Final answer:
Step 3 is useful for demonstrating the effect of exercise on the demand for oxygen, which in turn activates feedback mechanisms to maintain homeostasis, causing changes in heart rate and breathing to adjust the levels of oxygen and carbon dioxide in the blood.
Step-by-step explanation:
Step 3 of Ed's investigation, which involves having a test subject exercise vigorously for 1 minute, is useful for understanding how feedback mechanisms maintain homeostasis in the human body during physical activity. When a person exercises, the body's demand for oxygen increases, as the cells need more oxygen to produce the energy required for the physical effort. As a result, the autonomic nervous system's sympathetic division becomes active to cope with the increased demand, causing an increase in heart rate and respiration. The heart will tend to beat faster than its homeostatic set point, which without autonomic input would be about 100 beats per minute (bpm). Exercise-induced stress pushes the resting heart rate from approximately 70 bpm to 120-140 bpm during vigorous activity.
Increased respiration to supply the extra oxygen leads to an increase in the removal of carbon dioxide (CO2), which in turn affects the blood pH. A feedback loop involving sensors in the carotid body adjusts the breathing rate and volume to maintain the correct levels of blood gases and pH. Exercise, therefore, is a stimulus that triggers various homeostatic mechanisms, like increased breathing (hyperpnea), to regulate the internal environment despite the external conditions pushing the system away from its set point.