system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can spiders hang upside down for long stretches of time? When if we did that the blood would rush to our head and cause us pain.
gpt
Because invertebrates like arachnids and insects don't have a closed circulatory system with blood like we do. They are essentially hollow exoskeletons filled with fluid. Their "blood" is already everywhere it can be so there is no sense of it rushing to one part of their body or another.