Answer:
The answer is "homeostasis."
Step-by-step explanation:
Homeostasis is the dynamic state of equilibrium which allows an organism to have a stable internal condition while resisting natural changes in the external environment. Having a stable condition allows the organism to function normally in order to survive. This also allows the body to maintain certain levels of oxygen, pH and core body temperature.
In the case above, although the temperature outside is 31 degrees, John maintains a temperature of 98 degrees, which is higher than the outside environment, in order for his cells, tissues and organs to function normally.
If his body fails to achieve a homeostatic condition, then he'd die.