Adult fish have a pair of gills. Each gill is covered by a boney lid (removed from the picture). A fish draws in water by closing the lid over its gills and opening its mouth. When the fish closes its mouth and opens the gill lid the water is forced out and over the respiratory surfaces of the gill filaments.
These lamellae are made of extremely thin membranes (1 cell thick) and are the primary sites of gas exchange. Water flows across the gill filaments and oxygen is removed and passes into the blood by diffusion. To increase the efficiency of oxygen uptake a countercurrent method is used (the same principle as used in force air furnaces); blood flows through the lamellae in a direction opposite to the water flow through the gill filaments. Countercurrent flow insures a steady oxygen
The respiratory system of sharks is markedly different from that of bony fishes. Where bony fishes usually have five gilled arches and only one external gill opening, sharks may have as many as seven openings, but the most common number is five. Also, where the gill arches of bony fishes are protected by an opercle, or plate, the gills of sharks are not.
Sharks generally inhale most of the necessary water through their mouths, but they are also able to inhale water by way of spiracles, which are opening located close to the gills. When resting, sharks propel water over their gills using the muscles of their jaws and pharynx. Oxygen from the incoming water is absorbed into the blood system by way of the gill filaments. Water exits through the gill slits .(Davies, 1964).
- most vertebrates develop an outpocketing of pharynx or esophagus that becomes one or a pair of sacs (swim bladders or lungs) filled with gases derived directly or indirectly from the atmosphere. Similarities between swim bladders & lungs indicate they are the same organs.
hearing - some freshwater teleosts (e.g., catfish, goldfish, & carp) 'hear' by way of pressure waves transmitted via the swim bladder and small bones called Weberian ossicles (see diagram below)
sound production - muscles attached to the swim bladder contract to move air between 'sub-chambers' of the bladder. The resulting vibration creates sound in fish such as croakers, grunters , & midshipman fish .
respiration - the swim bladder of lungfish has number subdivisions or septa (to increase surface area) & oxygen and carbon dioxide is exchanged between the bladder & the blood