In the stage of general adaption syndrome the body is working at a much increased level of resistance, using resources until the stress ends or the resources run out

The given blank can be filled with Resistance.

According to Selye's theory of stress, during the condition of the stress, the body undergoes the general adaptation syndrome, which occurs in three stages. These three stages are fight or flight, resistance reaction, and exhaustion.

The flight or fight reaction is stimulated by the hypothalamus. It results in the excitement of the sympathetic division of the autonomic nervous system. The resistance reaction is caused by the hormones released by the hypothalamus; this is long lasting and provides ATP molecules for the counter reaction to the stress.  The resistance phase lasts up to the end of the stressor or complete exhaustion of the resources.

As the result of use of ATP and other energy resource from body during the resistance reaction, the body becomes deprived of energy. This causes the exhaustion in the body.  

a physical change is being made.