How a frog can breathe through its skin
A frog does not rely on lungs alone. Oxygen dissolved in water can pass through its moist skin and into tiny blood vessels close to the surface; carbon dioxide can move the opposite way. This exchange is called cutaneous respiration. Skin folds increase the area available for exchange, much as a larger window gives more room for air to pass through.
The exchange depends on a difference in oxygen concentration: water next to the skin must contain more oxygen than the blood beneath it. As oxygen crosses into the body, the thin layer of water touching the skin can become depleted. If that water sits still, the supply right at the surface may dwindle even when some oxygen remains nearby.
Why the push-ups help
The frog’s repeated body movements stir the water around its skin. That motion carries water with more dissolved oxygen toward the folds and sweeps away water that has already lost some of its oxygen. In effect, the frog refreshes the fluid at the exchange surface, helping maintain the concentration difference that drives oxygen into its blood.
The motion is not a replacement for the skin’s breathing ability; it makes that ability work better under difficult conditions. When oxygen is scarce, the frog is managing the boundary between its body and the surrounding water—an invisible layer that can matter as much as the water beyond it. Its push-ups turn a patch of still, depleted water into a renewed supply at the skin.
