What is one significant effect of microgravity on the human body?

Microgravity has a profound impact on the human body, leading to physiological changes that can be quite severe, particularly after extended periods in a low-gravity environment. One significant effect is muscle atrophy and decreased bone density, which occurs because muscles and bones experience less mechanical loading in microgravity conditions. In a normal gravitational environment, the body's muscles and bones are constantly under stress from weight-bearing activities, which helps maintain their strength and density.

In the absence of gravity, these structures do not need to work as hard. Consequently, muscles begin to weaken and shrink—a condition known as atrophy—as the body adapts to the lower demand for movement. Similarly, bones lose density due to reduced stress, making them more susceptible to fractures and other skeletal issues. This phenomenon poses considerable concerns for astronauts on long-duration missions, as countermeasures such as resistance exercises are needed to help mitigate these adverse effects and preserve astronauts' physical health during and after their missions.

In contrast to other options, increased muscle mass, reduced metabolic rate, and enhanced cardiovascular health are not typical outcomes of extended exposure to microgravity. Instead, the loss of muscle and bone density stands out as a critical concern in aerospace physiology that must be addressed to ensure astronaut health and mission success.