Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.
--Wikipedia, "The Rime of the Ancient Mariner", Samuel Taylor Coleridge, 1798 accessed 18 August 2012
The poem on the surface explores violation of nature and its resulting psychological effects on the Mariner, who interprets the fates of his crew to be a direct result of his having shot down an albatross.----Wikipedia, "The Rime of the Ancient Mariner: Interpretations", Samuel Taylor Coleridge, 1798 accessed 18 August 2012
Spoiler alert! (although there's a decent chance you already had to read this poem in elementary school, so in that case, you already know what happens to the Ancient Mariner and his crew).
The Ancient Mariner is a sailor who commits an unnecessary act of cruelty, even a crime in Coleridge's estimation--with his bow and arrow, he shoots the albatross (a water bird, like a seagull) who had led his lost ship out of dangerous waters.
With his cruel bow he laid full low
The harmless Albatross.
After the killing of the bird, more bad things continue to happen to the ship and crew. All the other crew members are eventually killed, but his punishment is to remain alive, tormented, and to wander the earth telling his story as a warning to others.
He partially redeems himself when--seeing the sea creatures that he had earlier despised, he recognizes how beautiful they really are. He wanders the earth eternally, trying to reach others with his lesson before it's too late for them to learn from it.
There is a metric boatload of things we could discuss about this poem, anywhere from conservation biology, history, psychology, and literature aspects, just to scratch the surface--but I want to talk about my trip to Padilla Bay estuary today, and what living in a littoral environment means for kidney function in animals--and Coleridge's observation about water is the perfect jumping-off point for that discussion.
An estuary is a body of water where fresh water from rivers and oceanic saltwater come together and mingle. So it's a complex transition zone, and organism functions that work one way in fresh water and another way in saltwater have to be covered in both cases there.
The littoral zone is the area of a body of water closest to the shoreline. So in an estuary, the littoral zone is where the fresh water and the saltwater mix--it's saltier than the fresh water is, but it's also less salty than the ocean water.
You probably know that, if you're ever lost on a boat on the ocean, you shouldn't drink seawater--not even a little bit, not even if you're very, very thirsty.
The reason is that your body tries to maintain a balance between the concentration of solutions (like dissolved salt) inside your cells and outside of them. Globetrooper has some good diagrams of what the process looks like, both when it's going right and when it's not.
Globetrooper's diagram shows what it looks like when things are balanced inside the cell and outside of it: a situation called "isotonic": the "same pressure" by water on the cell membrane from both sides.
Source: http://globetrooper.com/notes/wp-content/uploads/isotonic-state.png accessed 18 August 2012
The large gray circles represent salt ions (to be more specific, sodium ions and chloride ions), and the small blue circles represent water. The proportion of salt dissolved in the water--the concentration--inside the cell is about the same as the proportion of salt dissolved in the water outside the cells. The inside and the outside of the cell are in equilibrium (isotonic), and there is no pressure from the water either to leave or to enter the cell.
Globetrooper's next illustration shows a situation that is no longer isotonic. Salt ions (the gray circles) are dissolved in the water (the blue circles) inside the cell, but outside the cell, there are no salt ions--the water outside the cell is pure water, with no salt in it. This situation is called a hypotonic solution.
Source: http://globetrooper.com/notes/wp-content/uploads/too-much-fresh-water.png accessed 18 August 2012
Since the inside and the outside of the cell are no longer in balance, a process called osmosis occurs--the movement of fluid (in this case, water) across a semipermeable membrane (a membrane that substances can move through) from an area of lower concentration (in this case, of salt) to a area of higher concentration.
The effect is to bring the concentrations more into balance (isotonic).
The green arrow in this figure shows the movement of water (its osmotic pressure) from the area of low salt concentration outside of the cell to the area of higher salt concentration inside the cell. That movement of water into the cell dilutes the salt concentration, making it lower inside the cell.
This is what happens when we drink fresh water.
The opposite situation occurs when the water outside the cell is more salty (has a higher salt concentration: lots of gray circles, very few little blue circles) than the water inside the cell. This situation is called a hypertonic solution.
Source: http://globetrooper.com/notes/wp-content/uploads/too-much-salt-water.png accessed 18 August 2012
In a hypertonic solution, water flows out of the cells--the osmotic pressure of the water is toward the higher concentration of salt.
