Adult hellbenders lose their external gills at metamorphosis and retain small lungs, but the great majority of oxygen uptake happens across the skin. The distinctive lateral folds increase surface area, and capillaries run close beneath the surface. Hellbenders have been observed swaying or rocking in place, a behaviour interpreted as moving the boundary layer of water away from the skin to sustain diffusion.
That physiology sets a hard habitat constraint. Cutaneous respiration works only in cold, fast, highly oxygenated water, which is why hellbenders occupy riffles and hide under large flat rocks rather than in pools. Silt that fills the spaces beneath those rocks removes both shelter and oxygenated flow at once.
The lungs are commonly described as vestigial or useless. They are reduced and contribute little to respiration, but they are retained and are thought to serve mainly in buoyancy control. Hellbenders are also harmless: the folklore that they are venomous or that they poison trout streams has no basis.