Red blood cells keep much more creatine inside them than is floating in the blood, even if blood levels change — it’s like they have a special pump that keeps the levels steady.

Even though blood cells from people with kidney disease let in more creatine, they don’t end up with more creatine inside — probably because something in their blood is blocking it from building up.

People with kidney disease have red blood cells that pull in more creatine using sodium than healthy people do, but their cells work the same when sodium isn’t involved — this suggests their bodies handle creatine differently because of the disease.

In regular human red blood cells, about 40% of the creatine that gets inside does so using a special sodium-powered process, which means this system plays a big role in controlling how much creatine is inside the cells.

Your muscles need sodium to pull in creatine, kind of like a battery-powered door — if the battery's dead or there's no sodium around, creatine can't get inside, even if you're drinking plenty of water.

Creatine gets into human cells in two ways: one is like a precise lock-and-key system that works well at low levels, and the other is a more general process that kicks in when there's a lot of creatine around.