system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: People with liver disease have a unique smell. What is it?

I can walk into a room and smell that my brother was just there.
gpt
Most veins lead towards venae cavae (the big veins in your torso that lead directly into the heart) but there's a system of veins (known as portal veins) that go from your intestines to your liver instead for detoxification and storage of nutrients. Then the blood continues on to the inferior vena cava. Blood can't flow as easily through a cirrhosed liver so it gets backed up and eventually is forced backwards into the rest of the circulatory system, normally through some veins around the lower oesophagus (your food pipe) and the stomach.

This way you end up with blood going from your intestines (carrying normal dissolved chemicals from your food) to the general circulation without the liver ever cleaning it up, so to speak. Some of these chemicals end up in the lungs and are breathed out, like how when your blood has alcohol in it some of it comes out in your breath (which is how breathalysers work and how we can smell when someone is really drunk). The technical term is foetor hepaticus and a main chemical compounds responsible are suspected to be dimethyl sulfide, found in several vegetables, and thiols or mercaptans, which are the chemicals added to natural gas to give it a distinctive smell.