By Dr Alice Roberts
Presenter, Don't Die Young
The kidneys are fantastic organs. They make sure we have exactly the right volume and composition of blood.
Dr Roberts surveys the work of the kidneys
They filter about 180 litres a day, but return most of this straight back into the blood, just taking out the waste and extra fluid - which forms urine.
I wanted to find out more about how the kidneys do their job - and what we can do to help them.
Armed with weighing scales, glass jugs, some bottles of water and a little salt, I set off to Bath Rugby Club to explore the theory behind isotonic drinks.
This theory suggests that drinking water with just a pinch of salt in it can help athletes rehydrate quicker than just drinking pure water - by getting the kidneys to hold on to more fluid.
What I needed was some willing guinea pigs, who would test out different drinks after rugby training, and let me examine their urine afterwards!
One rugby player drank water, one drank water with a bit of salt in it, and one drank nothing at all.
When I compared their urine samples a couple of hours later I found the kidneys had done their job of conserving water in the rugby player who had drunk nothing - he peed the least.
Of the other two a pinch of salt had certainly seemed to do the trick in encouraging the kidneys to hold on to fluid and produce less urine.
Neglecting her health
We may not be aware of our kidneys constantly working away inside us, but when I met Grainne O'Neill during the course of filming the programme, I wanted to persuade her that her kidneys were well worth looking after.
Unhealthy kidneys can be bad news
When Grainne turned 40 she started thinking seriously about how her lifestyle might be affecting her health.
With a busy life, balancing looking after two children and running the family business, Grainne had been neglecting her own health for some time.
She also had a family history of diabetes - a condition that can damage the kidneys - and Grainne wanted to reduce her own chances of acquiring the disease.
Diabetes causes high blood sugar levels which can damage the kidneys as well as other organs.
Type 2 diabetes is on the increase in the UK - and it is linked to an unhealthy diet, physical inactivity and obesity.
So there are certainly things you can do to reduce your risk of getting this disease.
There are also symptoms you can look out for: feeling more thirsty or weeing a lot more than usual, weight loss, and itching around the genitals. It is worth getting checked out by a GP if you have these symptoms.
Grainne came to meet me in my lab to take a closer look at the kidneys.
As well as dissecting a kidney to look at its inner workings, I showed Grainne her own kidneys - using an ultrasound scan.
Armed with new knowledge and respect for her kidneys, Grainne knew she needed to lose weight to reduce her risk of diabetes - and embarked on a new healthy eating regime and a weekly exercise class.
She also decided to get herself tested for diabetes; fortunately, the result was negative, and Grainne is now taking the right steps to ensure it stays that way.
Dr Alice Roberts: Don't Die Young will be broadcast on Tuesday 16 January on BBC Two at 2000 GMT.