Question

The aquatic animals can breathe under water due to the presence of dissolved
________________.
A. Carbon dioxide
B. Oxygen
C. Nitrogen
D. None of the above

Answer 1

Hint: Just like humans, other animals living on the land have lungs to inhale the gases from the atmosphere, the aquatic animals for example fish etc. have gills to breathe.

Step by step answer:With the help of the gills the aquatic animals are capable to inhale the oxygen dissolved in the water. The gills are necessary for the process of breathing underwater because it is especially designed to separate the dissolved oxygen from the water. The lungs are not able to perform this type of function.
Gills have a feathery structure and are made up of blood vessels. This structure of gills helps them to filter out the dissolved oxygen from the water and make the oxygen available for the purpose of breathing. The process of breathing in aquatic animals starts from the intake of water inside the mouth and forcing it out through the gill passages. The dissolved oxygen gets filtered by the thin walls of gills as the water passes through the gills. The oxygen then moves into the blood of the aquatic animals like fish and reaches to the cells of the fish.
But there are some exceptions among the aquatic animals which do not utilise the dissolved oxygen for breathing. They are mammals living in the water for example; whale, dolphin, etc. They come to the surface of water for breathing. This is because they have lungs for breathing rather than gills. So, they are not able to inhale the oxygen dissolved in the water and intake the oxygen from the atmosphere.

Hence, option (B) oxygen, is the correct option.

Note: The aquatic animals breathe underwater due to the presence of the dissolved oxygen in the water. They inhale the dissolved oxygen with the help of a feathery organ named gills. Gills are composed of blood vessels. The thin walls of the gill filter out the oxygen dissolved in the water and make it available to the cells of the aquatic animal.