Final answer:
The brain and heart are the first to show signs when oxygen delivery to the extremities is insufficient due to their critical need for oxygen, potentially causing severe functional impairment.
Step-by-step explanation:
When O2 delivery to the extremities becomes inadequate, the brain and heart are the first to exhibit signs of distress due to their high demand for oxygen. Ischemia, or reduced blood flow, can lead to hypoxia—decreased supply of oxygen to tissues. This condition is particularly dangerous for the cardiac muscle and brain tissue, as hypoxia in these areas can lead to cell death and severe impairment of function. Tissues exposed to severe cold, like the fingers and toes, may also suffer from frostbite when blood flow is significantly reduced, potentially leading to permanent damage or gangrene.