To prevent their blood vessels from bursting due to lack of air pressure.
Explanation
The Moon lacks a significant atmosphere, resulting in a near-vacuum environment. On Earth, the atmospheric pressure exerts force on the human body, which is counterbalanced by the internal pressure of the body (including blood pressure). In the absence of external atmospheric pressure, this equilibrium is disrupted.
Detailed Analysis:
- To protect themselves from extreme cold only. is incorrect: While space suits do provide thermal insulation against extreme temperatures (both hot and cold), the specific requirement for the suit to be "filled with air" is primarily to maintain pressure and provide oxygen, not solely for temperature regulation. Furthermore, the term "only" makes this statement factually inadequate.
- To prevent their blood vessels from bursting due to lack of air pressure. is correct: In the vacuum of space or on the Moon, there is no external air pressure to push back against the body's internal pressure. Without a pressurized suit, the internal pressure of the body would cause blood vessels to expand rapidly and burst. The air-filled suit creates an artificial atmospheric pressure around the astronaut's body to maintain the necessary physiological balance.
- To provide weight to their bodies in zero gravity. is incorrect: While the suit adds mass, its primary purpose is life support (pressure, oxygen, temperature control), not to artificially increase weight to counter low gravity.
- To protect against meteor showers. is incorrect: Protection against micrometeoroids is provided by the tough outer layers of the suit (such as Kevlar and Mylar), not by the air pressure inside the suit.
Key Takeaway:
Astronauts require pressurized space suits primarily to simulate atmospheric pressure; without this counter-pressure, the body's internal fluids would boil and blood vessels would rupture due to the vacuum of space.