A long-distance cyclist completes a gruelling race. During the race, the cyclist's muscles work at high intensity for several hours. Explain how the cyclist's muscle cells obtain energy during the race and why the cyclist's breathing rate increases.

During the race the cyclist’s muscle cells use glucose as the main fuel.
1. Aerobic respiration in the mitochondria oxidises glucose with oxygen to produce ATP, CO₂ and H₂O – the energy for muscle contraction.
2. Because the intensity is very high, the oxygen supply from the blood cannot keep up with the demand, so the cells also carry out anaerobic glycolysis, converting glucose to lactate and producing a small amount of ATP.
3. The lactate produced is eventually converted back to glucose in the liver (Cori cycle) or used by other tissues.
4. To increase the oxygen available for aerobic respiration, the respiratory system raises the breathing rate, delivering more O₂ to the blood.
5. The increased ventilation also removes the excess CO₂ produced by the heightened rate of cellular respiration more rapidly.