More subtle and powerful forces can also affect our homeostasis. Because the hypothalmus links our nervous system to our pitutary system, it is a key organ. It can be stimulated by smells, blood-born stimuli, steriod compounds, and input from our nervous sensory sytems.
When external stimuli are encountered, the brain evaluates the perception of the stimuli. Using the nervous system, the body responds instictively and immediatly. The connection to the endocrine system releases a longer-term response.
That response may be to release neurotransmitters (chemical messengers in the blood stream), which are designed for fight/flight.
If you have been living in a lifestyle where the things in your life (job, children, financial obligations, relationships, etc.) are bringing you stress, your body may be responding subconsciously by elevating the levels of adrenaline and cortisol. These powerful chemicals affect our bodies in many, many ways.
They are designed for a good purpose. And when the perceived threat is passed, our bodies are designed to return to peace and homeostasis.