 Dear students, in this topic we shall discuss gas exchange across the hills of fishes. The fishes live in aquatic environment which has only about 2.5% oxygen content as compared to that present in the air. That is fishes live in an environment which has very little concentration of oxygen. In this way fishes have specialized structures which are called gales which are efficient respiratory organs which can extract oxygen from water containing very low oxygen in dissolved form. However, to maintain an adequate flow or extraction of oxygen in blood, large quantities of water must pass across them. To maintain continuous flow of water over the respiratory surfaces that is gales, the tiliost fishes have a muscular brinquial pump to move water. This brinquial pump is powered by skeletal muscles of pharynx and upper cooler cavity. This pump works as the water is drawn by it into the mouth then water is passed over the gales and it exits through the gale clefts or gale openings. In this way water flows from the mouth to the gales because of a pumping mechanism of brinquial cavity. In cartilaginous fishes as there is no upper coulomb, so there is no brinquial pump. However, some cartilaginous fishes have gill bars with external flaps. These gill bars with external flaps work in the same way as brinquial cavity is done. And in this way they form a pumping cavity. However, the fishes who do not have gill bars with flaps, they usually move in water or swim by keeping their mouth open. For example, tuna fish, elasmobranks, different sharks, swimming by keeping their mouth open is called ram ventilation. Ram ventilation causes water to enter in the mouth and it flows over the gills in this way. However, these fishes must keep moving in order to survive. Now we shall discuss the mechanism of gas exchange across the gill surface. Gill surfaces are very efficient in gas exchange. In them blood and water move in opposite directions on either side of the epithelium of lamellae. This makes a counter current gas exchange system. This system maintains oxygen concentration gradient between blood and water over the entire length of capillary bed. If we look at the length of the capillary bed, the oxygen in the blood and the oxygen in the water becomes a concentration gradient established. And the benefit of this is that it provides great test possible extraction of oxygen from water.