Final answer:
During a warm-up, the respiratory system experiences an elevated respiratory rate, referred to as hyperpnea, which prepares the body for increased oxygen demand by increasing the frequency and depth of breaths.
Step-by-step explanation:
During a warm-up, the correct respiratory system change that occurs is d) Elevated respiratory rate. As exercise begins, our body prepares for increased oxygen demand by increasing ventilation, known as hyperpnea. This involves an increase in both the rate and the depth of breaths, which ensures an ample supply of oxygen to the muscles and the clearing of carbon dioxide. Specifically, the respiratory rate increases to allow more frequent breathing, and the volume of air per breath can also increase, allowing a greater intake of oxygen with each breath. Therefore, the answer d) Elevated respiratory rate is the most accurate for the changes occurring during a warm-up according to the context of exercise physiology.