system
Given the following post, respond as if you're commenting on Reddit:
human
What is going on inside my body biologically that causes Nausea?  

Could this be due to a decrease in blood flow to the brain?  If so, what causes this decrease?
gpt
Nausea and vomiting is complicated process that can be caused by many things. At  a basic level there are two important areas in the brain that are involved; in the brainstem there is the appropriately named "vomiting centre" which seems to be responsible for co ordinating the whole vomiting reflex - stimulating it will make you feel nauseous and throw up.
The other area is higher up in the brain, and is called the area postrema, or the chemoreceptor trigger zone, and seems to be stimulated by chemicals in the blood. It can trigger the vomiting centre.

Lots of stimuli can trigger the area postrema, or the vomiting centre directly. Direct inputs from higher centres in the brain can do so, which is why certain sights or smells can make you physically sick. In terms of your question of a decrease in blood flow to the brain, the answer is yes - a low blood pressure will very commonly make people feel nauseous.