system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the body decide when to send blood to a person's genitals?

What process increases that blood flow? Why does it happen randomly when I'm learning about mass-damper-spring systems?
gpt
There is always blood flow otherwise the cells there would die. An erection occurs when blood is restricted from leaving so it backs up in the genitals. Vessels leading in expand to increase blood flow while those leading out constrict.

Why this happens without stimulation can just be due to normal fluctuations in testosterone, and is seen as normal healthy activity of the reproductive system. That or those mass dampers are particularly attractive today.