various types of osmoregulation in main groups of Vertebrates.
All cells are filled with charged proteins, dissolved salts, and other molecules that exert an osmotic force. Yet the internal environment must remain constant for the cell to function properly. This means that a cell must either have many mechanisms that keep its internal environment constant, or it must find a place to live that has a very constant environment. Having so many mechanisms in and on a cell that it can keep its internal environment constant in a wide range of external circumstances is very expensive to a cell. Most of these mechanisms involve moving chemicals against their gradient, and therefor require energy, usually in the form of ATP. The other option, finding a nice quiet place without external disruptions has the advantage of being cheaper (in terms of energy expenditure) but these places are very scarce and might have other disadvantages (low food supply etc.). It would mean leaving 90 % of the available living space unused.
So if these two solutions to the homeostasis problem are not good enough, what third solution presents itself?
If each one of these cells has trouble getting on alone, maybe colonies could do better. Imagine a flat colony on a rock surface submerged in the ocean. Most of the cells are surrounded by other cells, and therefor protected from sea-currents and salt water. Only the outer ones are subjected to the stresses of the external environment. The problem here is that the bottom layer of cells probably doesn’t get any food, and is bathing in the waste products of the cells above it. Clearly a transport mechanism is needed to distribute food and to remove waste products. The colony could “build” some tubes that allow seawater to penetrate the colony, but this would negate some of the advantages gained in colony forming. A more versatile idea can be seen all around us, and even inside us. A complex multicellular body, with a transport system (blood and lymph) that is as stabile as the cells it supports. Now there remains the problem of controlling and regulating the physical properties of blood and lymphfluid within as narrow as possible boundaries. The solution to this problem lies in the orchestrated actions of internal organs, which all play a role in the maintenance of homeostasis.
Now let me focus on the mechanisms that regulate osmolarity in an important group of these multicellular organisms, namely Vertebrates.
In Vertebrates, the most important organs in osmoregulation are the kidneys. These organs are responsible for regulating the composition and volume of body fluids (Smith, H. 1953).
I shall briefly describe first, the anatomy, and next the workings of a mammalian kidney, to illustrate the modifications of the kidney structures that will be discussed later on.
Mammalian kidneys are built up in two parts, the cortex and the medulla. Each kidney consists of many of individual functional units, called the nephrons. A nephron consists of a Bowman's capsule, a proximal convoluted tubule, and a distal convoluted tubule, all three of which are located in the renal cortex. The two convoluted tubules are connected by the loop of Henle, which lies in the renal medulla. Distal convoluted tubules empty into collecting tubules, which are also located in the medulla. These collecting tubes follow the minor and the major calyx, to empty into the renal pelvis, which is connected to the ureter that leads to the bladder.
Functionally a kidney can be described thus.
Arterial blood gets pumped under pressure into the kidney, entering through the median depression of the kidney through the renal artery. This artery branches out in the medulla, creating many arterioles that go all the way through the medulla into the cortex. In the cortex each of the arterioles branches into a capillary bed, the glomerulus, which is contained within a Bowman's capsule. Water, urea, ions, glucose and amino acids, can pass from the glomerulus to Bowman’s capsule, and the arterial pressure of the blood makes sure it does so. Cells and proteins are left behind in a smaller amount of plasma. The filtrate passes into the proximal convoluted tubule, where some of the water is resorbed. Next comes the intermediate tubule (or the loop of Henle), which is the place where most of the ions are actively resorbed. The remaining fluid passes through the distal convoluted tubule into the collecting tube that leads through the region where the ions were resorbed, creating a hyperosmotic region, where most of the remaining water is resorbed.
Vertebrates as a group probably evolved some 550 million years ago from Chordates. They can be divided into phylogenetic groups (Class Agnatha (Jawless Vertebrates), Class Placodermi (Extinct Jawed Fish), Class Chondrichthyes (Rays & Sharks), Class Osteichthyes (bony Fish), Class Amphibia (Frogs, Toads, and caecilians), Class Reptilia (Snakes, Lizards, Alligators, Turtles and the Sphenodon), Class Aves (Birds), and Class Mammalia (Mammals)) but they can also be divided according to other criteria.
