The stress response starts with the amygdala, which acts as a sensor at the base of the brain by vetting every input for possible threats. When it senses danger, it shuts down the entire brain operation (now is not the time for, say, creative thinking) and prepares the body to pool all of its resources for survival, allowing you to react before you even have time to think about what is happening. When constantly, severely stressed, the amygdala can become overly sensitive and hyper-vigilant, making even relatively harmless events (such as a whiny child or a snoring spouse) seem like a threat. That explains those instinctively snappy over-reactions we can all have when feeling strung out.

Amygdala

After picking up on a potential threat to safety, the amygdala sends impulses to an area of the brain called the locus coeruleus, which awakens your whole body to prepare for either fight or flight. “It’s like you took four big lattes and just infused those in your brain,” Sood says. The locus coeruleus triggers the release of a chemical called norepinephrine, which mobilizes sugars from around the body to fuel an effective stress response and activates the sympathetic nervous system.

Locus Coeruleus

Once the amygdala communicates with the locus coeruleus to fuel an immediate stress response, it sends signals to the prefrontal cortex, located in the higher cortical brain. The prefrontal cortex is more rational, and can help to evaluate how real a stressor is — it learns from experience and tells the amygdala when a response is unwarranted. “It’s like your elderly grandpa in your brain,” Sood says. Just as the amygdala can condition itself to become over-vigilant, we can strengthen the prefrontal cortex through mindfulness practices — like meditation — to help avoid overreactions.

Prefrontal Cortex

Ever find you get the sustained attention span to finish a project in the final hour before a deadline? Here’s why: During times of stress, a part of the brain called the anterior cingulate cortex acts like a telephoto lens, allowing you to zoom in on a problem and ignore everything else that’s happening. This area of the brain can help you to detach when necessary to take stock of what needs to be done. This also explains why some people are able to stay strategic and calm during a serious crisis, such as being attacked or fighting in war (and why later people sometimes describe feeling as if the events were unreal or dreamlike when they were actually happening).

Anterior Cingulate Cortex

This is a crucial area of the brain that helps with memory — it doesn’t actually store all the memories, but decides where you will store what. “It’s the director of the orchestra,” Sood says. But when excessive cortisol (a stress hormone) is in your system, it can kill the hippocampus nerve cells, causing memory problems (that’s why you can never find your keys when you’re perpetually frazzled). The good news is that you can get those nerve cells back. What works? Meditation, relaxation, a generally healthy lifestyle and cultivation of compassion, gratitude, forgiveness and acceptance.

Hippocampus

Once the amygdala picks up on a threat, it talks to a part of the brain called the hypothalamus, which in turn releases chemicals telling the pituitary gland, a small gland near the brain, to react. The pituitary gland then tells the adrenal glands, located on top of the kidneys, to pour out steroids (including the stress hormone cortisol) and adrenaline to fuel the sympathetic nervous system and mobilize sugars from various parts of the body to give it more energy.

Adrenal Glands

The cortisol released from the adrenal glands functions mainly to raise glucose levels to energize the body. And that glucose is secreted from the liver.

Liver

The flight or fight reaction pushes blood flow to the muscles, and away from the skin, to prepare you for war. This redistribution of blood flow, depending on the person, can either cause you to look paler or to flush (plus the autonomic nervous system can control the size of small arteries in the skin, causing blushing). This response is helpful when you really need it. But when it’s happening at a low grade all the time in response to chronic stress, it can actually cause the skin to age faster. You might also start perspiring, as your body wants to cool off in case you need to start running. What’s more, stress can aggravate certain skin conditions, such as psoriasis or rashes. And cortisol, the stress hormone, increases oil production and can stimulate acne. For some people, stress triggers a release of histamine in the body, which can then cause an inflammatory response that shows up in the form of hives or rashes.

Skin

There’s a reason you get a “gut feeling.” The intestines are like a second brain, Sood says, packed with nerve cells. In moments of acute stress, gastrointestinal functioning decreases so that the body’s energy goes toward fighting or fleeing instead of digesting food. That means the body doesn’t always properly process the food, which can dis-regulate activity in your gut. And when people are chronically stressed, it can affect the motility of the gastrointestinal system, which might lead to constipation, diarrhea, indigestion, reflux or bloating. Extreme acute stress can cause people to lose control of their bowels, which is where the term scared — well, you know — comes from.

