Due to their permeable integumentary system, amphibians are vulnerable in ambient osmotic environments. Nevertheless, some species inhabit hyperosmotic and limited water potential environments by allowing for considerable changes in body fluid and plasma osmolality, a strategy that could be termed heterostasis, rather than striving to maintain homeostasis (*). Among amphibians, only adaptable species show increased urea levels in the plasma and body fluids. Herein, we review the present understanding of the role of urea in heterostasis and the absence of a counter molecule and emphasize the need for progress concerning the control mechanisms and functional significance of active urea transport across the skin.
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