Osmoregulation

Osmoregulation as the regulation of the osmotic pressure of the body fluids of an organism is known in biology. Their biological function is the homeostasis of water content; each organism must prevent both that the solute concentration is too high ( loss of water) and that too much water is absorbed.

The osmotic pressure ( osmotic or value ) is an essential parameter for the water potential of a solution. If two solutions separated by a semipermeable membrane and have different water potentials on different, water moves by osmosis from the higher to lower water potential ( the highest potential is pure water). By the application of pressure on the side having the higher osmotic pressure of the movement of the water can be prevented and even reversed.

In all environments, aquatic or terrestrial, organisms must maintain the concentration of solutes as well as the water content of the body fluids in a suitable area for them; to the excretion ( elimination) or elimination of undesirable substances such as metabolites or hormones is required that would be toxic if too high a concentration. All processes and mechanisms in this context are referred to as osmoregulation.

The osmotic regulator and conformer

As the two major types of osmoregulation are osmotic conformers (English osmotic conformer ) and osmotic regulators (English osmotic regulators ) distinguished. Osmokonformer (also Osmokonforme ) adjust the osmolarity of their body tissue to their environment, they are poikilosmotisch. This can be done either passively ( without additional energy consumption ) or active ( consuming energy ). Most marine invertebrates are conformer. Also hagfish and Plattenkiemer (sharks and rays) are conformer, but different by electrolyte composition of the seawater from. In particular, due to the Donnan effect, which ( for example, proteins ) occurs in the presence of permeable materials not inside the organism, passive conformer are always weak hyperosmotic relative to the external medium.

More widespread in the animal kingdom are the Osmoregulierer. Keep the osmolarity of the organism in tight, constant limits and regulate the salt content of their body fluids, regardless of the salinity of the environment. One calls such organisms homoiosmotisch. Freshwater fish take electrolytes with the gills actively from the surrounding water, while the mitaufgenommene, excess water is excreted in the urine, which is therefore very diluted. In seawater, living organisms have a lower osmotic value inside the body than their surroundings, which would without counter-regulation lead to a permanent loss of water. They differ therefore from active salts via the gills. Most fish species are restricted to fresh or salt water environments ( stenohaline ). As a contrast, euryhaline species are known, their osmoregulation makes life possible in a wide salinity range.

Osmoregulation in plants

Most higher plants have no specific organs for osmoregulation, an exception are the salt glands in mangroves and some pioneer plants, including halophytes. Water delivery and consumption are determined by the internal and external influences that affect plant transpiration.

Just as animals need plants absorb water and release the excess again. Many species have developed methods for water storage. As Xerophyte plants of dry habitats are called, can withstand extended periods of drought. Succulent plants such as cacti store water in extensive parenchymal tissues. Other plants show adaptations to their leaves, which keep the water loss through transpiration. These include needle- shaped leaves, sunken stomata, and a thickened cuticle as in the pines. The beach grass has rolled leaves with inside stomata.

Osmoregulation in unicellular organisms and animals

Some single-celled organisms such as paramecium, amoebas or the alga Euglena have one or more contractile vacuoles, which absorb excess water from the cytoplasm by osmosis. The content of the contractile vacuole, either through a pore ( paramecium ) or exocytosis are removed from the cell. The pulsation of the contractile vacuoles, depending on the type between 5-10 seconds ( Paramecium caudatum ) up to 30-40 min (while ciliates Spirostomum ) and is influenced by a number of external factors such as the ion concentration gradient and temperature.

In vertebrates, kidneys have an important role in osmoregulation: they regulate the amount of water that is excreted along with metabolites in the urine. The human body can regulate the permeability of the collecting duct of nephrons of the kidney using the body's own hormones such as antidiuretic hormone, aldosterone and angiotensin II. With increased permeability of water is increasingly absorbed from the primary urine and thereby excreted less liquid.

The control of water excretion in the animal kingdom is the main mechanism for osmoregulation.

The excretory system of vertebrates

Waste products of protein metabolism

As a byproduct of protein metabolism produced cytotoxin ammonia. This is before the precipitation is converted into less toxic substances: urea in mammals, which is soluble in water, in birds and reptiles insoluble uric acid.

Mechanisms of osmoregulation

There are four processes can be distinguished:

• Filtration - the liquid portion of the blood ( blood plasma) is ultrafiltered in the glomeruli and reaches as primary urine in the subsequent tube system ( renal tubules ) of the nephron, the functional sub- unit of the kidney.
• Absorption - the largest part of this filtrate is absorbed by the surrounding blood vessels back in the system of tubules and Henle shear loop.
• Secretion - the remaining final urine flows via the efferent passages of the renal medulla from.
• Excretion - in mammals, the urine is stored in the bladder and excreted through the urethra. In other vertebrates, the urine in the cloaca is mixed with other waste before it leaves the body. ( In frogs there is also a bladder ).