How does the human body figures out what is “food” and what is a “liquid”, how does it sort it out?



This is something that has always puzzled me.

How does your body understands that these minuscule substances (like spices or seeds) aren’t liquid but instead considered food, and, how does it manages to sort out everything when your stomach has a bunch of things on it at the same time?

In: Biology

Just mashes it down, add some acidity and enzymes, then absorbs the amino acids, specially packed fat, and simple carbs into the bloodstream

It doesn’t do anything like that. Your body absorbs water when the tissues have a lower concentration of water than what is around it. That is usually the case when you eat because most of our food contains a good bit of water, and we usually drink more liquid to go along with it.

While you are absorbing water, you also absorb any nutrients that are dissolved in the water. Eventually you end up absorbing most of the nutrients, and what is left is the indigestible solid material that later becomes feces.

It doesn’t sort it out. Everything gets dumped into a pool of acid (stomach), which breaks everything down into small chunks. The acid-food-soup is passed on to a long tube (intestines), where the acid is neutralized (bile), and the nutrients and most of the water gets sucked out. The remaining solids get packed together into a larger mass for disposal later on.

Your stomach doesn’t sort anything. It just mashes it all together and breaks solids down then passes it into your intestines. Your intestines work to extract nutrients (to include water) from that mash. Water molecules are really small so they get absorbed rather quickly/easily.

Now as to how your body separates out various nutrients from water, that is going to include your kidneys, liver, and other parts of your body (like fat cells to store energy).

Solid and liquid food/drink enter the body through the mouth and are chewed into a fairly smooth mixture there. From the mouth they pass down to the stomach, where a combination of acids, enzymes, and physical churning breaks the food down further, both chemically and physically. From the stomach, the food enters the small intestine. A bit of further chemical breakdown happens here, and nutrients are absorbed from the food into the blood through the walls of the intestine. From the small intestine the remaining unabsorbed and indigestible parts (fiber, seeds, etc), as well as the water enters the large intestine. The large intestine absorbs most of the water, passing it into the blood, and the remainder is passed out as feces.

Meanwhile, your kidneys are filtering your blood to eliminate wastes produced by your cells and to maintain your blood at the right level of saltiness. As they filter, they excrete water filled with wastes and salt, which goes to the bladder for storage and is then excreted as urine.

TL;DR…there’s not a single step where everything gets sorted, it’s more like an assembly line where different parts of the food get absorbed (or not) at different stages. And urine production happens off on a separate assembly line.

It doesn’t. Everything goes in the same space (your stomach and then your gut). Things that don’t get absorbed by the intestinal walls remain in the gut and are eventually expelled as solid waste. All other waste is transported by your blood to your kidneys, filtered out and expelled as urine.