How our bodies handle stressful situations involves a circuit of chemicals called the hypothalamic-pituitary-adrenal axis (or HPA axis). This axis essentially represents the relationship between your brain and your stress hormones as they interact in what we call a feedback loop, when your body reacts to the information it is receiving.
In the case of this axis, what happens is that stressors cause the hypothalamus (the part of your brain that's the H) to release certain stress hormones, which then stimulate your pituitary gland (that's the P, also in your skull) to release more hormones. That cascade of chemicals signals your adrenal glands (the A, located on top of your kidneys) to release the biggest stress hormones of all - cortisol and epinephrine (otherwise known as adrenaline).
These chemicals are designed to help you. Adrenaline raises your blood pressure and heart rate, and cortisol increases glucose, giving your muscles the energy to move more swiftly and suddenly (this is the fight-or-flight response). That's helpful when a lion is charging (which is a form of acute stress), but not so much so when you're facing more long-term chronic stressors such as a negative job situation or a bad marriage (or even pregnancy, due to the stress it puts on your body over nine months).
Find out more about this book:YOU: Having a Baby: The Owner's Manual to a Happy and Healthy Pregnancy