How Gills Work:
1. Water Flows Over Gills: Fish actively pump water over their gills, which are feathery structures located behind their heads.
2. Oxygen Diffuses: The water contains dissolved oxygen, which diffuses from the water into the blood vessels within the gills. This happens because the concentration of oxygen is higher in the water than in the blood.
3. Carbon Dioxide Diffuses Out: At the same time, carbon dioxide from the blood diffuses out into the water, following the concentration gradient.
4. Blood Carries Oxygen: The oxygenated blood is then carried throughout the fish's body to deliver oxygen to its cells.
Other Aquatic Animals:
* Marine mammals (whales, dolphins, seals): These animals breathe air through lungs like land mammals. They need to come to the surface to breathe.
* Some invertebrates (crabs, lobsters, snails): These animals have gills or other specialized structures that extract oxygen from the water.
* Some amphibians (tadpoles): Tadpoles use gills to breathe underwater, but they develop lungs as they grow into frogs and toads.
Key Points:
* The amount of dissolved oxygen in water varies depending on factors such as temperature, salinity, and depth.
* Fish and other aquatic animals have adapted specific mechanisms to extract oxygen from their watery environments.
* Gills are highly efficient organs designed for gas exchange.