How do fish take in oxygen from the enviroment?

The route of respiration for aquatic animals is via the gills. Oxygen absorption takes place as water, drawn in through the mouth is pushed out through the gill slits so flow is unidirectional (unlike mammalian lungs).  Water is actively pumped though the mouth (buccal cavity) by closing the mouth, which raises the pressure and forces water out through the widened gill slits. The gills themselves are made of four 4 major arches enclosed within the gill cavity. Each arch has 2 rows of gill filaments, which give rise to rows of lamellae to increase the surface area. Immediately below is an extensive vascular network so that the diffusion distance for respiratory exchange is minimal. Efficiency is further enhanced as the direction of blood in the lamellae is opposite to that of the water passing over the lamellar surface. Water flowing past the gills in the opposite direction to the blood flow maintains high concentration gradient, and is called countercurrent flow, the water with the highest concentration of oxgen will encounter the blood with the highest oxygen content. This is how fish achieve ~80% oxygen utilisation compared to 25% in people. The diffusion distance is also kept minimal as endotheliem (wall of the blood vessel) is only one layer of cells thick, and the membrane over the lamella is also only one cell thick.