The brain is usually approximately 1/50th of your total body weight but it gets 18-20 per cent of all your blood circulation and it needs that to keep it functioning. When you start drinking more water you wake up your body's other systems for regulating water and they send out their own signals to call for water.

Exactly how they do this is a mystery, still but basically when the body needs water, sensors are alerted and they send a message to the brain. They nudge the brain into telling us to drink. The hypothalamus, deep inside the brain, hears the call and gives you the idea to drink.

Say you ignore your brain telling you to drink the body gets a bit annoyed and sends out a send, heftier hint. It slows down its production of saliva to see if that will give you the push you need. That's why you get a dry mouth but as you can see this symptom is quite a way along the road towards being dehydrated. It's usually what people consider the first sign of thirst but you should be drinking before this happens.

Then your brain sends in the big guns, reducing water in the blood, because two-thirds of your body's water supply is held in your blood cells. This reduction in water in the blood obviously causes blood cells to shrink, like water bombs deflating - so the spaces around them are less full too. The brain knows this and again it asks you to drink but less politely, this time. You may be beginning to feel confused and have a headache, as thickened blood, with less water in it, moves more slowly. It's a long way up to your brain so less of your blood bothers to make the journey, which impairs your mental abilities.

But you didn't think it would be that easy, did you? Things rarely are and so it is with water in your body. The kidneys complicate things by excreting or retaining water at different rates. This is affected by how much salt there is inside you, in and around your blood cells. Your body tries to not let levels of salts rise in your body, even when you're not drinking enough to dilute them. It also hangs on to the salts you have by reducing the amount of salt in your sweat, when you sweat a lot, so salt pills are rarely needed.

One of the major causes of dehydration in the developed world is excess salt intake. This extra sodium floats around outside your blood cells until the water in your cells yearns to neutralize it. Eventually, the water in your blood cells can stand it no longer and races around the sodium straight out of your blood cells to try to dilute the sodium. That leaves your blood with a higher concentration of salt in it and too little water. Your body craves water again and screams at you to drink as your kidneys try desperately hard to kick the excess sodium into your bladder so they can get rid of it.

Clever, huh? Yes but only if you heed these calls for you to drink.