We all know that the humans and mammals breathe through lungs, but for the lungs to work properly it is essential that they must be dry. Lungs cannot function if they contain any kind of external fluid. That is the reason humans and other animals cannot breathe underwater. Now we have to wonder how do the fish breathe under water? Do they have something else to breathe underwater, instead of lungs? The answer to that question is ‘Yes’, the fish have gills that help them breathe underwater.
Almost all the creatures in the world need oxygen to breathe. The gills actually separate the oxygen from the water, which is utilized by the fish’s body. In fish, the water enters the mouth and is forced over the gills. Gills are very light and feathery organs, that are divided into groups called epithelium. Epitheliums have countless tiny blood vessels. A set of 4 gills is present on either side of the fish’s mouth. The concentration of oxygen in the water is very high. According to the concentration gradient, something moves from a point of higher concentration to a point of lower concentration. The concentration of oxygen in the water is higher than that present in the vessels. So the dissolved oxygen present in the water moves through the blood vessels, into the blood and the waste products like carbon dioxide are transferred into the water.
The gill filaments are also known as lamellae. They have a large surface area so that maximum oxygen can be absorbed. The blood flows through the lamellae in such a way that the flow of water is always opposite to the flow of blood. This ensures that the concentration of oxygen in the water is always higher than the concentration of oxygen in the blood.
The blood becomes oxygenized. This oxygenated blood is transported to all the parts of the fish’s body. The carbon dioxide in the blood is passed out through the gill flaps and into the water. Active fish have highly developed gills while the less active ones have small gills.