Complex sentence: In fact, under anaerobic conditions, muscles generate lactic acid so quickly that pH of the blood passing through the muscles will drop to around 7.2, which causes haemoglobin to begin releasing ~ 10% more oxygen.
Split into simpler sentences: In fact, muscles generate lactic acid so quickly that pH of the blood passing through the muscles will drop to around 7.2. As lactic acid releases its protons, pH decreases, which causes haemoglobin to release ~ 10% more oxygen.