Water is essential for life. We take it in mainly by drinking and eating. But as well as gaining it, the body also loses it. You breathe out water vapour.
And water vapour evaporates from your skin. It's most noticeable when you sweat. More obviously, you lose water through urine and faeces.
But despite these daily gains and losses, the body works hard to keep its internal body fluids at a constant concentration. The kidneys play an important role, producing more or less urine depending on how much water you've gained or lost. As well as sweating profusely for the past couple of hours, Lisa has had nothing to drink since yesterday. By now, she'll be extremely dehydrated. One obvious sign is that she's producing very little urine and it's dark in colour.
On the other hand, Joanne has taken in lots of liquid over the past few hours. Her urine is plentiful and very pale. Her body is losing water, which means that she must be fully hydrated.
The kidneys regulate the amount of urine produced. It's their job to keep body fluids, like your blood, at a constant volume and concentration. Blood flows in through the renal artery and out through the renal vein.
Inside the kidney are millions of microscopic devices called nephrons. Blood entering each nephron is filtered. Most of its fluid content, represented here as yellow, passes into the nephron tube. As it trickles down, exactly the right amount of water is taken back to give the blood its correct volume. The remaining waste solution passes out as urine.
So what will happen to Lisa and Joanne's urine levels if they both drink a litre of water? They continue to give samples every half hour. Lisa's urine is gradually getting paler but she's still producing less than Joanne. In just two hours Joanne has lost nearly all the water she took in.
Her kidneys are getting rid of any excess fluid. When she drank a large quantity of water, her body fluid increased. This excess fluid appears in the blood.
When it reaches the kidney, very little is reabsorbed into the bloodstream. Most of the water passes out as urine, bringing the blood volume back to normal. So if you're dehydrated, what's happening inside your kidneys?