Adrenalin is a powerful hormone which your adrenal glands release into your bloodstream in response to stress. This flood of adrenalin creates ideal conditions for a "fight or flight" response. This enables you to physically perform better than you ever could under normal circumstances. Adrenalin has medicinal uses in treatments for cardiac arrest and anaphylactic shock (allergic reactions). Excess adrenalin remaining in your bloodstream in response to constant stress can make you chronically sick.
Endocrine and Nervous Systems
When it perceives a threat, your brain stimulates your sympathetic nervous system to stimulate the endocrine system. The adrenal medulla--the area inside the adrenal glands, which are walnut-shaped structures atop the kidneys--releases a great burst of adrenalin and other stress hormones into your bloodstream. The nervous system immediately becomes desensitised to pain.
Circulatory and Respiratory Systems
The adrenalin rush sends a barrage of oxygen-rich red blood cells to the heart, lungs and muscles to prepare them for quick action. The heart pumps harder to get more blood flowing faster. The bronchioles of the lungs relax, allowing them to move oxygen faster than normal.
The Musculoskeletal System
The muscles contract harder than they do normally, dilating your pupils, causing your hair to stand erect and imbuing your skeletal muscles with sometimes superhuman strength. Emergency workers, such as firefighter Chris Hickman of Ocala, Fla., are a perfect example of how adrenalin creates that burst of strength: when responding to a motor vehicle accident in 2008, Hickman lifted a Chevrolet Blazer a foot off of the ground so that other firefighters could free the driver pinned between the truck and the pavement.
Constant stress can cause your body to hum along at a low-level state of alarm all the time. Your adrenals leak adrenalin and another stress hormone, cortisol, into your blood, causing anxiety, fatigue, weight gain; this may also lead to heart disease and stroke.
Adrenalin to Combat Stress
Engaging regularly in activities that produce an adrenalin rush, including adventure sports such as mountain biking and rock climbing, conditions your body to respond positively to stress. This can happen when adrenalin responses become routine and are associated with positive feelings. Your adrenal glands also learn to produce adrenalin at the right times instead of constantly producing low levels of the stress hormone.