Organ System Integration and Homeostasis (3B) Practice Test

During a heat-stress study, healthy volunteers walked on a treadmill for 30 minutes in a warm room. Core temperature rose by 1.0C, and sweat rate increased. Despite water intake, plasma osmolality increased slightly, and urine output fell over the next hour. The investigators concluded that multiple organ systems coordinated to conserve body water while maintaining blood pressure and heat dissipation. Which interaction is most critical for maintaining homeostasis in this setting?

A lab models chronic obstructive pulmonary disease (COPD) by increasing airway resistance in an animal model. Over days, arterial $P_{CO_2}$ increases and arterial pH initially decreases. After compensation, arterial pH moves closer to normal despite persistently elevated $P_{CO_2}$. Which change would most likely restore homeostasis in this compensated state?