A student runs a 400 m race. During the race, her muscles begin to respire anaerobically. Describe what happens during anaerobic respiration in human muscle cells and explain why this type of respiration cannot continue indefinitely.

Anaerobic respiration in muscle cells occurs when oxygen is unavailable.
1. Glucose is broken down by glycolysis to pyruvate.
2. Pyruvate is converted to lactic acid (lactate) by lactate dehydrogenase.
3. Only 2 ATP molecules per glucose are produced, far less than the 36‑38 ATP from aerobic respiration.
4. Lactic acid accumulates in the muscle and bloodstream.
5. The build‑up of lactic acid lowers pH, causing fatigue, muscle pain and cramps, so the muscle cannot continue to work efficiently.