Is there a difference between being thirsty and dehydrated? Like scientifically, biologically – by the time you notice that you are thirsty, are you already dehydrated?
In: Biology
Thirst is regulated by two things: the volume of fluid in your body and the balance of salts/electrolytes in that fluid, mostly sodium. You have sensors/receptors in certain areas of your body specifically tuned to each. If the volume receptors sense that you have less fluid in your blood stream, they will release signals to other parts of your body to change how your body balances water and salts so that you retain more water and pee out less. If your sodium level is way out of balance, other signals will try to regulate your salt levels. Since water and salt really like to follow each around, it’s not super clean and regulating water levels oftentimes means some changes to salt levels, with a few exceptions. One of the responses to these signals is behavioral: thirst. You feel the behavioral urge to drink.
Side note: there are certain behavioral problems wherein a patient drinks and drinks and drinks if they have access to water, and can do so to the point that they can dangerously drop their salt levels and suffer bad consequences like seizures. The term for that is psychogenic polydipsia. (Poly roughly means many and dispsia roughly refers to drinking).
Dehydration refers to having less than the necessary amount of fluid volume in your body. You can be dehydrated with low sodium levels, normal sodium levels, or high sodium levels. Depending on how intact and functional your thirst regulators are, you may or may not feel thirsty, or if you’re unconscious or otherwise altered, you may not be able to drink anything despite your body trying to tell you too.
Your body goes through a certain amount of fluid every day just taking care of its basic functions. You get fluid from drinking but also from the foods you eat that have liquid. Healthy people with functioning kidneys should drink when they’re thirsty. If it’s hot outside or you’re doing something active, drink a little bit more. Trust your kidneys. If they’re healthy, they know what to do better than some arbitrary fluid goal.
Edited to add: since I didn’t say it outright, to get to the heart of your question. Thirst is a behavioral response/feeling that is most frequently driven by overall fluid balance, but which can also be driven by other factors, like salt balance. Dehydration is a physical state of low body fluid status compared to where it needs to be to support your body’s internal functions.
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