The stress response of the body is somewhat like an airplane readying for take-off. Virtually all systems (eg, the heart and blood vessels, the immune system, the lungs, the digestive system, the sensory organs, and brain) are modified to meet the perceived danger.

External and Internal Stressors
People can experience either external or internal stressors.

• External stressors include adverse physical conditions (such as pain or hot or cold temperatures) or stressful psychological environments (such as poor working conditions or abusive relationships). Humans, like animals, can experience external stressors.  
• Internal stressors can also be physical (infections, inflammation) or psychological. An example of an internal psychological stressor is intense worry about a harmful event that may or may not occur. As far as anyone can tell, internal psychological stressors are rare or absent in most animals except humans.