Final answer:
The primary function of gills in aquatic animals is gas exchange, where gills allow oxygen to diffuse from seawater into the blood and carbon dioxide to diffuse from the blood into seawater. Gills have a large surface area due to their thin, folded structure, facilitating efficient diffusion based on concentraion gradients.
Step-by-step explanation:
The primary function of gills in aquatic animals is to facilitate the exchange of oxygen and carbon dioxide between the animal's blood and the surrounding seawater. Gills are composed of thin tissue filaments that are highly branched and folded, providing a large surface area for gas exchange. When water passes over the gills, oxygen dissolved in the water diffuses across the gill surfaces into the bloodstream due to a higher concentration of oxygen in the water compared to the blood.
Conversely, carbon dioxide in the blood, being at a higher concentration, diffuses into the water. This process is known as diffusion and is driven by the concentration gradients of these gases.
Diffusion ensures that the fish's blood becomes oxygenated as it travels through the gills. Likewise, carbon dioxide, a waste product of metabolism, is removed from the blood and released back into the water. The circulatory system then transports the oxygen-rich blood to various parts of the body, supporting the fish's cellular processes and overall metabolism. Gills are thus critical for the respiratory function of fish and many other aquatic organisms, including some mollusks, annelids, and crustaceans.