Exchange of materials in other Organisms
Fish can't get Oxygen directly from the water, as their bodies are covered in protective scales.
However, they have a very effective respiratory system to allow them to live well in water.
Fish have Gills, made up of lots of thin layers of tissue (This makes a large surface area) with a rich blood supply. They're thin, so there is only a small distance for gas to diffuse across, and the surfaces are always moist as they work in water.
In bony fish, water is pumped over their gills continuously to maintain a concentration gradient. These fish, like some shark, have to keep moving so water is constantly pumped across their gills, and they can breathe.
Gills don't work out of water, so a fish out of water will suffocate. Without water surrounding the gills, they stick together. There then isn't a big enough surface area available for fish to get the Oxygen it needs to survive.
Exchange in other Organisms 2
Tadpoles and Frogs:
Tadpoles have frilly external gills, that work in the same way as fish's gills. They have a large surface area and rich blood supply. Oxygen diffuses into their system from the water around them in which they live, while C02 diffuses out of their bodies into the water.
When they turn into frogs, they spend most of their time on the land, but can still breathe underwater!
This is why they undergo a Metamorphosis, where their gills disappear. Instead, they have moist skin with a rich bloody supply, and most of its gas exchange takes place through this skin.
Frogs also have a simple pair of lungs that help with gas exchange by increasing the surface area for gas exchange to take place, in case the frog becomes hot or very active on land
All gas exchange takes place through the skin when it is in water
Exchange in other Organisms 3
Many insects are very active, and so their muscles need a lot of oxygen. To get this oxygen they have an internal respiratory system to provide oxygen and remove C02.
Along the side of an insect, you can find tiny little things called Spiracles
These open up when the insect needs a lot of oxygen, but close when they don't. This is to prevent water loss.
The Spiracles lead into a system of tubes that lead directly lead to the cells. Most of the gas exchange takes place in the Tracheoles. These tubes are freely permeable to gas. They are very moist and air is pumped in and out of them by the insect to maintain a concentration gradient
There is no bloody supply in an insect, but the Tracheoles have a large surface area and come into close contact with the individual cells in the insects body, so they are very effective at gas exchange.