The criterion I’m going use to divide all vertebrates is their osmolarity relative to their environment. That means I’m going to divide them into those groups that have body fluids that are saltier than their environment (hyperosmotic), those that have body fluids that are as salty as their environment (isosmotic), and those that have body fluids that are less salty than their environment (hyposmotic). I made no mention of those groups that do not live in watery surrounds (tetrapods). They resemble hyposmotic aquatic vertebrates in some the respect that they are always in danger of losing water, but they don’t have the excess salt problem to deal with (see §3.4), but will be discussed as a separate group. Hyperosmotic organisms and hyposmotic organisms are collectively known as osmoregulators, isosmotic organisms are known as osmoconformers.
Fish and amphibians that live in sweet water are usually saltier than the water. This means that the surrounding water is constantly trying to get into their bodies, or perhaps its better to say that their salty bodies are always trying to absorb water. If this would be allowed to happen, the extracellular fluids would become more dilute, and this would be a disruption to homeostasis.
To avoid this happening, these groups could develop a very waterproof integument, or surround themselves with a thick layer of mucus. This would stop water from getting in and solve the problem. Or would it? Water is ingested with food and gills for breathing must be permeable to some extent (the gills are in fact were most of the water gets in). So if the incoming water cannot be stopped, it must be dealt with.
The waterproof integument is in fact present in most freshwater fishes, and they have slightly lower NaCl concentrations in their blood than their marine counterparts, which reduces the osmotic pressure.
As I have just described, the mammalian kidneys are very good at resorbing water. Freshwater dwellers do not need this, so their kidneys are better suited to produce large amounts of dilute urine. Freshwater fishes extrude urine that is about ten times more dilute than comparable marine fishes. Freshwater fishes and amphibians have large, well-developed glomeruli, which produce large amounts of filtrate, and since only mammals have a loop of Henle, the reverse osmosis re-uptake of water in the collection ducts does not take place. In this way, only a relative small portion of the water in the primary glomerular filtrate can be resorbed. This solves the problem of having too much water, and as most (but not all) of the salts and other solutes are resorbed in well-developed proximal tubules, little loss of these solutes occurs. To compensate for the salts that they do lose and cannot replace from their food, freshwater fishes have specialised cells located in their gills that absorb sodium and chloride ions from the water. Amphibians can absorb ions through their skin. Note that these specialised cells are moving ions against their concentration gradient, so this transport requires energy.
Marine fishes are less salty than their environment, which means that they are constantly in danger of losing water (dehydration). To solve this problem, many adaptations have arisen.
As in freshwater fishes, the integument is hardly permeable to water, so that most water and ion exchange occurs across the gills.
In the group of marine teleosts, the glomeruli and the distal tubuli are small or absent. Not having well-developed glomeruli, the large amount of watery glomerular filtrate is never formed. To rid themselves of nitrous waste, ammonia is selectively secreted into the remaining tubules of the kidney.
The distal tubuli in most vertebrate kidneys are designed to resorb salts, while sacrificing water. Salt shortage is not a problem in a marine environment, so the absence of these tubuli doesn’t create a salt deficit, but it does save some water.
Another adaptation seen in marine teleosts, is that they drink seawater. This would seem to be a bad idea, as seawater is saltier than marine teleosts, but they have a very efficient way to rid themselves of excess salt. This leaves only the water. In their gills, marine teleosts have specialised cells called chloride cells, which actively pump sodium and chloride ions back into the seawater. This is so successful that some reports say that certain species drink up to 25% of their bodyweight in seawater, retaining 80% of the water.
The simplest solution to the osmosis problem might be that of Hagfish (a group of marine Agnathans) Elasmobranchs and the Coelacanth Latimera. These marine dwellers are collectively known as osmoconformers, because their body fluids contain approximately the same amount of salt as their surroundings. This way there is no osmotic pressure forcing water in or out of the body fluids.
