Aquatic animals, such as fish, use the oxygen dissolved in water for respiration. The process of respiration in aquatic animals is different from that of terrestrial animals. In terrestrial animals, respiration occurs through lungs, while in aquatic animals, it occurs through gills.



Gills are specialized respiratory organs that allow aquatic animals to extract oxygen from water. Gills are made up of thin, flat filaments that are covered in tiny blood vessels called capillaries. Water flows over the gills, and oxygen diffuses from the water into the capillaries. At the same time, carbon dioxide from the animal's blood diffuses into the water and is carried away.



One of the key features of gills is the countercurrent exchange system. This system ensures that oxygen is efficiently extracted from water and that the animal's blood is fully oxygenated. In this system, water flows over the gills in the opposite direction to the flow of blood in the capillaries. This causes the oxygen concentration gradient to be maintained throughout the entire length of the gill filaments, ensuring that oxygen diffuses from the water into the blood as efficiently as possible.



Once oxygen has diffused into the animal's blood, it is transported to the body's cells, where it is used for cellular respiration. In fish, oxygen is transported by a protein called hemoglobin, which is contained in the red blood cells. Hemoglobin binds to oxygen in the gills, and then releases it in the body's tissues.



In conclusion, aquatic animals use gills to extract oxygen from water for respiration. The countercurrent exchange system in gills ensures that oxygen is efficiently extracted from water, and hemoglobin in the animal's blood transports oxygen to the body's cells. This process of respiration in aquatic animals is essential for their survival in their aquatic habitat.