 The oxygen and nutrient uptake in the circulatory system is transported to these swimming muscles rather than to tissues required to pump water over the gills in other teleost fish.Based on the principles of the Fick equation, the rate of the gas diffusion across the gas exchange membrane is directly proportional to the respiratory surface area, and inversely proportional to the thickness of the membrane.Tunas have highly specialized gills, with a surface area 7–9 times larger than that of other aquatic environment organisms.