https://www.wired.com/story/are-humans-fit-for-space-a-herculean-study-says-maybe-not/

Here’s how you test your intracranial pressure in space. First, you collect baseline samples of your blood, saliva, and urine, and take ultrasound images of the vessels in your heart, neck, head, and eyes, lining up the scanning device on black dots tattooed on your body before you left Earth.

Then, you clamber into the Chibis, Russian for “lapwing,” a pair of hard, corrugated-rubber pants whose waist can be sealed. The pants suck: A vacuum imitates how gravity on Earth pulls blood, mucus, the water in cells, and cerebral and lymphatic fluids from our skulls to the bottom half of the body.