Osmoregulation is the active regulation of osmotic pressure to maintain the balance of water and electrolytes in an organism. Control of osmotic pressure is needed to perform biochemical reactions and preserve homeostasis.
How Osmoregulation Works
Osmosis is the movement of solvent molecules through a semipermeable membrane into an area that has a higher solute concentration. Osmotic pressure is the external pressure needed to prevent the solvent from crossing the membrane. Osmotic pressure depends on the concentration of solute particles. In an organism, the solvent is water and the solute particles are mainly dissolved salts and other ions, since larger molecules (proteins and polysaccharides) and nonpolar or hydrophobic molecules (dissolved gases, lipids) don't cross a semipermeable membrane. To maintain the water and electrolyte balance, organisms excrete excess water, solute molecules, and wastes.
Osmoconformers and Osmoregulators
There are two strategies used for osmoregulation-conforming and regulating.
Osmoconformers use active or passive processes to match their internal osmolarity to that of the environment. This is commonly seen in marine invertebrates, which have the same internal osmotic pressure inside their cells as the outside water, even though the chemical composition of the solutes may be different.
Osmoregulators control internal osmotic pressure so that conditions are maintained within a tightly-regulated range. Many animals are osmoregulators, including vertebrates (like humans).
Osmoregulation Strategies of Different Organisms
Bacteria - When osmolarity increases around bacteria, they may use transport mechanisms to absorb electrolytes or small organic molecules. The osmotic stress activates genes in certain bacteria that lead to the synthesis of osmoprotectant molecules.
Protozoa - Protists use contractile vacuoles to transport ammonia and other excretory wastes from the cytoplasm to the cell membrane, where the vacuole opens to the environment. Osmotic pressure forces water into the cytoplasm, while diffusion and active transport control the flow of water and electrolytes.
Plants - Higher plants use the stomata on the underside of leaves to control water loss. Plant cells rely on vacuoles to regulate cytoplasm osmolarity. Plants that live in hydrated soil (mesophytes) easily compensate for water lost from transpiration by absorbing more water. The leaves and stem of the plants may be protected from excessive water loss by a waxy outer coating called the cuticle. Plants that live in dry habitats (xerophytes) store water in vacuoles, have thick cuticles, and may have structural modifications (i.e., needle-shaped leaves, protected stomata) to protect against water loss. Plants that live in salty environments (halophytes) have to regulate not only water intake/loss but also the effect on osmotic pressure by salt. Some species store salts in their roots so the low water potential will draw the solvent in via osmosis. Salt may be excreted onto leaves to trap water molecules for absorption by leaf cells. Plants that live in water or damp environments (hydrophytes) can absorb water across their entire surface.
Animals - Animals utilize an excretory system to control the amount of water that is lost to the environment and maintain osmotic pressure. Protein metabolism also generates waste molecules which could disrupt osmotic pressure. The organs that are responsible for osmoregulation depend on the species.
Osmoregulation in Humans
In humans, the primary organ that regulates water is the kidney. Water, glucose, and amino acids may be reabsorbed from the glomerular filtrate in the kidneys or it may continue through the ureters to the bladder for excretion in urine. In this way, the kidneys maintain the electrolyte balance of the blood and also regulate blood pressure. Absorption is controlled by the hormones aldosterone, antidiuretic hormone (ADH), and angiotensin II. Humans also lose water and electrolytes via perspiration.
Osmoreceptors in the hypothalamus of the brain monitor changes in water potential, controlling thirst and secreting ADH. ADH is stored in the pituitary gland. When it is released, it targets the endothelial cells in the nephrons of the kidneys. These cells are unique because they have aquaporins. Water can pass through aquaporins directly rather than having to navigate through the lipid bilayer of the cell membrane. ADH opens the water channels of the aquaporins, allowing water to flow. The kidneys continue to absorb water, returning it to the bloodstream, until the pituitary gland stops releasing ADH.