So you see what would happen? If you drank even a little salt water, it would draw water out of your cells. Instead of quenching your thirst, salt water would leave you more dehydrated than you were when you started.
At the cellular level, this is what it would look like:
Source: http://upload.wikimedia.org/wikipedia/commons/7/76/Osmotic_pressure_on_blood_cells_diagram.svg accessed 18 August 2012
In the hypertonic solution, the osmotic pressure comes from the water leaving the cells, leaving behind shriveled, badly dehydrated, cells (left side of image).
In the isotonic solution, the osmotic pressure balances out to zero, as equal amounts of water enter and leave the cells. The red blood cells in an isotonic state look normal and healthy with the indentation in the center that is typical of them.
In the hypotonic solution, the osmotic pressure pushes water into the cells, stuffing and waterlogging them--the normal indentation starts to disappear as the cell is too full of water.
These microphotographs of real red blood cells show how they really look, as they react to the different solutions we just described.
Source: http://upload.wikimedia.org/wikipedia/commons/6/62/Human_Erythrocytes_OsmoticPressure_PhaseContrast_Plain.svg accessed 18 August 2012
So now that we've discussed how cells behave in different kinds of solutions (hypertonic, isotonic, and hypotonic), does Coleridge's verse
And all the boards did shrink;
make even more sense now?
Why is that?
What is he describing, and why did the boards shrink as a result?
Osmoregulation (the control of osmotic pressure in body fluids, such as blood) is controlled in humans and other vertebrates by the kidneys--that's an important function they have in addition to production and excretion of urine.
Hormones produced by the pituitary gland and the adrenal glands, among others, signal to the kidneys what state the solutions in the blood are--hypertonic, isotonic, or hypotonic--and the kidney responds accordingly by reserving water or by releasing it to restore the balance.
As we are land animals, our kidneys have to respond to fluids we drink or take in in other ways (like an IV solution in the hospital, for example), but that's basically it. Fish, on the other hand, are surrounded by fluid, and their kidneys have to respond to that fluid and balance the water that their cells take in.
Too much water, and the cells swell up and get waterlogged--the hypotonic solution in the previous pictures.
Too little water, and the cells shrink and dehydrate and rupture--the hypertonic solution in the previous pictures.
The fish kidneys have to get the solution just right, and in a situation where the fish is surrounded by fluid of a different concentration.
Freshwater fish adapt to this situation by not drinking very much, and by urinating a lot; ocean fish (except sharks, which are a whole different story for another time) adapt by drinking a lot and not urinating very much.
So freshwater fish and ocean fish have adapted to this problem in pretty much opposite ways--that would seem to make a lot of sense.
But salmon live part of their lives in fresh water, and part of the time in salt water--so how can they have adapted to both, when the adaptations are the opposite of each other?
Salmon have adapted to both lifestyles--they can barely drink and urinate a lot when they live in fresh water, and then change to drinking a lot and barely urinating when they're out in the ocean.
Specialized cells in their body can work in opposite ways, depending on what they need at which stage they are in their lives.
But they can't turn it on a dime--they need days or weeks to make the transition between fresh water and ocean water.
And that's where the importance of the estuarine environment, like Padilla Bay, comes in--as an intermediate zone between the two other environments, it provides a place where salmon can make the transition.
In a region where the salmon can move around in the littoral zone to find the right amount of salt concentration they need, estuaries ensure the survival of those salmon leaving the fresh water where they were born, to go out and spend a large part of their lives in the ocean.
And they also ensure a place where--when it's time to go back up the freshwater river and breed--salmon have a place to adjust back from the ocean to the river environment, so that they can give birth to the next generation, and continue the cycle.
So often, in massage school, we don't have time to teach anatomy this deeply, and that's a real shame. If you just have time to memorize the fact that the kidneys control osmoregulation, so that you can recognize it when the MBLEX or the NCBTMB/NCBTM asks you about it, then that doesn't give you any particular preparation for clinical practice.
But if, at a deeper level, you understand what is going on, and you can draw a line from how the kidneys are involved in salt balance to what happens when that balance gets out of control one way or another, then you can understand what is going on with people living with renal failure or other kidney disease, and you are better equipped to know whether or not it's safe for you to provide massage under the circumstances.