Gills: Fish have specialized organs called gills that are responsible for extracting oxygen from water. Gills are composed of thin filaments that increase the surface area for efficient gas exchange.
1. Water Flow: Fish open their mouths and take in water through their mouths. The water then flows over the gills.
2. Oxygen Diffusion: As the water passes through the gills, oxygen from the water diffuses across the thin gill filaments into the bloodstream of the fish. The capillaries in the gills facilitate the exchange of gases.
3. Gas Exchange: Inside the gills, specialized cells called chloride cells actively transport chloride ions from the water into the bloodstream. In exchange, sodium ions and bicarbonate ions move out of the fish's body into the water. This process creates a flow of water over the gills, ensuring a constant supply of oxygenated water.
4. Carbon Dioxide Release: As oxygen is absorbed from the water, carbon dioxide, a waste product of cellular respiration, diffuses out of the fish's blood into the water through the gills.
5. Regulation of Blood Flow: The blood flow to the gills is regulated to optimize oxygen uptake. When the demand for oxygen is high, such as during exercise or stress, blood flow to the gills increases, allowing more oxygen to be extracted from the water.
It's important to note that while fish can breathe underwater, they cannot breathe air directly like humans. Fish have evolved with specialized respiratory adaptations that enable them to extract oxygen from water, making them well-suited for their aquatic environments.