All animals need a large amount of energy (also known as ATP) to be able to carry out the activities they do to look for food, migrate, escape predators, reproduce, and so on.

For example, muscle cells need a large amount of energy to be able to contract and relax, which gives animals the ability to escape predators. The large amount of energy needed is produced inside animal cells through a process called aerobic CELLULAR RESPIRATION. Without aerobic cellular respiration, animals would not be able to survive in their niche.

It just so happens that one of the molecules absolutely needed for aerobic cellular respiration is OXYGEN. Without oxygen, aerobic cellular respiration cannot happen, which means the animal would not get the energy it needs to survive in its niche. So, animals absolutely need oxygen for aerobic cellular respiration to make energy.

Gas Exchange

All animals need to absorb OXYGEN from air or water, and release carbon dioxide back into air or water through a process called GAS EXCHANGE. Gas exchange is the process where different gases (oxygen and carbon dioxide) get exchanged/transferred/swapped in opposite directions.

• Gas exchange is the process that supplies the organism with oxygen, as oxygen goes from the outside of the body/cell to the inside of the body/cell.

• Gas exchange is also the process that removes carbon dioxide from the organism, as carbon dioxide goes from the inside of the body/cell to the outside of the body/cell.

Diffusion

Oxygen and carbon dioxide get exchanged in opposite directions, through the process of DIFFUSION.

Diffusion is the passive movement of molecules from an area of high concentration to low concentration.

For diffusion to occur, there must be a SPECIALISED RESPIRATORY SURFACE that is semipermeable. There must also be a concentration gradient across this specialised respiratory surface to drive the process of diffusion. In other words, the concentration of oxygen must be higher on one side of the membrane, and lower on the other side of the membrane.

• Oxygen moves across the semipermeable membrane from an area of high concentration outside of the body, to an area of low concentration inside of the body.

• Carbon dioxide moves across the semipermeable membrane from an area of high concentration inside of the body, to an area of low concentration outside of the body.

Ventilation

Before GAS EXCHANGE and DIFFUSION can happen, air/water first needs to get from the outside of the body, to the SPECIALISED RESPIRATORY SURFACE which is located inside of the body. So animals must first take in the air/water from their surroundings, through a process called VENTILATION.

Ventilation is the flow of air or water in and out of the gas exchange system, to increase the rate of gas exchange.

I like to use the analogy of getting a bad smell out of a room. When you want to get a bad smell out of the room, you open windows and get a fan going, to get some air flowing through the room. The open window lets new air in and allows old smelly air to escape, while the fan circulates the air faster.

So animals like HUMANS, CRICKETS, and SNAPPER ventilate to increase the flow of new air/water in and old air/water out, to increase the rate of gas exchange.