In most animals, the majority of cells are not in direct contact with the external environment but are bathed by an internal body fluid. Insects and other animals with an open circulatiory system have an internal "pond" composed of hemolymph, which bathes all body cells. In vertebrates and other animals with a closed circulatory system, the internal pond is interstitial fluid serviced by blood.
Cells require a blance between water uptake and loss
We lose water by urinating and defecating. For aquatic animals, evaporation is unimportant, but these animals experience the uptake and loss of water across the bosy surface by osmosis. Even if an animal is proctected by a covering that impedes water loss or gain, specialisex epithelia that must be exposed to the environment in order to exchange gases (such as gills, lungs, and tracheae) cannot by waterproof. The cells of the animal cannot survive a net water gain or loss. It must be balanced. If there is an uptake of water, the cells swell and burst. If there is a loss of water, the cells shrivel and die. In osmosis, the movement of water across a selectively permeable membrance occurs whenever two solutions separated differ in total solute concentration, or osmolarity (total solute concentration expressed as molarity, or moles of solute per liter of solution.)
Osmoconformers and Osmoregulations
There are two basic solutions to the problem of balancing water gain with water loss. Once is to be isotonic with a saltwater environment. Such animals, which do not actively adjyst their internal osmolatiry are known as osmoconformers. Animals whose body fluids are not isotonic with the outside environment must discharge excess water if they live in a hypotonic environment or continuously take in water to offset osmotic loss if they inhabit a hypertonic environment. They are called osmoregulators. Most animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes in external osmolarity. Such animals are said to be stenohaline. However, some animals do survive radical fluctuations of osmolatiry in their surroundings. They are known as euryhaline.
Maintaining Water Balance in Different Environments
Marine Animals Most marine invertebrates are osmoconformers. It does not drink water, and the water that enters its body by osmosis is disposed of in urine. (the waste fluid formed by the excretory organs, the kidneys.
Freshwater Animals These animals are constantly taking in water by osmosis because the osmolarity if their internal fluids is much higher than that of their surroundings.
