If I would drink 4 liters of water in one go on an empty stomach and with an empty intestine, why doesn’t this water fall directly through the intestine back to the ground?

Answer

There is a coordinating system between the stomach and intestines that controls the rate of gastric emptying. Alone a salt solution of 9 grams per liter flows unhindered through the stomach into the intestines.

Liquids that are more concentrated or contain less or no salt leave the stomach more slowly. If there are calories in the liquid, gastric emptying is also slower.

The purpose of this system is to avoid that too large amounts of fluid are adsorbed via the intestines in too short a time. It is therefore impossible for a lot of fluid to enter the bloodstream in the short term.

Whatever moisture does end up in it does indeed expand the circulation, but this is limited.

When this happens, the heart immediately secretes a hormone that prompts the kidneys to remove excess fluid (due to increased urine production).

in other words, the body has different braking systems to prevent such phenomena.

During gastric surgery it can sometimes be that the coordination between stomach and intestines is disturbed. Larger amounts of maladjusted fluid then end up in the intestines. This is usually concentrated and temporarily withdraws moisture from the blood compartment by osmosis.

This causes the intestines to expand, causing bloating, nausea and sometimes vomiting. The circulating blood volume decreases temporarily, which can lead to weakness, dizziness and fainting. This will recover.

This situation illustrates well what can go wrong. However, truly life-threatening conditions do not occur.

Food colorings are usually broken down during digestion. They do not stain blood and urine.

The blood has an intensely coloring substance (hemoglobin) that dominates everything. Dyes that are nevertheless included (such as methylene blue) are not noticeable. If they end up in the urine, it will turn green. Other dyes can color it orange.