Every living creature needs water to survive. Yet sweating, peeing, vomiting, or having diarrhea can cause a loss of fluid, says MedlinePlus, further increasing your fluid needs, threatening your survival, and, in a complex physiological process described in a May 2018 article in Current Biology, making you feel thirsty.

If you’re thirsty, that’s the most obvious sign you’re dehydrated, meaning your body doesn’t have enough fluid to function properly.

According to MedlinePlus, being dehydrated doesn’t just mean your body is losing water — it also means you’re losing electrolytes, such as salt and potassium, which help your body breathe, move, talk, and do all the other things it needs to do to stay up and running.

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As MedlinePlus points out, certain health conditions, including diabetes, can put you at an increased risk for dehydration. If you’ve been sweating too much due to heat or overexertion, throwing up or having diarrhea because of the flu or another acute illness, or urinating frequently, it’s important to watch your fluid intake.

People who are especially vulnerable to losing fluid include those who are unable to quench their thirst because of disability or disease, those who are athletes, and those who are simply too young or too old to replace fluids on their own,  according to NHS Inform. Men who are middle-aged or elderly may also be at an increased risk of complications from dehydration, according to a small study published in September 2020. (The study did not involve women.) The researchers found that over time, the body becomes worse at detecting markers of dehydration (such as high levels of salt in the blood), and without these signals, older adults may not realize they are dehydrated or take steps to rehydrate. Untreated dehydration can cause the heart rate to increase, straining your ticker.