Why, when the skin can absorb medications and ointments for pain, does soaking in a bath not hydrate the body?

Your skin is quite literally made to be water resistant. This is a very good thing, as the kind of water our skin is likely to come in contact with (rain) is going to be littered with all sorts of nasty things the body doesn’t want.

Medications are in creams (fat) so it can go past your fat layer on your skin. Water and fat dont mix.

Water and fats don’t mix. It’s like oil and vinegar. Neither dissolves in the other.

Your skin is made of fats. Your bathwater can’t enter your body through the skin the same way a glass of water with some oil on top will never evaporate. The same way you could put some water in a tub of vaseline and it would never “soak in”.

And it’s a darn good thing! If you could hydrate in a bath, that would mean whenever you weren’t in the bath you would be dehydrating by just seeping water through your skin! It definitely has to be waterproof for a %60-water human body to exist on dry land.

Now why do the medicines absorb? Because ointments have the active medicine stuff dissolved in a fat/grease/oil suspension. Our skin, being made of fats, is somewhat permeable to other fats.

ELI15 TLDR: “Like dissolves like”. Water is polar, your skin (and ointments) are fats (nonpolar). Opposites don’t mix.

Some researchers believe wrinkling is your skin becoming overly hydrated. Either way, the skin is just really good at resisting water like an umbralla