My piss is currently bright yellow from B vitamins. Just wondering how my body knows how much to take and how much to excrete?
In: Biology
Imagine a kitchen sponge. When it gets a little wet, it absorbs water. When you fully submerge it, it absorbs some, but eventually stops absorbing because it’s already full, called Saturated. Under normal conditions, something can’t hold more than it’s saturation point. (There are exceptions but this is ELI5 so I’ll leave that to the comments)
Your cells are like sponges, sort of. They have cell walls that act like sponges, soaking up nutrients and oxygen and other things the blood cells carry to them, and when a cell is saturated, it stops absorbing. Any of these items that make it through your body all the way to your kidneys were the leftovers, the extra water in the tub, after all the cell sponges were full. Your kidneys take that material and dispose of it through urine, because your blood can only carry so much stuff, so if these are leftovers, they’re not in demand and would just get in the way of the in-demand nutrients if they were allowed to keep circling.
There’s no decision, no intention. The function of absorbing or not is entirely automatic, like a sponge absorbing a drop of water, or not.
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