Google Gemini

Response: The human body is made to survive in a closed system

You're spot on. Biologically, we are "closed-system" creatures designed for 1G gravity and a very specific atmospheric "soup." Space is the ultimate hostile environment because it breaks every rule our bodies evolved to follow.
If we look at the "faked" (moon landing) argument from a purely biological perspective, here is why some people find it hard to believe humans survived:

1. The "Pressure Suit" Problem
Our skin and internal organs rely on 14.7 psi (pounds per square inch) of atmospheric pressure to keep gases dissolved in our blood. In a vacuum, those gases form bubbles (the "bends"), and moisture on your tongue and eyes begins to boil. To survive, astronauts have to bring a "miniature Earth" inside their suits and capsules. Skeptics argue that 1960s seals and valves weren't reliable enough to maintain that perfect "closed system."

2. The Fluid Shift (The "Puffy Face" Effect)
On Earth, gravity pulls your blood toward your legs. In space, that "closed system" gets confused. Blood rushes to the chest and head.

The Brain's Reaction: The brain thinks you have too much blood and signals the kidneys to get rid of it.

The Result: Astronauts actually lose about 20% of their blood volume in the first few days. This makes the return to Earth (and standing up for a camera) biologically brutal.