Osmoregulation

In a strictly osmoregulating animal, the amounts of internal salt and water are held relatively constant in the face of environmental changes.

A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt.

Flounder have been observed to inhabit two disparate environments—marine and fresh water—and it is inherent to adapt to both by bringing in behavioral and physiological modifications.

Abscisic acid is an important hormone in helping plants to conserve water—it causes stomata to close and stimulates root growth so that more water can be absorbed.

Xerophytes are plants that can survive in dry habitats, such as deserts, and are able to withstand prolonged periods of water shortage.

Other plants have leaf modifications to reduce water loss, such as needle-shaped leaves, sunken stomata, and thick, waxy cuticles as in the pine.

These plants do not face major osmoregulatory challenges from water scarcity, but aside from species adapted for seasonal wetlands, have few defenses against desiccation.

The salt thus secreted by some species help them to trap water vapours from the air, which is absorbed in liquid by leaf cells.

For example, a decrease in water potential is detected by osmoreceptors in the hypothalamus, which stimulates ADH release from the pituitary gland to increase the permeability of the walls of the collecting ducts in the kidneys.

The kidneys of pinnipeds and cetaceans are lobed in structure, unlike those of non-bears among terrestrial mammals, but this specific adaptation does not confer any greater concentrating ability.

[3] In teleost (advanced ray-finned) fishes, the gills, kidney and digestive tract are involved in maintenance of body fluid balance, as the main osmoregulatory organs.

However, the Plotosidae dendritic organ may be of limited use under extreme salinity conditions, compared to more typical gill-based ionoregulation.

[4] Amoeba makes use of contractile vacuoles to collect excretory wastes, such as ammonia, from the intracellular fluid by diffusion and active transport.

Bacteria respond to osmotic stress by rapidly accumulating electrolytes or small organic solutes via transporters whose activities are stimulated by increases in osmolarity.