Like, what exactly happens when you take a pill of vitamin A compared to vitamin D? Or a tiny grain of fentanyl? How exactly do our body recognize these extremely complex substances and not confuse one from another?
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It’s like those kids toys where you have to put the right shape in the hole that matches its shape. Squares go in the square hole etc. Chemicals have unique structures and properties. If I throw a bunch of triangles at the holes, the triangles will enter the triangle holes only and not the holes for the other shapes. Chemicals compounds work like this, certain molecules are too big to pass through membranes on their own or are the wrong shape to bind to certain molecules. A square can’t fit in a triangle etc.
So Vitamin D will go where it fits, Vitamin C will go where it fits, etc.
They do confuse each other. All the time.
You mention fentanyl. Fentanyl is an opioid, meaning it acts on the opioid receptors. These are little receotora on the nerves that activate upon contact with endorphins, and on activation it calms the nerves, makes it harder for the nerves to send signals, like to send a pain signal. Sometimes it’s necessary for the body to increase its pain tolerance so the brain releases endorphins into the blood which acts on all nerves throughout the body.
Fentanyl tricks the opioid receptors by looking like natural endorphins but not actually being one. It’s actually more powerful than the natural ones because endorphins are a lot bigger cause they were made through the imperfect process of evolution, fentanyl is much smaller but fits into the part of the opioid receptor that matters and is better at triggering it. It’s a mistake that fentanyl works here, but there’s been little evolutionary need to prevent this so it just happens. And even if there was an evolutionary need to, there are a lot of chemicals out there, many which still fit the lock here.
So the actual thing often recognized isn’t the entire substance, it’s the individual pieces that make it up. When you start dealing with big molecules like proteins a lot of their parts start to look alike and we have proteins, little chemical machines, that can recognize these parts, not just the individual molecules, and deal with them however it needed to be dealt with.
You need to stop visualizing the body as one entity, it’s billions of smaller entities that work together by chance, unaware of their purpose. If you understand this, everything becomes clearer.
So the vitamins aren’t recognised instantly, they are absorbed in your intestines and get into your bloodstream, where they find molecules tailor-made to complement their shape that bind them. That’s how they are recognised, there’s a specific molecule made to bind them.
It’s not like your body goes “oh, that is vitamin A! Vitamin A is good for me, therefore I will be happy!” Each chemical has a particular set of effects based on its properties and how it’s digested and absorbed, and sometimes these effects can indeed be similar between chemicals. Nothing is “recognised” unless we’re talking about viral/bacterial infections and immunology, but that’s not digestion. A chemical will react or not in a certain way in your stomach acid plus the various enzymes that are present there, and that process carries on through various chemical environments lower down your digestive tract. At some point, it is absorbed into your bloodstream, where it might do something or it might do nothing. Once it’s in your bloodstream, it is transported everywhere your blood goes, and some cells in your body might absorb it for their internal use or perhaps it sticks to some exterior part of some cells and affects their function.
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