Final answer:
Polly's body will increase her breathing rate and heart rate to supply more oxygen to her muscles during her run. This happens to repay the oxygen debt that is created during intense physical activity, ensuring ATP and other energy substrates are replenished for the muscles to function.
Step-by-step explanation:
When Polly goes on a run, her muscles require more oxygen to support the increased demand for energy. During intense muscle activity, an oxygen debt is created which needs to be repaid to restore adenosine triphosphate (ATP) and creatine phosphate levels, convert lactic acid to pyruvic acid, both in the muscles and in the liver by converting lactic acid back into glucose or glycogen. Polly's body will respond by increasing her breathing and heart rate to supply more oxygen to the muscles. This process is facilitated by myoglobin in muscle cells which stores a small quantity of oxygen to improve efficiency. Over time, aerobic training can increase the circulatory system's efficiency, allowing for more sustained oxygen delivery during prolonged periods of exercise.