Salt and water is some of the toughest stuff in Medicine. I prob spent untold hours studying this stuff, but until you see it in a lot of patients its tough to put in perspective.
I have found the easiest way to understand salt and water is at the bedside. First I evaluate the patients volume status (TB salt), toughest thing to do by far but- estimation of JVP, rales, and intracardiac filling pressures, ascites, peripheral/sacral edema, skin turgor, mucus membranes, blood pressure and heart rate, history of GI/renal/other losses- along with focused historical clues, usuing all of this and your clinical judgement is the first and hardest step. Then its easy from there...
TB water can be figured out if you can clinically estimate the TB salt.
Hyponatremia = more water than salt (but could have too much/too little of both compared to "normal levels")
Hypernatremia = ALWAYS a water deficit regardless of salt level
Where this matters in relation to your question is relevant clinically. Basically, Hyponatremia = hypotonicity in your plasma, over time brain gets jealous and sucks water from your plasma to dilute its higher osmolar concentration. Acutely, no big deal to rapidly correct this disorder (volume depleted patient with recent GI illness - you can correct their serum sodium from 120 to 140 without penalty). Chronically this can be bad as brain has had time to adapt to serum levels and if you suddenly create a more hypertonic plasma, all that water the brain is swimming in with all its jealousy suddenly gets pulled out and "demyelination" "CPM" happens. I saw this in an alcoholic with end stage liver- walked into ED with serum Na of 112, altered, admitted to ICU and sodium corrected to 135 over the course of a day -we got him on the floor about a week later and he was lethargic, blamed on "hepatic encephalopathy" MRI sev days later showed abnormal enhancement in the central pons. ooooops, sorry bout that.
Hypernatremia is a bit different. Acutely, also no big deal, the plasma becomes transiently hyperosmolar, brain gets jealous and starts to spit out water, but wait, he is much smarter than that and starts to reabsorb some organic solutes ("idiogenic osmoles") after a couple of hours; thus he is able to keep big bully plasma from stealing his kiddy pool of water. So septic heroin shooter in the ICU with endocarditis who is breathing 50 x per minute and sweating bullets of water does ok if you rapidly correct his sodium from 160 to 140. But grandma Sally is screwed, since she has been in a nursing home demented and the staff is mean and haven't been giving her access to water. Her serum sodium is 160, but has been that way for weeks, and her happy brain is swimming in her happy kiddy pool of water because it is buffed up with extra solutes. But then comes along Joe intern in July, who thinks he is smart because he looks at some numbers on a computer- "ahhh, I need to fix that now" to impress his Attending. Grandma Sally's serum Na is 140 in the morning. The attending wasn't paying attention in the morning because he just broke up with his wife and is distracted by the ICU nurse in pigtails with the beautiful backside. Grandma sally stops breathing a couple of days later and seizes. A CT head revealed midline shift and herniation because pissed off plasma suddenly became hyposmolar and happy brain said- "hey buddy, there is an osmolar party in here with tons of nekkid hookers" water now becomes the jealous one and jumps inside to get freaky in the brain party, which subsequently becomes a block party orgy so large that it can't be contained and spills out into the street. And that is why you should use a condom in situations like that.
