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. This increased surface area allows more oxygen to be in contact with the respiratory surface and therefore diffusion to take place more quickly (as represented by the direct proportionality in the Fick equation). This massive increase in surface area of the gills of the southern bluefin tuna is due to a higher density of secondary lamella in the gill filaments.