where does gas exchange take place in the gills
Fish gill Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area. The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does. A cubic meter of air contains about 250 grams of oxygen at STP. The concentration of oxygen in water is lower than air and it diffuses more slowly. In a litre of freshwater the oxygen content is 8Â cm3 per litre compared to 210 in the same volume of air.[8] Water is 777 times more dense than air and is 100 times more viscous.[8] Oxygen has a diffusion rate in air 10,000 times greater than in water.[8] The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life.[8] Rather than using lungs "Gaseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water."[8]
Epiglottis The epiglottis is a flap in the throat that keeps food from entering the windpipe and the lungs. The flap is made of elastic cartilage covered with a mucous membrane, attached to the entrance of the larynx. It projects obliquely upwards behind the tongue and the hyoid bone, pointing dorsally. It stands open during breathing, allowing air into the larynx. During swallowing, it closes to prevent aspiration, forcing the swallowed liquids or food to go along the esophagus instead. It is thus the valve that diverts passage to either the trachea or the esophagus.