Why is it that people cannot breathe under water but a fish can?

It's all about how we extract oxygen from our environment! Here's the breakdown:

Humans Need Air:

* Lungs: We have lungs, specialized organs that extract oxygen from the air we breathe.

* Oxygen: Our bodies need oxygen to fuel our cells and keep us alive.

* Water Problem: Water doesn't contain enough dissolved oxygen to support our lungs. Even if we could force water into our lungs, the tiny air sacs wouldn't be able to absorb enough oxygen. Plus, water is much denser than air, making it difficult to fill our lungs with it.

Fish Breathe Underwater:

* Gills: Fish have gills, which are feathery structures that extract oxygen from the water.

* Efficient Design: Gills are designed to maximize surface area, allowing them to absorb oxygen from the water very efficiently.

* Blood Flow: Blood flows through the gills, picking up oxygen and releasing carbon dioxide.

In Summary:

Humans and fish have different respiratory systems perfectly adapted to their environments. Humans have lungs for breathing air, while fish have gills for breathing underwater.