Intestines

Craving a big, fat piece of chocolate cake after a tough day of work? There are several complicated factors that can trigger stress eating. Among them, feeling anxious can cause you to crave chocolate or carbohydrates, both of which will trigger a release of serotonin, a feel-good chemical, in the brain. And those calorie- and fat-packed foods make us feel better, even if it’s just temporary. Some research has also indicated that stress might up the hunger hormones in your body. Even worse? Increased levels of the stress hormone cortisol have been linked to the storage of excess abdominal fat.

Weight Gain

With the activation of the sympathetic nervous system, the body is primed to fight — it can’t tell the difference between a grizzly bear or an annoying boss. And that means your muscles will tense up, as your body prepares itself to either fight the attacker or escape it. That constant tension can aggravate existing muscular conditions, or cause pain all its own. “You need that if you’re going to start running very soon,” Spiegel says. “You don’t need it if you have a bad back and your boss is yelling at you.”

Muscles

The fight or flight response causes muscles to tense up, preparing you to either fight or run. But over time, maintaining constant tension in your neck can cause your head to ache.

Head

With the activation of the sympathetic nervous system, the heart starts pumping out blood faster and harder to the rest of the body, fueling it for action. “Initially the changes are functional and reversible,” Sood says. “But eventually, the changes become structural and irreversible.” Your body will start to think you need to constantly maintain a high blood pressure and the heart muscles can become thicker. Unchecked, chronic stress has been linked to increased risk of heart disease, heart attack and stroke.

Heart

During sleep, the relaxed parasympathetic system is in control and the active sympathetic nervous system gets very quiet. But when you’re acutely stressed, the sympathetic nervous system doesn’t want to shut itself down. “You still want to protect yourself from whatever the danger is,” Spiegel says. Plus, when your brain is hyper-aroused from ruminating over all those daytime anxieties, it tends to be more difficult to drift off to sleep.

Sleep

Because the body is diverting resources to the systems required for immediate survival, the immune system can become suppressed. And not only does that make you susceptible to new viruses (one study found that people who recently experienced a major stressful life event were more likely to catch a cold), but it can also activate existing infections and viruses. Stress can affect the regulation of inflammation, and increased inflammation can, in turn, exacerbate conditions such as asthma, inflammatory bowel disease and rheumatoid arthritis. That inflammatory response also means we’re less able to fight off infections, making symptoms worse.

Immunity

It’s a no-brainer that the last thing we feel after a long, stressful day is sexy. The stress response is a physical one, and when we’re feeling threatened, sex is the least important priority. Stress activates the sympathetic nervous system (read: an uptick in heart rate and blood pressure), while the more relaxing parasympathetic nervous system is involved in sexual arousal. And, at the most basic level, people typically need to feel comfortable and relaxed to be in the mood for sex, not strung-out and distracted.

Libido

When you’re under pressure, the body is diverting all of its resources toward immediate survival, not future goals like reproduction. “Fertility is a long-term evolutionary privilege we have to transmit our genes,” Sood says. “We don’t want to have babies if I don’t know if I’m going to live through the next two minutes.” Not to mention: Stress can really zap sex drive.

Fertility

While the long-perpetuated myth that stress can turn your hair gray is yet to be conclusively proven, we do know that too much stress can stimulate hair loss. Hair naturally cycles through growth and rest phases — but when we’re experiencing acute, severe stress, the body might start to divert resources away from the hair, meaning it can spend a longer amount of time in the resting phase, a condition called telogen effluvium. The result is less hair growth and, at the same time, more hair loss. Chronic stress can also trigger a disorder called trichotillomania, where people feel compelled to pull or twist hair — whether it’s from the scalp, eyebrows, eyelashes or other body parts — until it falls out.

Hair

When we’re stressed out, our muscles tense up to prepare for the fight or flight response — and this includes the muscles in the jaw. Why some people are more likely to tense up in the jaw than others “is less clear, but we commonly ‘clench our teeth’ when stressed, fearful or angry,” Spiegel says. Tension can also trigger teeth-grinding.

Jaw

The body needs more oxygen to fuel the fight or flight response, which can cause us to start breathing more rapidly. And in order to breathe quickly, we take shallow, more superficial breaths instead of fewer, deep ones. Before you know it, you’re short of breath and, in severe cases, maybe even hyperventilating. Consciously taking deep breaths can have the opposite effect by activating the calming parasympathetic nervous system. Stress also predisposes us to inflammation, which can make asthma attacks more likely.

Lungs