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Osmoregulation in Terrestrial Environment

When emerging from aquatic to terrestrial existence, all organisms face the dangers of dehydration and desiccation. A terrestrial environment for vertebrates leads to both a passive water loss to the air and solute gain through food. Consequently, terrestrial environments would tend to increase the osmotic concentration of vertebrates, Land animals have evolved many adaptations to counteract the drying conditions of terrestrial life.

These adaptations include a waterproof coating, such as the exoskeletons of arthropods and the skin of humans. Animals cannot be completely waterproof they inevitably lose some water from urine, feces, sweat, and gas exchange breathing. This water is replaced by water from drinking, eating food, and cellular respiration. In order to minimize the water loss into the air through the respiratory epithelium and also other parts of the body, certain adaptations have evolved.

Water movement through the integument:

The integument of most terrestrial animals is relatively impermeable to water and very little water is lost through the skin. For example, insects lose very little moisture through the integument due to the presence of waxy cuticle over it, which makes it highly impermeable to water.

The water permeability property of vertebrate integument varies widely. For example, reptiles, some desert amphibians, birds and many mammals have relatively impermeable skins. But due to the water loss during perspiration, they can become dehydrated at low humidity. Animals having highly permeable skin cannot tolerate very hot, dry environments.

Most amphibians avoid desiccation by staying close to water during hot, dry times of the day. Toads can oversize urinary bladders in order to store water. The epithelium of the bladder can actively transport Na⁺ and Cl^– from the bladder lumen into the body to compensate the salts lost during times of excessive hydration.

Many anurans have specialized regions of the skin on the abdomen and thighs called seat patches, which when immersed can take up water at a rate of three times the body weight per day.

Water loss during air breathing:

Water is lost through the respiratory surface but it is reduced in the terrestrial vertebrates because of the presence of the lungs. During the lungs ventilation also, the respiratory epithelium causes evaporation of the moisture wetting the epithelial surface. Such evaporative loss is enhanced in birds and mammals because of the difference between the body temperature and ambient temperature.

Since the expired air is warmer than the inspired air, water is lost during expiration. It can be minimized in vertebrates through a mechanism known as temporal counter-current system. Temporal counter-current heat exchange in the nasal passage reduces expired air temperature below body temperature.

Nasal temporal counter-current heat exchange reduces total water loss sufficiently to allow maintenance of water balance using metabolic water production alone. The major cause of water loss in terrestrial insects is due to its tracheal system. Certain terrestrial arthropods have the ability to extract water vapor directly from air.

Water loss during excretion:

In terrestrial animals, body water is also lost during excretion of nitrogenous wastes. The excretory system plays a major role in the movement of solutes and water between internal body fluids and the environment. Solute concentrations regulate movement of water into or out of the filtrate leading up to excretion, which is the discharge of urine containing nitrogenous waste. In terrestrial vertebrates, kidney is the chief organ of osmoregulation and excretion.

The production of highly concentrated urine prevents water loss from the body of most invertebrates. Amphibians and reptiles unable to produce a hyperosmotic urine do not produce urine during the period of osmotic stress.