system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do our bodies absorb things like caffeine and alcohol at a set rate or do we absorb faster if we consume more?
gpt
Essentially, we absorb them faster - up to a point.

Think of digestion like pouring water through a funnel. Up to a point, things pass through into the bloodstream faster, but there's an upper limit on that speed. In a funnel, that's based on how narrow or wide the opening is - in your body, it's based on how many cells are doing the work of passing things into the bloodstream. Caffeine and alcohol don't just seep into the bloodstream, they have to be carried across the gut lining and blood vessel walls before they can make it to the blood.

In pharmacology, this is called the dose-response curve, and it usually looks vaguely S-shaped: at low doses, the absorption (and the response to the drug) is quite slow, and it speeds up quickly as the dose is increased, but then it will usually level out again at high doses, and anything above that level will just stay sitting in your stomach.

Concentration can also have an impact on how fast you absorb a drug (including things like caffeine and alcohol; in this sense, "drug" means anything that has an effect on your body's metabolism). This is why certain alcoholic drinks (such as poitín) are notorious for getting you drunk *the next day* - because it's at such a high concentration that your body recognises it as poison and doesn't absorb very much of it, but when you get up and drink some water, it dilutes the alcohol and a whole lot more alcohol gets into your system.