By Sarah Klein; Photography by Pexels

Let’s hear it for the H to the O.

Before we get all scientific, a request: Please seek emergency medical attention for anyone you think is dehydrated who loses consciousness, has a fever higher than 39°C, becomes confused or less alert, or shows symptoms of heatstroke like rapid breathing or pulse. This also goes for anyone who gets worse despite having stopped their activity, moved to the shade, and had something cold (and non-alcoholic, hello!) to drink.

Your body can’t control its temperature

Every little movement you make generates heat in your body, but our core temps actually stay stable, says Dr Timothy Lightfoot, director of the Huffines Institute for Sports Medicine and Human Performance. When sweat evaporates from the skin, it takes some of the heat along with it. But when you’re dehydrated, you don’t have enough water inside to sweat any out, which means you miss out on that brilliant built-in cooling mechanism. The heat stays trapped in your body, and your core temperature continues to rise. Should it get too hot in there, you’re at risk for heat stroke, which can, frighteningly, be fatal.

Your body can stop sweating

And not even in a good way. One minute, little rivulets will be trickling down the sides of your nose; the next, the flow has slowed and there’s a film of dried sweat across your cheeks, despite the fact that you’re still running. Not good, Lightfoot says. “This is the body trying to regulate; we don’t have enough fluid to both sweat and pump blood to the muscles, so it starts selectively shutting down some processes. When your body is trying to provide adequate blood flow to your muscles and provide enough fluid to sweat to remove some of the heat, sweating will lose.” Same deal if you turn a ghastly shade of grey: “That’s the body saying, ‘I need that fluid elsewhere,’ so it shuts off the blood vessels in the face,” he says. This is an pretty extreme case: the average person going about their daily business won’t stop sweating from a little mild dehydration, he says.

Your heart has to work overtime

About 60 percent of the human body is water and one of the biggest reservoirs we’ve got is our blood, Lightfoot says. When you start to get dehydrated, your blood volume decreases, but your heart has to pump the same amount through your body to provide the same cooling relief and the same nutrients to working muscles, he says. “Reducing the volume available makes the heart work harder.” When your heart can’t keep up, you could be at a greater risk of heat exhaustion, a less dangerous cousin of heatstroke. To maintain steady blood pressure with that decreased volume of blood, your vessels constrict. This tightening may be why some people experience dehydration headaches, Lightfoot says.

Read More: Three Signs You’re Dehydrated

You get toddler-level cranky

Ever feel forgetful, confused, mentally sluggish, whiny—then suddenly realise you’re thirsty? Our brains don’t like being low on liquids, although researchers aren’t entirely sure why that can lead to a hissy fit, Armstrong says. One (disturbing) theory is that specific areas of the brain physically shrink when you don’t have enough fluids in you. Armstrong’s research has shown these symptoms can start pretty early on in the dehydration process, too—and that they affect men and women differently. In one study, women noticed their moods dropped, tasks became more difficult, and they had headaches and a harder time concentrating after losing just 1.36 percent of their body weight after some treadmill walking. In another, men reported feeling tense, anxious, and fatigued and noticed their memory slipping after losing just 1.59 percent of their body weight.

Your pee won’t look like lemonade anymore

If you can even go, that is. A severe loss of fluids may make a bathroom break impossible, but if the result in the toilet bowl is dark yellow, you should be concerned, and if it’s brown or reddish in colour, you need to get yourself to a doctor. “If you can see that your urine is very concentrated, that indicates your body’s fighting dehydration,” says Dr Stavros Kavouras a hydration and fluid balance researcher. Taking a peek at your pee is about the easiest way to tell if you’re dehydrated, since very few other measures provide effects you can physically see. You don’t want to be going every 15 minutes, but if you’re hitting the head fewer than five or six times a day, you’re probably not getting enough H2O.

Read More: Are you dehydrated? Use This Equation 

Your body will get thirsty

Yes, this is a bit of a no-brainer. But! We actually don’t get thirsty all that easily, Kavouras says. Water wasn’t always readily available to our hunter-gatherer ancestors, so we’re programmed to survive without it for a significant amount of time; thirst kicks in when we’re already in a bit of a water deficit, he says. Which naturally leads us to the age-old conundrum: How much water are we really supposed to drink? Sorry to break the unsatisfying news, but there’s really no one-size-fits-all recommendation. The Institute of Medicine estimates men need about 3.7 litres a day and women need about 2.7, but that includes the fluids we get from the foods we eat, which accounts for roughly 20 percent of our daily water intake. “The most honest answer to the question of how much water you should drink is ‘it depends,’” Kavouras says, on everything from the weather to how active you are to how high you have your air con cranked.

Since most of us won’t sip it unless it’s right there in front of us, keep a bottle of H2O by your side in the office. Increased water intake is a very inexpensive way to improve your health. At the very least, refilling it is an excuse to get up from your chair.

Looking for more info on dehydration and your water in-take? Here are three reasons why you should be drinking more water, plus tips that’ll help you figure out how much water is right for your workout.