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Aquatic Respiration
In fish, respiration takes place in the gills. Gills collect dissolved oxygen from the water and release carbon dioxide. Gills are much more complex than just a slit in the cheeks of a fish.
Gills are comprised of gill arches with hundreds of gill filaments extending from them. Each filament is lined with rows of lamellae, and the gas exchange takes place as water flows through them. The frills and flaps increase the surface area to allow more gas exchange to take place, just as the alveoli do in the lungs.Fish utilize a countercurrent exchange pathway (except for cartilaginous fish), which means that their arteries are arranged so that blood flows in the opposite direction of water movement against the gills. By having their respiration pathway in this orientation, maximum gas exchange can take place.
If the blood and the water were moving in the same direction, the blood would always be next to the same bit of water which would soon be depleted of oxygen. By setting up a countercurrent pathway, the blood is always passing water that is still oxygenated. This allows the blood to gather as much oxygen as possible.
Since water must be flowing over the gills to provide a continual source of oxygen, fish have developed several ways to keep them ventilated. Some fish swim with their mouths open almost all of the time. Other fish have a special flap called an operculum, which is used to force water across the gills.
Like all good rules, there’s an exception. While all fish have gills, one fish also has lungs. The lungfish can survive when its water habitat dries up from seasonal drought. What an aptly named fish. There’s also certain land crabs that have both lungs and gills, and can breathe both under the sea and on land.Every cell in an animal requires oxygen to perform cellular respiration. Cellular respiration is the process by which animals take in oxygen and exchange it for carbon dioxide and water as waste products. Animals have specialized systems that help them do this successfully and efficiently. Even a fish will drown if it can’t breathe underwater.
Gas Exchange
The actual exchanging of gases is dependent upon important structures such as lungs or gills, and the principle of diffusion. Diffusion is the process where molecules or particles move from an area where they are very concentrated into an area where they are less concentrated.
Below is an illustration showing the process of diffusion. A container is separated with a semipermeable membrane, dividing an area with a high concentration of a molecule (red dots) from an area with lower concentration. The membrane allows the molecules to move from one side to the other. Over time, the molecules will move from an area of higher concentration to an area of lower concentration. The molecules will continue to migrate across the membrane until there is an equal amount on both sides. This is called equilibrium, or when both sides of a membrane have equal concentrations.
