If C D is accelerated by an increased temperature faster than B C, then C will be depleted. Since C is needed for E, E will no longer be available. If E is critical to life, the organism will die. Another reason for a lethal temperature is that high temperature may actually change the structure of cell membranes and directly cause death. Some organisms can tolerate extensive freezing but most cannot.
Core temperature is what is maintained homeostatically by the organism.
Surface temperature may fluctuate considerably depending upon environmental temperature and the activity level of the organism. Behavior is one major mechanism for regulating heat gain and loss in both ectotherms like lizards and endotherms like humans. The second major mechanism for regulating heat gain and loss in many animals is to control the level of blood flow to the skin and other exposed surfaces of the animal. Heat exchange between the body and the environment takes place by radiation, conduction, convection, and evaporation . The regulation of body temperature in complex animals is a function of the hypothalamus and involves thermoreceptors located in the core and on the surface of the body.