Gills: The Key to Underwater Breathing
Fish don't drown because they possess specialized respiratory organs called gills. Unlike lungs, which are adapted for breathing air, gills are designed to efficiently extract oxygen dissolved in water. This process is entirely natural and essential for their survival in aquatic environments.
How Gills Work: Countercurrent Exchange
Each gill is made up of numerous feather-like filaments covered in lamellae, which are thin, plate-like structures containing a rich network of blood vessels. Water flows over these lamellae in one direction, while blood flows through them in the opposite direction. This "countercurrent exchange" maximizes the efficiency of oxygen transfer from the water into the fish's bloodstream.
The Role of Water Flow and Oxygen Extraction
To keep oxygenated water flowing over their gills, most fish constantly pump water through their mouths and over their gills (a process called buccal pumping). Some fast-swimming fish, like sharks, practice "ram ventilation," meaning they must continuously swim with their mouths open to force water over their gills. This constant flow ensures a steady supply of oxygen.
Adaptations for Aquatic Life
The ability of fish to "breathe" water is a highly evolved adaptation. Their circulatory system is also specialized to transport oxygen absorbed by the gills throughout the body and remove carbon dioxide. This intricate system allows them to thrive in environments where terrestrial animals would quickly suffocate.