Gas Exchange in a Fish🐟
Fish cannot get oxygen directly from the water they live in because their bodies are covered in protective scales. Gills are made up of many thin layers of tissue with a rich blood supply.
- The gills are thin so there is only a short distance for the gas to diffuse across.
- The gills are also always kept moist because they work in water.
- In bony fish the gills are contained in a special gill cavity, and water is pumped over them constantly to maintain a concentration gradient.
- Fish, e.g. some types of shark, have to keep swimming constantly to keep water moving over their gills. If a fish is taken out of water, or if it stops swimming, the gill stacks stick together, and the oxygen does not have enough surface area to diffuse into the fish.
🐸Tadpoles and frogs
Frogs are amphibians and have a very strange life story. The eggs hatch into tadpoles which spend all their time in the water. Young tadpoles have frilly external gills with a large surface area and a rich blood supply. The tadpoles get all their oxygen by diffusion through these gills. In the same way, the carbon dioxide diffuses out along a concentration gradient into the water. When tadpoles turn into frogs they can breathe both on land and in water.
The external gills disappear and are reabsorbed into the body, we say that the tadpole undergoes metamorphosis. An adult frog has very moist skin with a rich blood supply, and underwater and on land, gas exchange takes place under the skin.
The Respiratory System in Insects 🐛🦟
Insects are very active, so their muscles need a lot of oxygen, but gas cannot diffuse across their tough outer shell. Along the side of an insect there are Spiracles. These can open when an insect needs oxygen but close when they don’t. This prevents water loss, like the guard cells of plant.
The spiracles lead into the Trachea which feed into smaller Tracheoles which run right into the cells of the tissues themselves. In I/GCSE Biology, most of the gas exchange takes place in these Tracheoles. The trachea is not permeable, so no gas escapes, but the Tracheoles are permeable, moist and able to pump air in and out.
Key Points of materials exchange
That's the end of the topic!
Drafted by Joey (Biology)