There are two classes of environment commonly investigated in biology: aqueous (water-based) and terrestrial (land-based). Aqueous animals can live in fresh or seawater, each of which gives rise to almost opposite adaptations. Euryhalines can tolerate a wide range of salinities whereas, stenohalines can only tolerate a narrow range.
Generally freshwater animals are hyperosmotic i.e a higher osmotic pressure, to their surrounding environment, and seawater animals are hypoosmotic. There are two problems associated with freshwater habitation: the animals are subject to swelling due to an inward H2O movement, and a loss of salts to the surrounding environment. These problems are overcome by the possession of a low permeability integument to salts and water. Animals living in water also refrain from drinking water. This reduces their need to expel water, and so they usually get water from diffusion across a certain part of their skin. They often produce dilute and copious urine, as most salts are reabsorbed into the blood from the ultrafiltrate. Some salts will be replaced by food ingestion. A special adaptation of freshwater animals though, is the active transport of salts from the external dilute medium across the epithelium into the interstitial fluid using a mechanism such as ‘transport epithelia’ in amphibian skin and fish gills. These epithelia have ‘tight junctions’ which reduce the permeability of a paracellular pathway so that salts absorbed pass through the cells that need them.
On a cellular level, adaptations have even occurred in membranes according to the need for water transport. Membranes in many animals contain many water-permeable channels or aquaporins, because permeability of water through the lipid bilayer is not high enough to support the rate of water transport needed for the animal to survive. Vesicles containing aquaporins can actually be inserted into the collecting duct in kidney nephrons under the influence of ADH in mammals, when water concentration drops in the tissues.
The integument is the outer covering of in vertebrates and amphibians. It acts as a large barrier between the extracellular compartment and the environment. The permeability of the integument varies between animals. Amphibians generally have moist, highly permeable skins, through which they exchange water, oxygen, carbon dioxide and ions by passive diffusion. Salts lost this way are actively transported into the skin from the surrounding aqueous environment to compensate. Gills in fish have a similar role. Due to this involuntary water diffusion across the skin, amphibians such as frogs and toads can never venture very far from puddles and ponds to prevent desiccation. Even animals with low permeability integument will seek cooler, damper environments in hot arid weather.
There are numerous ways in which water is lost in animals. All animals excrete urine in some way, using a kidney or bladder system. Water is lost via evaporation of sweat from the skin in mammals, and water is lost via the lungs in ventilation, as air is humidified by water in the nasal passages. Salts are also lost through the skin in sweat, and a fairly large amount in the urine. Most animals have adapted a way to control this. The bladder of frogs and toads, for example, is oversized. It acts as a water reservoir during times of dehydration, passing water into the surrounding tissues, and as a source of salts in times of excessive hydration.
Osmoregulation in marine elasmobranches, reptiles and teleosts does not expend much energy because most of these animals have cellular fluid compositions similar to that of the water surrounding them. They are said to be isosmotic. They can be broken into Elasmobranches e.g. sharks, which do not drink seawater, and have excrete hyperosmotic NaCl from the rectal gland. Marine teleosts such as mackerel, drink seawater and secrete salt from the gills. Freshwater teleosts such as salmon, by comparison, drink absolutely no water and absorb salt from the gills.
The habitat of the animals is so important in the extent for their need to osmoregulate. In hot, arid regions, the extreme heat causes more water to evaporate from the skin and to be lost in ventilation. Cells become easily dehydrated and so in these regions, most animals have adaptations to conserve water. A salt conservation technique is also important to restore vital minerals lost in sweat. Some animals avoid the problem in the first place by not drinking much or any water, so their osmolarity is never greatly changed. For example, Desert mammals such as the desert rat, drink no water, and instead depend solely on water from the metabolism of fats. They often have special adaptations such as an extra long loops of Henle designed from the reabsorption of more water.
Amphibians, which tend to live close to or in water, e.g. salamanders absorb salt through the skin. This balances the high osmolarity caused by diffusion of vast quantities of water through the integument. They sometimes secrete acidic substances onto their backs to encourage water to evaporate rapidly. This prevents over-dilution, which can lead to cell rupture in extreme cases, and malnutrition due to a lack of vitamins lost in the sweat. Marine reptiles such a turtles and tortoises drink seawater and possess a hyperosmotic salt-gland secretor, specifically designed for ridding extra salts. Marine mammals such as whales do not drink seawater to prevent water loss. Marine birds such as seagulls, drink seawater and have a hyperosmotic salt-gland secreter, and terrestrial birds such as pigeons drink freshwater, and have low problems in osmoregulation.
In order to cope with changes in osmotic pressure, osmolatory organs are found, which are specifically designed for this function. Simple organs exist in some animals, such as salt-excreting cells in the nasal gland of birds and reptiles, and the rectal gland of elasmobranches. Similar cells are found in the mammalian kidney, the most well-known example of an osmolatory organ. The epithelium is specially adapted to cope with the specific requirements of the animal concerned. Energy in the form of ATP, for example is generated and hydrolysed using different ATP pumps. F-ATP synthases found in mitochondria and chloroplasts, use a proton electrochemical gradient to make ATP, whereas V-ATPases or vacuolar type ATPases hydrolyses ATP to generate electrochemical gradients. This is the similar to P-type ATPases.
