Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. When this happens, the cell must divide into smaller cells with favorable surface area/volume ratios, or cease to function.
Well this is more heart rate but they're technically the same thing; it's because people who go over their target heart rate are pushing themselves too much, which can lead to dehydration and dizziness/possible fainting.