Frogs and toads belong to the class amphibia (they live in water and on land).One of the biggest problems faced by these animals is Osmoregulation. When they are in water they face the same problem as freshwater fishes; they have to expel excess water from their body. When on land they have to conserve water.
They do not replace water by intentional drinking nor do they have the impermeable skin characteristic of other tetapods. Above all their kidneys are not capable of producing hypertonic urine.
Amphibians limit their water loss by their behaviour that reduces exposure to desiccating conditions. When they come to land foraging during night, they lose water and this is compensated by water uptake across the skin. Paradoxically, the skin, the most important source for dehydration is also the most important structure for rehydration.
The frogs and toads have specialized patches of skin called seat patches on their abdomen and thighs which can absorb water very quickly allowing them to rapidly rehydrate. They flatten their body on moist surfaces and absorb water through seat patches.
Neurohypophyseal(ADH) and adrenocortical hormones have special roles in regulating water and salt transfer across epithelial membranes of tetrapods. Such effects are seen in the skin and urinary bladder of frogs and toads.
Amphibians on the whole are intolerant of saline condition. Their kidneys are not capable of producing hypertonic urine. Only amphibians and mammals excrete urea. Had the amphibians not inherited the Ornithine-urea cycle from their crossopterygian ancestors, they would not have succeeded in the movement onto dry land.
Source: The Hindu
