Osmoregulation

Osmoregulation means the physiological processes that an organism uses to maintain water balance; that is, to compensate for water loss, avoid excess water gain, and maintain the proper osmotic concentration (osmolarity) of the body fluids. Most humans are about 55 to 60 percent water by weight (45 percent in elderly and obese people and up to 75 percent in newborn infants). Many jellyfish are 95 percent or more water.

Osmoconformers and Osmoregulators

Not all organisms osmoregulate. Some marine animals such as the sea stars are osmoconformers; their body fluids are similar to seawater in osmolarity, so they gain and lose water at equal rates and have no need to expend energy expelling water or salt from the body. However, if they are placed in water more or less concentrated than seawater, their tissues shrink or swell, their organelles and cell membranes are damaged, and they die. This is why echinoderms are not found in estuaries, or river mouths where fresh and salt water meet and the salinity fluctuates greatly. Osmoconformers are stenohaline (steno means “narrow range,” and hal means “salt”), unable to tolerate much variation in environmental salinity.

Osmoregulators, on the other hand, maintain a more or less stable internal osmolarity by physiological means. Terrestrial animals must os- moregulate because they unavoidably lose water by evaporation and excre­tion, and replacement water is not always immediately available. Marine osmoregulators maintain an internal salinity lower than that of seawater, and freshwater osmoregulators maintain an internal salinity higher than that of fresh water. Euryhaline (eury means “broad”) animals, those able to tol­erate a broad range of environmental salinity, must be good osmoregula­tors. The blue crab, Callinectes sapidus, for example, thrives in estuaries and requires efficient osmoregulation to survive there.

Osmoregulatory Mechanisms

Water cannot be actively transported across cell membranes because there are no carrier proteins capable of binding and transporting it. Water can, however, pass directly through membranes in response to changes in ion concentration. Water movement is therefore controlled indirectly, by pumping ions such as sodium and potassium across cell membranes, cre­ating a concentration gradient that causes water to follow by osmosis. If sodium is excreted from the body, for example, water tends to follow it. The rate of water loss can thus be regulated by hormones that con­trol the rate of sodium excretion or the water permeability of the excre­tory ducts.

Osmoregulation is usually achieved by excretory organs that serve also for the disposal of metabolic wastes. Thus, urination is a mechanism of both waste excretion and osmoregulation. Organelles and organs that carry out osmoregulation include contractile vacuoles, nephridia, antennal glands, and malpighian tubules of invertebrates, and salt glands and kidneys of verte­brates.

Contractile vacuoles are organelles in the cells of sponges and freshwa­ter protozoans. In the freshwater Amoeba proteus, for example, the bubble­like contractile vacuole swells with excess fluid from the cytoplasm. Its membrane then pumps valuable ions back into the cytoplasm, leaving mainly water in the vacuole. Contractile proteins surrounding the vacuole then abruptly compress it, squirting the water out of the cell through a pore in the cell membrane. The vacuole then slowly begins to refill, repeating the process with a rhythm superficially resembling a heartbeat.

Nephridia are tubular structures that filter body fluids other than blood, found in flatworms, annelids, and many other invertebrates. Beating cilia or flagella draw fluid into the tubular system, leaving cells and proteins behind in the tissues. The tubules then reabsorb useful substances such as glucose and amino acids from the fluid and return them to the tissues, while se­creting excess ions into the fluid. Finally, the excess water, ions, and metabolic wastes are expelled from the body by way of nephridiopores in the body wall. Nephridia are called protonephridia if the inner end of the tubule is closed, like a porous bulb, and extracts liquid from the tissue fluid. These occur in flatworms such as planarians and tapeworms. Metanephridia have a funnel like opening at the internal end, through which they draw in fluid from the body cavity. Earthworms have metanephridia.

Antennal glands occur in crustaceans such as crayfish. They receive a blood filtrate, modify it by the reabsorption of some substances and se­cretion of others into the fluid, and then expel the modified fluid (urine) from a pore at the base of the antenna.

Malpighian tubules are found in spiders and insects. Numbering from two to several hundred, they are attached in clusters to the digestive tract between the midgut and hindgut and hang freely in the abdominal cavity. They absorb water and ions from the coelomic fluid and pass the fluid to the gut. The hindgut reabsorbs most of the water, leaving excess ions and metabolic wastes to be excreted with the feces, which are often dry.

Salt glands are associated with the eyes, nostrils, or tongue of marine reptiles (sea snakes, sea turtles, marine iguanas, saltwater crocodiles) and birds (gulls, albatrosses). These animals ingest excess salt with their food and water and excrete it by way of these glands.

Kidneys are vertebrate osmoregulatory organs in which blood pressure forces fluid to filter through the walls of blood capillaries into tubules that process the filtrate into urine. Each human kidney has about 1.2 million tiny balls of capillaries called glomeruli, where the blood pressure is very high. A filtrate of the blood plasma, free of cells and protein, seeps from these capillaries into a hollow ball called a glomerular (Bowman) capsule. From there, it flows into a series of tubules that remove most of the salt and wa­ter along with useful material such as glucose and vitamins, while secreting hydrogen and potassium ions, urea, and drugs (for example, penicillin and aspirin) into the tubular fluid. A final tube in the pathway, called the col­lecting duct, adjusts the salinity of the urine by reabsorbing variable amounts of water, before the urine leaves the kidney for storage in the urinary blad­der and eventual elimination from the body.

Two hormones, aldosterone and antidiuretic hormone, regulate the amounts of salt and water reabsorbed, enabling the human kidney to adjust water loss or retention to the body’s state of hydration. Human blood plasma and tissue fluid normally has an osmolarity of 300 milliosmoles per liter (mOsm/L); that is, 0.3 mole of dissolved particles per liter of solution. Hu­man urine can be as dilute (hypoosmotic) as 50 mOsm/L when the body is voiding excess water, or as concentrated (hyperosmotic) as 1,200 mOsm/L when conserving water.

Freshwater fish, by contrast, cannot produce hyperosmotic urine, but they have no need to. Surrounded by water, they can afford to produce abun­dant, dilute urine to flush away their metabolic wastes. Among mammals, the ability to concentrate the urine is also little developed in aquatic forms such as beavers and muskrats. Kangaroo rats, by contrast, are desert rodents that need never drink water (they obtain it from food), and can concentrate their urine to as much as fourteen times the osmolarity of their blood plasma (compared to four times for humans).

Referencesٌ

Harris, C. L. Concepts in Zoology, 2nd ed. New York: HarperCollins, 1996.

Willmer, P., G. Stone, and I. Johnson. Environmental Physiology of Animals. Oxford: Blackwell Science, 2000.