molecule exchangers that pass Oxygen molecules through a thin membrane of the gill. The ‘breathing’ begins when the fish open their mouths to pump water onto their gills. The Gill membrane allows the oxygen to pass directly into the bloodstream of the Fish much like how air passes into our blood in the Lungs through the Alveoli. While air-breathing animals achieve molecular exchange of Oxygen through their Alveoli, the aquatic animals achieve their molecular exchange through ‘Lamellae’. The gills permit liquid water around the fish to enter the layers of membrane and allow Oxygen to diffuse through them into their bloodstream.

how fish breathe.3We must remember that the Gills are not instrument to separate Oxygen from the two hydrogen molecules. They are just instruments to allow diffusion of Oxygen into the bloodstream via a process called ‘selective Osmosis’. Once the oxygen is in the bloodstream, the Aquatic animals function as similarly in water as all air breathing animals do on land.

Watch the video below to understand the breathing mechanism in better detail.



Written by Aru Raghuvanshi

Aru Raghuvanshi