The body fluid pools:
Osmoregulation: regulation of salt and water balance, essentially in the extracellular body fluid pool.
The concentration of the intracellular fluid pool remains largely unaltered apart from circumstances requiring the generation of synthesised osmolytes.
Life evolved in the sea with fw interludes.
Water has idea physical properties for supporting life.
The intracellular ICF environment to this day carries the aqueous heritage of the primeval sea and generally depends on an association with a form of extracellular ECF fluid.
ECF could just be the surrounding aqueous medium in the case of single celled animals.
The other fluid pools are the intracellular, and in higher animals the interstitial and plasma fluid pools.
The ability to withstand osmotically unfavourable environment (too dilute, concentrated, dry) has been achieved by more advanced groups of animals by evolving a stable internal environment.
The creation of an internally stable environment has been a driver for evolution via geographical dispersion and speciation.
If arthropods and vertebrates has not found a means of regulating the solute concentration in their extracellular compartments they would not have colonised fresh water and terrestrial environments.
Simpler forms of marine invertebrates tend to osmoconform: ECF is iso-osmotic with the surrounding sea water.
This does not imply a simple approach of conforming via passive diffusion. These animals can regulate ion concentrations using same membrane pumps found in higher animals.
Fresh water animals must osmo and ionoregulate their ECF to protect their ICF.
Multicellular organisms employ a mixture of osmoregulatory strategies: osmoconforming and osmoregulating or both.
Hypertonic regulation: producing a urine more concentrated than the environment.
Hypotonic regulation: producing a urine more dilute than the environment.
Epithelia in osmoregulatory function:
The osmoregulatory capabilities of metazoans depend on epithelial tissue: trasnport epithelia of gills, skin, kidneys and gut.
Epethelial cells are bi-polar: 2 membranes of differing channel functions joined by a tight junction.
Apical side faces lumen (gut, kidney) e.g. the outside world.
Basal side involved in osmotic transport of water and is bathed in extracellular fluid.
The proper composition of the ECF depends…