In Hagfish this is achieved by having elevated levels of sodium and chloride ions in their blood and extracellular fluids. This means that their tissues must be resistant to these ionic pressures, bringing the problem back to a cellular level. The kidneys are very simple, having virtually no proximal, intermediate, or distal tubuli. This modified (or is it just primitive?) kidney excretes large amounts of dilute urine, as this helps clean the blood and water loss is not a problem. Large renal corpuscles play a role in the regulation of bivalent ions.
Elasmobranchs and coelacanths have a different method. The levels of sodium and chloride ions in their blood and extracellular fluids are comparable to those of marine teleosts. They do however have high amounts of urea in the blood, high enough to elevate the osmolarity of the blood to be equal to that of seawater. This means that although the cells are in an environment that has optimal sodium and chloride concentrations, they must have evolved mechanisms to cope with the high urea concentrations. In fact these animals synthesise methylamines, like trimethylamine oxide, which neutralise the toxicity of urea. The retention of methylamines also contributes to increasing the osmolarity of the body fluids, and some species of cartilaginous fish may actually be hyperosmotic to seawater
To cope with excess salt in the body, Hagfish and Latimera have highly developed salt excreting glands in the gills. In Chondricthyans, these cells are not so well developed. Instead, they have special rectal glands to excrete excess salt.
Terrestrial vertebrates do not have a watery environment, so speaking of osmolarity in relation to their surroundings is not adequate. They do live in an environment where water loss through evaporation and elimination is a problem. Also, contrary to aquatic organisms, excess heat produced by their bodies is not as easily lost, as air conducts thermal energy less efficiently than water. This is in some groups solved by sweating, resulting in an increased loss of water and salt.
Terrestrial vertebrates have evolved a divers series of adaptations to compensate for these losses. The modifications to the mammalian kidney have been briefly discussed in §2, as the main water saving structure in the mammalian kidney is the loop of Henle. No water is actually resorbed in the loop of Henle, but because sodium ions are actively pumped out of the glomerular filtrate here, a region with a very high osmotic value is created. When the collecting duct passes through here, the fluid inside is less salty than the surrounding tissue, which causes the water to pass from the filtrate into the tissue. In this tissue, blood capillaries are located, that take up the water. These are called the Vasa recta.
Other modifications include uricotelism, which is an alternate way of excreting excess nitrogen. In the kidneys of birds and most living reptiles, nitrogen is fixed in the form of uric acid. This large molecule can be eliminated with a minimum of water, so all the water can be resorbed.
Also, mammals, reptiles and birds have a very waterproof integument, and lack gills.
To conclude, I must first express my astonishment at the diversity of structures, adaptations and solutions that have been found to conquer the problem of osmosis. Too much water, too little water, too salty water or too sweet water, it doesn’t really matter, nature found a way to cope with it. The problem stated in the introduction, where the choice seemed to be between living in a few favourable places or working very hard to survive in a hostile environment, seems to have been solved or circumvented. In a sense, a middle way has been found in which each cell is working on one aspect of creating favourable circumstances for other cells. Whether that is providing food, proving transport, keeping unwanted guests out, or maintaining a proper water/salt balance, all cells are working for the common good. And most cells are better off this way than if they would be alone.
Although the solution of isosmosis seems the simplest answer to the osmosis problem, it is not by far the most successful, or the most elegant. Nitrogen excretion seems to be one of the major problems, but a diversity of kidney structures has evolved to cope with this. Salty water or sweet, they both present their own problems but again, a diversity of specialised glands, organ and skin adaptations have risen to meet the challenge.
Frogs and lizards can adapt to live in deserts, fish can adapt to survive droughts, and birds can “sneeze” out excess salt. And through it all, there is the little single celled organism, fighting to survive in its hostile surrounds. And it survives…
Most of the information in this essay is an interpretation of chapter fourteen of “Vertebrates”, by Kardong K. V. (second edition, 1998), various parts of “Vertebrate Life”, by F. H. Poug, C. M. Janis, and J. B. Heisser (Fifth edition, 1999) and various parts of “Human Physiology” by S. A. Fox (fifth edition, 1996).