The that part of the total capacity of our body’s organs that we do not normally need to use.

The organ reserve is that part of the total capacity of our body’s organs that we do not normally need to use.

Organ reserve capacity will be used when your body receives stress and will cause a disease if it not enough. When people age, these reverse will be reduced. That's why old people are easier to catch a disease. A child also has lower reserve capacity but it will grow as they age.