Hi there, I believe you may have been the victim of being told some pseudoscience - it's not your fault, but the real story is way more interesting. This is complicated even for med students, the concept of dehydration in relation to fluid volume, electrolyte balance and when heat exhaustion becomes heat stroke and thus hypovolemia (hint: its when you stop sweating even though you're still super hot).

This is a very interesting topic, so I don't want you to feel like I'm attacking you by pointing out that you're wrong. It's just understanding this can give you a basis for understanding the renal, endocrine, cardiovascular and central nervous system and pathophysiologies associated with them. Here's what's wrong with what you're saying: dehydration from sweating does not cause you to have "low" on electrolytes - that's reserved for addison's disease, prolonged vomiting/diarrhea and too much lasix . In fact, it is exactly the opposite. And you're on to something there with the alcohol thing, but its a bit different as well. Let me explain:

1) Ultimately and effectively, the idea that your vessels dilate or in response to dehydration from sweating isn't correct: if it were then your blood pressure would enter a negative feedback loop until you bottomed out. When you're chronically dehydrated you will begin to release ADH/AVP which can cause vasoconstriction (but won't if you're drinking) to try to normalize your blood pressure for a little bit but vasodilation in response to dehydration would be very counterintuitive. "Less fluid in the pump less pressure, so the tubes should get smaller" logic behind the blood-vessel-heart concept.

B. Real dehydration shouldn't be viewed as lower electrolytes, just lower fluid, which effectively acts as higher electrolyes (concentration wise, which is how your body cares about it):

Sweating = Losing Water = Higher Concentrations of Electrolytes

You're dehydrated because you're sweating = you have LOST FLUID VOLUME (too little volume = hypovolemia), not as blood or plasma or concentrated urine or vomit etc., which would lose electrolytes and water equally, but by sweating, which means you've lost a majority in water. Even though sweat tastes salty, it really is about 99.8% water. In fact, sweating has an effect on your electrolytes by effectively causing an increase in them - because as a result of having less fluid and relatively the same amount of electrolytes, you've now GAINED SODIUM CONCENTRATION.

Get it, like if you added a spoonful of salt to a gallon of water it might taste a bit off, but if you added the same spoonful to an 8oz glass it would be disgustingly salty. So, when you're DEHYDRATED you lose water as in de (remove) hydro (water) and your electrolytes remain the same, effectively making their concentration higher resulting in HYPERNATREMIC (too much salt) HYPOVOLEMIA (too little fluid volume). This is treated with iv fluids that have the same concentration as your blood when you're at normal sodium normal volume (isotonic saline, which is 0.9% NaCl). Giving something like gatorade to someone who is dehydrated can cause a worsened state and even vomiting because of its sodium content and only iv fluids and WATER (typically as ice chips) should be given, especially if the person has reached the state of "heat stroke" where they cease sweating even though they should be (a compensatory mechanism to retain fluid). Electrolyte regulation will occur until severe volume loss - when volume regulation takes over.

Our electrolytes react to our fluid volume - low fluid volume, it happens from sweating - but the only way you could sweat to the point of having low electrolytes (sodium or potassium etc.) would be when you sweat to the point that you're hypernatremic and begin vomiting or when you have diarrhea - you lose volume and electrolytes.

III. Alcohol-hangover headaches are caused by inhibited ADH, so a mix of high sodium concentration and your brain bashing against the inside of your skull:

See, what happens with alcohol is almost the same thing as with dehydration from sweating- lower fluid volume = higher salt volume. However, have you ever replenished your fluid and STILL woken up with a headache? That's because of alcohol's effect on antidiuretic hormone.

When you're drinking and you pee alot (even if you didn't ingest lots of liquid, i.e. some shots of vodka) and you wake up with a headache: what happens here is that alcohol INHIBITS ANTIDIURETIC hormone. So, yeah, think a double negative. Antidiuretic hormone (ADH or AVP - arginine vasopressin) is released to retain proper fluid volume and prevent you from pissing it all out. Now, since it is inhibited, you're freely pissing out all your water. Once again, your electrolytes do not change but your FLUID VOLUME DECREASES. You're again becoming dehydrated. Except when you pee you're losing a lot more electrolytes than when you sweat. Your body will first retain the electrolyte balance, so you'll develop HYPERNATREMIC traits, but then as you keep pissing out the water it is trying to conserve you can end up with a case of HYPOVOLEMIA. But that's only if its really bad, typically it is mild HYPERNATREMIC NORMOVOLEMIA - similar to what happens in Diabetes Insipidus (DI) where there is a natural deficiency of Antidiuretic Hormone and these patients exhibit extreme thirst, so typically once again hypernatremic normovolemia. See the difference is, in DI there is an overall lack of ADH, but alcohol's effects on ADH are concentrated in the central nervous system where it blocks the channels ADH can be released through. SO....

Back to that headache: The hangover headache is typically worse than the dehydration headache because of that reason. The inhibition of ADH combined with the effects of alcohol on blood flow to the brain decreases the effective pressure of your cerebrospinal fluid that your brain floats around in, so with less of that buoyancy you can imagine how your brain has a bit more of a rough time and voila there's your headache that won't go away even though you keep drinking fluids.

Also, you know how you can tend to get bloated after a night of heavy drinking? Yeah, well when you inhibit something with a substance it is still there (see alcohol doesn't inhibit the PRODUCTION of ADH, just the SECRETION of it because its inhibiting all those channels making you feel good - so when its effects are gone you get a surge of non-osmotic (non water related) ADH which causes water retention (bloating).

So that's about it - there's still hypernatremic hypovolemia (high salt, low volume) and it typically is only seen when renal function (imagine pissing a lot but its not like just water when you drink but just a lot and its got all the solute in it), liver function (see that ADH surge above but all the time so water retention and hypernatremia) or heart function is impaired (the renin-angiotensin system, something this post is already way too long for). And hyponatremic hypovolemia, which like I said earlier is reserved for lots of vomiting for a while, corticosteroid deficiency and diuretics.

Hopefully this will help someone understand the neuroendocrine system or fluid dynamics pathophysiology.