The functional unit of the mammalian kidney is the nephron. Each kidney contains numerous nephrons which empty into a collecting duct in the medulla of the kidney. At the closed end in the cortex of the kidney, the nephron opens up into a Bowman’s capsule. A network of capillaries forms the renal glomerulus inside the Bowman’s capsule, and it is this which is responsible for the first stage of urine filtration.
The wall of the renal tubule is one cell thick and separates the ultrafiltrate in the lumen of the tubule from the interstitial fluid. In some areas the cells are specialised for transport and contain numerous microvilli to increase overall surface area for absorption. These form the brush border. The cells are tied together by leaky tight junctions which allow limited diffusion between cells.
The nephron can be divided into three main areas: the proximal convoluted tubule, the Loop of Henle and the distal convoluted tubule. The proximal convoluted tubule also includes the Bowman’s capsule and glomerulus. The Loop of Henle can be further broken into the ascending and descending limbs.
The basis to reabsorption in the kidney is the movement of important solutes and water across the epithelium cells of the nephron into the surrounding tissues. This is set up via a complex array of movements.
The proximal tubule begins the process of concentrating the glomerular filtrate, and is the most important in the active reabsorption of salts. About 70% of Na+ ions are actively removed from the filtrate here, and as a result a large proportion of water follows by passive diffusion. About 75% of the filtrate is reabsorbed before it reaches the Loop of Henle. The result is a fluid isosmotic to the surrounding interstitial fluids. By the time the filtrate reaches the distal tubule, it has reduced to a quarter of its original volume, so that solutes that have not been removed are now at a much higher concentration.
Glucose and amino acids are also reabsorbed here by a sodium-coupled mechanisms. Carrier proteins on the apical membrane cotransport sodium and glucose/amino acids from the ultrafiltrate into the cell. The uptake process (against the concentration gradient for the glucose/amino acids) depends on the maintenance of the electrochemical gradient created by the Na+/K+ pump. Once in the tubular cell, glucose and amino acids diffuse passively into the blood.
The permeability of the epithelium is the main determinant of the reabsorption of solutes from the filtrate. The descending limb of the Loop of Henle has a low permeability to NaCl and urea and hence there is no active transport of these ions across the membranes of the cells. The ascending limb also has low transport of NaCl even though this permeability is higher. The permeability to urea and water are however low.
The medullary Thick ascending limb of the Loop of Henle differs in that it actively transports NaCl outwards. This segment has however a low permeability to water, so water cannot follow by passive diffusion. This means that the fluid entering the distal tubule is hyposmotic to the interstitial fluid in the tissue surrounding it.
The walls of the collecting duct are permeable to water though, so water can leave via diffusion in this part. The permeability is controlled by the addition of ADH, an antidiuretic hormone, that cause vesicles containing aquaporins to fuse with the cell membrane increasing the permeability of the walls to water. This is the final stage of reabsorption, and acts a final control.
The Loops of Henle is said to be a counter-current multiplier. It acts cyclically to increase the concentration of the filtrate by the movement of filtrate in opposite but parallel directions. The descending limb has high water, low urea, and low salt permeability, whereas the ascending limb has low water, low urea and high salt permeability. The first step is that NaCl is actively transported into the tissues from the thick ascending limb. This increases the concentration of the tissue in-between the two limbs. This causes water to leak out via diffusion from the water-permeable walls of the descending limb. This causes filtrate entering the collecting duct to be highly concentrated. Up until this point, there has been low urea permeability. At the collecting duct, this changes. It becomes highly permeable to urea, so it leaks out by diffusion. This further adds to the osmolarity of the interstitial fluid around the bottom of the loop of Henle. In this way, it can be seen that the organs specifically designed to cope with osmolarity problems, work in a negative feedback way, without direct interception by the nervous system. When there is a lack of water, for example, more of a gradient is built up naturally because the concentration of the solutes increases if there is less volume, so this means more water is reabsorbed. This function in itself varies between species, and even between individual organisms. Although the kidney is relatively unique, osmoregulatory organs all contain similar features. For example, Malpighian tubules in insects contain similar one-cell thick tubules with brush borders and proximal and distal convoluted tubules.
Osmoregulatory systems are very specific to the animal concerned because differing environments produce different needs for salts and water. The general need for a constant osmolarity in cells is a basic fundamental for all living organisms though, animals or plants.
Word Count = 2517
Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P. (2002). Molecular Biology of the Cell. 4th ed. New York: Garland Science.
Jones, M., Fosbery, R. and Taylor, D. (2000). Biology 1. Cambridge, Cambridge University Press.
Jones, M. and Gregory, J. (2001). Biology 2. Cambridge: Cambridge University Press.
Randall, D., Burggren, W. and French, K. (2002). Eckert Animal Physiology: Mechanism and Adaptations. 5th ed. New York: W.H.Freeman and Company
Widmaier, E.P. (1999). Why Geese Don’t Get Obese (And We Do). New York: W.H.Freeman and Company
http://www.neoucom.edu/Depts/ANAT/Osmo.html
