For most humans today, the threat of encountering a wild bear in daily life is quite slim. And yet, the likelihood of experiencing a stress response as if you are evading or defending yourself from a bear remains high. In our bodies, this stress reaction can play out the moment we’re called upon at work to give a presentation, or perhaps during a nerve-racking first date: sweaty palms, bowel discomfort, the sensation of pins and needles pricking the skin. This is the reality of living as social, modern humans with biological systems that have evolved for millions of years. “The human social interaction is the most stressful thing that we do,” says Cliff Summers, a behavioral neuroscientist at South Dakota University. Despite the bodily discomfort, this acute stress response often means your nervous system and brain are functioning properly, carrying out beneficial and vital tasks. Though biologists and neuroscientists are still unraveling the complex interplay of hormones, neurotransmitters and physiological effects informing our behaviors and health, resulting in stress reactions. Acute Stress Response: Stress Hormones to Stress Reactions Stress gets a bad rap as a sort of villain to the human experience. But it’s often a beneficial and productive force for change and survival. For starters, one lens in the research field defines stress as “a state of homeostasis being challenged.” (This idea builds on the work of Hans Selye, who became the father of stress theory in the mid 1900s.) In effect, this version of stress is an element of daily life for most living things seeking security. So, what exactly takes place in the body in the face of stress, anxiety or nervousness when it comes to acute stress response? Read More: What is Anxiety and How Can Worries Overpower Us? Stress Hormones Hormonally speaking, a stressful state — whether involving an unexpected bear, or an angry boss in your office — boosts two key chemicals in the human body: adrenaline and cortisol, according to Summers. Together, these two stress hormones (and some neurotransmitters) coordinate to optimize the flow of blood sugar to the brain, and, in turn, carry out the fight-or-flight mechanics in the physical body. Let’s start with the effects of cortisol. When cortisol gets pumped into the bloodstream, the hormone blocks the uptake of sugar in most cells throughout the body. That leaves an influx of sugar in the bloodstream available for the brain to absorb, ultimately heightening cognitive function to manage a stressful encounter. While this is playing out, the stress reaction in the body also triggers the release of adrenaline (also called epinephrine) from the adrenal glands. The adrenaline hormone floods the bloodstream and is rapidly dispersed to all parts of the body, such as the eyes, heart, blood vessels and airway. The main function of epinephrine, or adrenaline, is to release and help convert reserves of sugar and nutrients stored in cells throughout the muscles and other organs. Stress Reactions The resulting boost of energy is something we can see play out in athletes, such as a runner participating in a marathon, the proverbial mother who displays a feat of strength to rescue her child or the unfortunate hiker who encountered a bear. “You actually have more energy to run away, because adrenaline and cortisol are working to release this energy into your system,” Summers says. It’s also this epinephrine that can prompt some of the localized reactions, such as sweaty hands or stomach pain, that many of us identify as feeling nervous or stressed. In part, this is because adrenal receptors are located in our glands and intestines. Also, when the adrenaline hormone attaches to receptors in the gut, for instance, it stops typical contractions from occurring, which causes discomfort. Read More: Can You Predict a Panic Attack? The Danger of Perpetual Stress In animals and humans alike, the frequency, severity and pace of this stress response engaging can bring about very different long-term effects on the body and mind, according to Summers’ research on mice. “We [tend to] think stress is a bad thing,” Summers says. “As it turns out, experiments that I have done show pretty definitively that having a strong stress response is a good thing, only if it lasts for a short time.” Ideally, the stress response should turn on quickly, and turn off quickly, switching between the sympathetic and parasympathetic nervous systems, respectively. If the stressed, fight-or-flight state stays engaged for prolonged periods of time, it can lead to numerous and severe health consequences. “A high level of stress will make you sick, and kill you,” Summers says.  Read More: How Your Brain’s “Fingerprints” Could Diagnose Disease Probing the Spectrum of Stress  One tenuous aspect of the research is that highly stressful events can inhibit cognitive function, rather than moderately stressful situations that optimize brain performance with elevated cortisol levels. These findings suggest that the balance of brain hormones and bodily response falls within a complicated spectrum that Summers and other researchers are still studying. Emerging research is also illuminating two distinct groups: stress-resilient individuals and stress-sensitive individuals. As one example, Summers says he watches this play out in his classroom during exams. While some students seem to perform better because of test-taking stress, plenty of others do worse under stressful pressure, despite whether or not they studied and learned the material. Other researchers have studied actual stress hormone levels in young medical faculty students preparing for an exam, and they measured cortisol levels up to nine times higher than during a relaxed period.  Summers says the focus of much of today’s research in the field is investigating “What makes some people sensitive to stress? And what makes other people calmer?” The answers to questions pertaining to stress reaction will likely point to key anti-stress tools and practices that could boost the overall well-being of a multitude of humans and animals. Read More: In the Brain, Romantic Love is Basically an Addiction