Gills are respiratory organs that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. They are an evolutionary adaptation of many aquatic animals that allow them to effectively breathe underwater. Gills usually consist of thin filaments of tissue, lamellae (plates), branches, or slender, tufted processes that have a highly folded surface to increase surface area. The delicate nature of the gills is possible because the surrounding water provides support. The blood or other body fluid must be in intimate contact with the respiratory surface for ease of diffusion.
Gills can be found in fish, crustaceans, worms, and mollusks. Water passes over the gills in one direction, and oxygen is absorbed by the capillaries while carbon dioxide is released. The walls of the gills are very thin and loaded with tiny tubes that transport blood (called “capillaries”). This means the capillaries come into close contact with the water outside, letting oxygen pass across the thin walls and into the blood. At the same time, carbon dioxide, which is a waste product of respiration, passes from the blood into the water.
