How is oxygen able to diffuse into skeletal muscles during physical activity?

During physical activity, oxygen is able to diffuse from
the capillaries to the muscle cells due to the concentration gradient, of higher oxygen
in the bloodstream, as compared to in the muscle cells. At moderate levels of activity,
the rate of diffusion of oxygen is enough for the mitochondria to meet the needs of the
muscle cells for ATP. As long as aerobic respiration can meet the energy demands of the
muscle fiber, it will continue. However, if the activity rate continues to increase in
the muscle fiber, and the oxygen can no longer diffuse fast enough into the cell and the
mitochondria alone can no longer meet energy needs, anaerobic respiration occurs. The
muscles have a reserve called glycogen, which is a storage product of glucose. In
situations where the mitochondria alone can't meet the energy needs, glycolysis produces
pyruvic acid which can't all be used in aerobic respiration. When it builds up faster
than the mitochondria can use it, levels of pyruvic acid rise in the sarcoplasm.
Eventually, pyruvic acid is converted to lactic acid and the cell gains 2ATP molecules.
This process occurs anaerobically and it adds extra ATP to muscles that are working hard
to supplement their energy needs.