I’ve always heard that if you’re stranded on an island not to drink the ocean water because it will just make you more thirsty. I’ve never heard a logical explanation for this though.
Will you get dehydrated faster? Is that what it means? Why? If you’re getting dehydrated/thirsty just drink more water. Its not like you’ll ever run out.
I could understand a volumetric comparison: Drinking 1L of saltwater will dehydrate you faster than drinking 1L of freshwater.
But you’ll never run out of water! So you can always just keep drinking more.
Something along the lines of needing a certain ratio of sodium and potassium chloride to water in the body…these electrolytes help regulate heartbeat and organ function. when the ratio is just slightly out of balance, ie too much sodium or potassium you get thirsty, if there is not enough, you get salt cravings…but to guzzle waaaay too much water you can die or to drink water with high sodium content,the body cannot flush it out fast enough and bam…heart and kidneys get out of whack and you will die.
Ok, so this is a barnyard explanation but it works for me.
How thirsty you are depends on the ratio of salt to water in your blood. Salt water has more salt that blood so the more you drink, the thirstier you are.
But…I salt regulation basically occurred via osmotic pressure in the kidneys. If too much salt in the blood, more water is taken up. If too little salt in the blood, less water is taken up.
Basically, I don’t see how you can end up with too much salt in the blood…since there is always excess water to ensure proper equillibrium.
But sea water has a much higher concentration of salt than your blood has. The sodium ions will flow from the higher concentration to the lower. In other words, your body can’t dump the excess salt because osmotic pressure will always cause sodium ions to flow from sea water into your blood stream.
Here’s a brief explanation.
You can get rid of excess salt via urine. But seawater has more salt than urine does. So your body attempts to flush the salt out of your body in the urine, which requires water…more water than you got from the seawater in the first place. Every drink of seawater takes more water from your body to dump the salt out than there was water in the seawater you drank. The more you drink the thirstier you get.
Marine mammals can produce urine that is saltier than seawater, and so they can safely drink seawater, but humans and other land mammals don’t have such effective kidneys. It takes energy to concentrate urine like that, and our evolutionary history never required it.
But eventually an equilibrium will be reached wherein the concentrations are the same. I gather then that this equilibrium concentration is an unhealthy level?
Sea survival trained ex-coast guard guy checking in:
The above responses are basically correct in that the salt content in seawater is too high to drink exclusively; however, in a survival situation seawater may be drank if supplemented by fresh water. You don’t need to eliminate all of the salt, but rather lower the average concentration to a level which can be handled by your body. By drinking fresh water in an amount 2 to 3 times the volume of ingested seawater, you can maintain a survivable electrolyte concentration in the bloodstream. Important to know if you ever find yourself lost at sea, since your only source of fresh water may be collection (rain) or solar distillation - slow and often ineffective processes. Some seawater will help extend your survival time.
I have a related question. Since sea water dehydrates you and plain water will hydrate you, there must be a point in between that does neither. So how much salt could be dissolved in water before any benefit from drinking it is eliminated?
If you’re talking about average salinity seawater, yes. Your body does require some electrolytes. Rehydration entails replacing lost salts as well as water. There was a man once who realized this, and analyzed human sweat to figure out exactly what it was made of. He re-created it from scratch (less the bacteria that thrive in human sweat and cause B.O.), added extra sugar and some orange flavouring, and invented Gatorade. Now, seawater is going to contain much more salt than you need, and certainly does if you follow that 1:2-3 ratio. That is a maximum tolerable value, since a deficiency of water is a much more likely and more serious problem than electrolyte deficiency. Having said that, in a lost at sea survival scenario, if you have to go a long time without food (an alternate source of salts), you will last longer with some salt intake than you would if you consistently ate nothing and drank only fresh water, but the required salt intake is small.
Incidentally, there have been cases of sailors lost at sea who survived by drinking their own urine. This is a good way to recycle water if you do not have another method of replacing lost body fluids. If you do that, you shouldn’t drink much seawater at all, since you are gaining back some of those lost salts. Expect to get quite ill over time, though. Urine is the body’s way of disposing of toxins, and by recyling it you are increasing the concentration of those toxins in your body. Again, this is a tradeoff, since dehydration is the more serious problem.
This discussion is really moot, though. Your best defense against dehydration is proper planning, knowing the weather, having the right emergency equipment and so on. Lost at sea incidents should never happen, but if they do, you stand a much better chance of survival with appropriate equipment and provisions.
I had a physiology professor who swore that this had never been scientifically investigated. I certainly believe all of the seamen and survival nuts – they have the cold, hard anecdotal evidence that one would not like to contest. I saw a special on the USS Indianapolis sinking during WWII this weekend, and the doctor (who survived and drifted with a group of 400, of which 90 or so were rescued 4 days later) said that if someone started drinking seawater, they started to hallucinate and eventually drowned. So if they saw someone drinking seawater, they would push him away from the group. That’s pretty strong.
Anyway, my physiology professor’s reasoning was that human blood was like 290 mOsm, and seawater was usually a little over 300 mOsm. A well-functioning kidney can excrete 1200 mOsm concentrated urine. So we should be able to at least partly “distill” seawater.
I don’t know if things break down before this. I’m sure that they do somewhere – hypernatremia is a bad thing on many levels, and perhaps a bulk ingestion of seawater would be enough to induce this before the kidneys even had a chance to concentrate urine.
I would just share that I recently had a kidney stone and subsequent urine analysis indicated high levels of calcium for me. Lay off dairy was my recommendation.
The urologist told me that, in rare cases, kidney stones could nucleate and obtain large sizes in a matter of hours. I thought this unlikely in my case and probably the majority of almost all cases, but it demostrates that the kidneys cannot handle everything thrown at them.
Maybe divalent calcium is different from monovalent sodium–and I realize this can play a role in passage through the nephrons during filtration, but the kidney can only do so much.
I’m guessing it doesn’t take long to overload the kidney’s filtration setup.
Isn’t it based on osmosis (and sodium pumps–I don’t remember).
If osmosis drives sodium out of the blood and into the urine, wouldn’t there be a point where urine sodium concetration and blood sodium reach an equilibrium across the nephron? Can’t the urine be overloaded with sodium to the point where anymore present remains in the blood because it can’t be drawn into the urine? And drinking seawater is what is concentrating sodium in the blood and then overloads the kidney?
Sorry–edwino–I don’t really know that much about this.
And, what about the cation potassium? A co-ion in some salts (and most of us think of salt as NaCl) ; too high levels of this are fatal, as it’s used in executions–what are its levels in salt water–or is ingestion too low to make a difference. Injection of potassium kills, but injestion of K isn’t what we’re talking about; ingestion of sodium does kill, in seawater doses (doses large enough to drink them) however.
Again, I’m not a physiologist, but how does it work?