system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Sperm are often referred to as “swimmers.” What exactly are they swimming through? Are the uterus and Fallopian tubes filled with liquid, or empty?
gpt
They are travelling through cervical fluid. When a woman ovulates and is most fertile, her cervical fluid becomes clear and mucous like, perfect for sperm to travel along. The consistancy of cervical fluid changes throughout the month and becomes less easy for said swimmers to travel along when she is not fertile.

The cavities do contain these cervical fluids but it's not as much as you think, its just enough to coat the surface of the womb. Aside from this, it is empty.