How does the reserve capacity of organs change during human aging?

During human aging, the reserve capacity of organs undergoes a gradual decline, which significantly impacts the body's ability to adapt to metabolic and physiological stress. This reduction in reserve capacity is primarily driven by two factors: a progressive loss of cells within vital organs such as the heart, kidneys, brain, and lungs, and a decrease in the functional efficiency of the remaining cells.1

Because of this diminished reserve, elderly individuals often require more time to recover from physiological disturbances compared to younger adults. For example, when blood acidity or sugar levels are altered, the regulatory mechanisms in older individuals take significantly longer to return to normal states. Similarly, the body's ability to perform physical activity and manage environmental temperature changes becomes less effective with age. Ultimately, this decline in reserve capacity makes the aging individual more vulnerable to disease and reduces the body's overall capacity to maintain homeostasis under stress.21