Answer: The main idea of the passage is what living in space feels like. According to the passage, "In microgravity, your blood is rerouted, flowing from the legs, which become thin and sticklike, to the head, which swells up." It also states, "The extra liquid in your head makes you feel like you're having upside down or have a stuffed-up nose."
Brief summary: Gravity largely affects how our bodies are designed and work, without gravity, our bodies undergo major changes such as redirected blood flow and muscle atrophy.