A. Gills
Other organisms use a localized region of the body that is golded or branched, which enlarges the area of the respiratory surface for gas exchange. The expanded respiratory surface for aquatic animals, external and bathed, are gills. A detterent in having water as a respiratory medium is that the concentration of oxygen in the water is lower than in the air; the dissolved oxygen concentratin in water is about .4% compared to about 20% in air. The warmer and saltier the water, the less dissolved oxygen it holds. Gills must be efficient in obtaining oxygen from the water. A process helps is ventilation, increased flow of the respirtory medium over the respiratory surface. Lobsters and crayfish use tiny appendages to beat a current of water over the gills. If ventilation didn’t ovvur, water around the gills would stagnate and become quickly depleted of oxygen.
The arrangement of capillaries in the gills increases gas exchange. Blood flows opposite to the direction in which water passes over the gills, a countercurrent exchange. With countercurrent exchange, gills can remove 80% if the oxygen in the water. Gill baskets are found in tunicates and lancelets. Tunicates hava na open circulatory system; their blood lacks hemoglobin. Gill baskets are highly branched structures with enormous surface area, but they probably don’t fuction in gas exchange. Instead they may strain sea water. Fish have a combination of respiratory and circulatory system. There is a thin walled, finely divided out-pocketing with extensive capillary beds through which blood flows, bringing carbon dioxide and carrying oxygen away. These structures are gills.
Gills in Fishes: In fish, gills have a solely respiratory function. There are five pairs of gills found on most fish. Gills are rows of fingers like rods from which feather filaments arise either side. Each supporting rod hoiuses an artery that sends capillaries into each filament. Oxygen and carbon dioxide can pass across the walls of the capillaries easily. Oxygenated blood passes back to the supporting rod and joins to form a dorsal aorta which branches off to the body for distribution.
Gills are composed if rows of supportive gills arches along with blood vessels and
Gill filaments form these arches. Gill rakers are stiff fingers-like protrusions from gill arches. These keep swallowed foood from passing over the gills. All active aquatic animals must keep water flowing constantly over the respiratory surfaces. Sharks swim constantly with their mouths open for the most part. Fish keep water moving by moving mouth muscles. Gills are closed off to the outside by the opeculum. The fish closes its mouth and swallows, the operculum opens and closes the watere flows out over its gills.