When a person is upside down, gravity affects blood flow, and blood can pool in the upper body, particularly in the head. However, the amount of blood that can accumulate in the head is limited by several factors, including the body’s circulatory system, the effects of gravity, and the body's natural mechanisms to maintain blood pressure and flow.
Typically, when someone is inverted, blood will move towards the head almost immediately, but in terms of total volume or pressure changes, it can take a matter of seconds to a few minutes for significant pooling to occur. Sustaining this position for extended periods (like several minutes to a few hours) can cause discomfort, headaches, or even more serious issues due to increased intracranial pressure, reduced blood flow to other parts of the body, or potential fainting.
If you're considering being inverted for a specific purpose, it's essential to listen to your body and avoid staying upside down for too long to prevent any adverse effects. Always consult a medical professional if you have concerns about blood circulation or health during such activities.