Answer:
Organ Reserve
Step-by-step explanation:
The ability of the organs of the body to convalesce and get back to their natural physiological state after being subjected to high stress over a period of time is referred to as organ reserve.
As per the clinical evidences, this phenomena correlates with the older individuals and their ability to cope up with the high stresses that they face.
Thus even after being under high stresses due to the incidents in John's life, he still feels active and his body continues to function properly is due to the organ reserve.