LO fricking L, I don't know where to start. Those are some pretty big statements coming from nonnephrologists, guys.
It all harkens back to the day when that dumb broad Orthopedist from UCSF went into spontaneous remission from Breast Cancer, then went around telling everyone water was the cure for everything under the sun, including Breast Cancer. I personally called her up and chewed her ass for as long as she could take it. She knew absolutely nothing. Nothing. Orthopedic doctors--strong as an ox, twice as smart, lol.
:thumb:
Back to the topic. NO ONE NEEDS to drink excessive amounts of water except stone formers. Healthy kidneys are PERFECT, and are able to change urine concentration over a very broad range from 50mOsm to approximately 1200mOsm, to keep PLASMA OSMOLALITY "tight" in a range from 285-295mOsm/L. What happens when you don't drink enough water? Your neurohypophyseal neurons sense MINUTE changes in plasma osmolality, and when the pOSM goes up even 2Osms, ADH is kicked out of the posterior pituitary, goes to the distal convoluted tubule, interacts with ADH receptors, which basically open channels (complex) in the urine flow side of the tubule to maximize free H2O reabsorption. The resulting drop in plasma Osmolality suppresses ADH (Vasopressin). The lack of ADH causes polyuria and a free water diuresis, raising pOsm, and on and on. These changes are likened to a feedback loop, not too dissimilar in concept from EFI with an O2 sensor in closed loop operation.
It also turns out that when the Blood Pressure drops, teleologically associated with volume depletion (dehydration), ADH is released. Now, in association with a host of other compounds like Angiotensin II, which stimulates Aldosterone release from the adrenal glands, etc., etc. ADH, and thus AII seem to be the major stimulators of thirst drive. ADH now pathologically is acting to reabsorb free water "too much" or "pathologically" in hypotensive conditions, and hyponatremia results (low Serum Sodium). This is why most REALLY volume depleted patients have hyponatremia.
ADH release causes distal tubular free water reabsorption, thus concentrating the urine (in humans, maximally to 1200mOsm) and ADH suppression (as occurs due to beer/EtOH) results in free water diuresis, diluting urine maximally (in humans, 50mOsm). What's this mean? Well, how much urine do you need to make? You need to make enough urine within the range of renal concentrating and diluting capacity to be able to rid yourself of your daily osmolar load. If you eat 300mOsm worth of solute (dietary), a maximally concentrated urine can rid these Osms in 300mOsm/1200mOsm/L equals 250cc of maximally concentrated urine. Given 500cc insensible water loss (sweat, stool, breath), that's about 750cc or .75L of po water intake per 24 hours to maintain pOsm in normal range. On the other end of the scale, if one drinks gallons of water, ADH is suppressed and urine is maximally dilute at 50mOsm. Therefore, their urine output to rid 300mOsm of solute is 300mOsm/50mOsm/L or 6Liters of urine. Again, with .5-1L of insensible water loss, you're looking at that person drinking about 6.5 to 7L of water.
So what happens when you drink TOO much water. Let's say you drink 15L of water and only have a solute load of 150mOsm. Everyone's in steady state, so what you take in, you've got to get rid of. However, now you've overwhelmed to kidney's capacity to maximally dilute urine and you end up with horrific free water retention. This state does NOT lead to volume overload, since volume status (extracellular fluid) is reflected only by the physical exam, but the body's free water content is typically determined by the serum Na. Hyponatremia ensues, and if severe enough, as correctly stated before, can result in death. Hyponatremia causes the ECF to "think" there's too much ICF osmolality (since Na rarely resides in the ICF), water follows its concentration gradient, goes INTO cells, causes cellular swelling (particularly brain), and results in seizures, arrhythmias and if severe enough--brain swelling and UNCAL (brainstem) herniation with death.
People who drink too much water chronically, have maximally dilute urines and chronic hyponatremia, are characterized as having "psychogenic polydipsia". They typically have severe psychological problems.
People who eat very little solute, drink a ton of alcohol and wash their medullary concentration gradient away, have chronic hyponatremia, isosthenuric urines (mid-concentration 300mOsm, eg) and are characterized as having "beerpotomania". These people don't eat enough solute, so they can't even generate a urea concentration medullary gradient (due to low urea generation rates and high urine flow rates--another long story), so they have serum-like urine concentrations and are at risk of both dehydration and easy water overload since their spectrum of concentration capability is now 300-400 mOsm instead of 50-1200mOsm.
The girl above died of water intoxication.
Kangaroo rats have kidneys that can generate urines in the 2400mOsm range. So what's that mean? You guessed it. THEY DRINK NO water. NONE. They don't have to, because they generate enough water from the metabolism of carbohydrates CHx---to CO2 & H2O to keep their serum Osm acceptable. To the best of my knowledge, they don't die of breast cancer from water deprivation. They also are obvioiusly desert dwellers. If you catch one drinking something, I'd like to see a pic. You can email it to me.
What's water good for?
1) constipation
2) stone formers
3) control of hunger (sometimes)
Sorry, the kidney's just too damn smart, and it doesn't need YOUR help. It can take care of itself just fine, thanks. Sorry if I hurt any feelings or offended anyone. That's about 1% of what I know on this topic, so fire away. :thumb: