system
Given the following post, respond as if you're commenting on Reddit:
human
Normally in the body fluids and minerals in those fluids flow from areas of high concentration to low concentration. However the kidneys are able to remove minerals/toxins from your blood and concentrate them into urine which is much more concentrated. How does it do this?
gpt
The nephron is a microscopic “unit” of the kidney that controls this. It’s essentially a series a tubes set up in a way to allow only some solutes to pass out - effectively concentrating it. These “concentrating” parts are not permeable to water, so you only pump out certain ions. How does it seemingly work against a gradient? By using energy elsewhere to build a temporary gradient of another ion (not found in high concentrations in fluid used to build gradient, so you don’t have to work against another gradient), and using *that* gradient to pump that solutes to very high concentrations. 

Then the kidney uses that very high concentration to then draw out water elsewhere, which forms the concentrated urine. In some desert animals, they have very long nephrons that are able to build up a much more concentrated gradient, and therefore concentrate their urine that much more.

Of note, kidney is a pretty energy-intensive organ, needing to build up that high concentration gradient and all. That’s why kidneys are easily injured during critical ICU-level illness (and you see a lot of people on dialysis after long hospital stays).