Science Questions

How do water tablets reduce blood pressure?

Kat - I’m assuming these are tablets which would make you wee more which would reduce your blood pressure. What do you think?

Chris - Yes. The simplest way to think about it is that when you take your diuretic, in other words something that makes your body lose water (that’s what water tablets are), then you’re reducing the volume of water in the body. Therefore you’re reducing the volume of water inside your blood vessels. Blood vessels are a fixed size and the more water which you put inside them, which is in your blood, then the more stretched they are and therefore the higher the pressure because liquids are incompressible. If you lose water from the body then this means that there’s less fluid in the vessels and, as a result, the pressure is a bit lower. That’s probably an oversimplification. There’s probably more to it than that. We know that some of these blood pressure tablets have other effects which affect metabolism and also affect the amount of sodium in the body. That’s principally how they work. Put simply, it’s because you’re reducing the volume of liquid inside a sealed container and if you take some liquid out then the amount of stretch, the pressure in the blood vessels must go down.

November 2008