Discuss the implications of microgravity on human physiology and the countermeasures required to maintain astronaut health during extended space missions.

Microgravity affects human physiology by causing muscle atrophy, bone density loss, fluid redistribution, and cardiovascular deconditioning. Countermeasures include resistance and aerobic exercise regimes, nutritional support, pharmacological interventions, and potentially artificial gravity through rotational habitats. These measures aim to mitigate adverse health effects and ensure astronaut readiness for both space and planetary environments.