The Physiology of Stress: How the Body "Amps Up" in Stressful Situations

May 30, 2018


Our Stressed Out Society


The American Psychological Association has been measuring stress in America since 2007. Over that time, overall stress levels among adult Americans have continued to rise. In 2015, the average overall stress level of adult respondents was a self-rating of 5.1 on a 10 point scale, where 1 is “little or no stress” and 10 is “a great deal of stress.” This same survey reported that adults consider 3.8 a healthy level of stress, meaning the average American rates his or her level of stress as unhealthy. 24% of adults reported stress levels of 8, 9 or 10, which is considered an extreme level of stress.


Simply put, America is stressed out. As an acupuncturist, I frequently work with patients to reduce their stress as a goal of treatment. Chronic stress can have wide-ranging effects on health, causing stomachaches and other digestive problems, headaches, depression, hypertension and even heart attack. Because stress is so pervasive and strongly affects a person’s health, I wanted to write about the physical mechanism of stress. If we’re going to address the stress epidemic in this country, it’s important for people to understand how stress manifests and how to reduce it. The physiology of stress is a big topic and will be covered in two parts. This first part will describe how nature has crafted the stress response to support survival, emphasizing the “fight or flight” response. The second part will take a look at the endocrine response to stress. In the future, I’ll discuss the health risks associated with stress and provide easy-to-use strategies for reducing stress.


The Evolutionary Stress Response


The human stress response has evolved over millions of years. Homo Sapiens emerged on the planet approximately 300,000 years ago, and behavioral modernity emerged about 50,000 years ago. We inherited our stress response from early ancestors that were fortunate enough to survive prehistoric times. As such, the human stress response is tuned to a much more dangerous and physically challenging environment than our modern world. It is tuned for survival, and the main threat we’re built to respond to is a physical one, such as a predator. It’s no exaggeration to say our standard “fight or flight” response is built to cope with big, scary animals trying to eat us.


Consider this: if you were alive in prehistoric times, out hunting or scavenging for food, and you stumbled across a scary predator like a saber-toothed cat, your physiological stress response would kick into high gear and follow a programmed response millions of years in the making. Unfortunately, this same stress response gets triggered today when we are cut off in traffic, have a disagreement with a colleague or family member, or get frightened by a scary movie. It’s easy to forget that our everyday responses to stress often overreact to the relatively benign challenges of modern life.


To understand what this overreaction looks like, let’s consider what we would need to survive a truly life-threatening situation. What is the body’s optimal response for surviving a violent encounter? First of all, in a dangerous situation, we need to produce lots of energy fast. Whether you end up duking it out with a violent predator or running like there’s no tomorrow (and there may not be), your skeletal muscle is going to need all the energy it can get. Your body’s primary source of energy is sugar, specifically glucose.


Glucose is a simple sugar derived from the digestion of other complex carbohydrates in our food, and it’s the “blood sugar” that provides energy to most cells of the body. Neurons in the brain and nervous system are particularly glucose-dependent. That’s why, if your blood sugar drops too low, you begin to feel faint and light-headed. In the face of danger, our stress response activates several mechanisms that dump lots of glucose into the blood stream, providing fuel to the heart, brain, and skeletal muscle.


Besides glucose, your body needs oxygen to make energy. Oxygen is a key component in the chemical reactions that utilize energy stored in glucose and other important fuels, like fats and other carbohydrates. In the face of an imminent threat, your body increases oxygen levels in the blood to make use of the energy stored in glucose. To do this, the heart and lungs work overtime. The branches of air tubes in the lungs, called bronchioles, dilate to pull more oxygen into the blood, and the heart works harder to move that oxygen-rich blood where it needs to go. In addition to saturating the blood with oxygen and glucose, your heart rate, force of heart contraction, and blood volume all increase to maximize the flow of fuel-rich blood throughout the body.


Besides fuel for energy, what else is needed to survive a dangerous encounter? Response and reaction times are critical, so the mind and reflexes need to be particularly sharp. There’s no time to stop for a prehistoric double-espresso, so mother nature has figured out ways to quickly boost our reflexes. Our vision needs to be sharp, so the eyes change to take in as much light as possible. Finally, it’s important that nonessential functions don’t use up energy while we’re trying to survive the next few minutes. When a significant stressor is encountered, the body puts systems like digestion and immunity into “low-power” mode to ensure they aren’t competing with the heart, brain and skeletal muscle for energy.


To summarize, the main objectives of the stress response are:

  • Increase oxygen and glucose levels in the blood.

  • Increase blood flow to the heart, brain and skeletal muscle.

  • Sharpen reflexes.

  • Optimize vision.

  • Downregulate non-essential functions, like digestion and immunity.

Though the endocrine system performs several of these functions, the sympathetic nervous system is the fastest, most immediate actor when it comes to achieving these goals. Because of this, the sympathetic nervous system is synonymous with the “fight or flight” response, and we’ll dive into a description of this system next.


The Sympathetic Nervous System


The nervous system is really really complex. Like crazy complex. Science has decided to divide the nervous system into a hierarchy to help make sense of its myriad functions and anatomical structures. The sympathetic nervous system is one of two divisions of the autonomic nervous system that send motor commands (as opposed to sensory information) to the internal organs. Nerves of the sympathetic nervous system exit the spinal cord and connect to all the major organs of the gastrointestinal tract, including the stomach, pancreas, spleen, liver, gallbladder, and intestines. But remember, we’re talking about promoting survival in a dicey situation, so sending commands to organs of digestion is a small part of the picture. The sympathetic nervous system also innervates the eyes, heart, lungs, skin, kidneys, adrenals, and other organs.


When activated due to stress or fear, the hypothalamus in the forebrain pulls the fire alarm and  signals all these organ to jump into survival mode. Nerves send signals to the heart to increase heart rate and increase the force of contraction. Nerve signals dilate arteries of working muscle for better blood flow. The pupils dilate for better vision, and the bronchi of the lungs dilate for better respiration. Within seconds, the heart is pumping faster and harder, the muscles are receiving more blood, the eyes are taking in more light, and the lungs are sucking in more oxygen. 


All this has happened, and we haven’t even gotten to effects on digestion yet! Nerve signals to the liver trigger a release of glucose into the bloodstream. All of the digestive organs get a message to go on break, slowing down to keep from stealing energy from the heart, brain and muscles while in crisis mode. 


The sympathetic nervous system also innervates and activates the adrenal glands during a  stressful encounter. Each adrenal gland can be thought of as two distinct glands, the exterior adrenal cortex and the interior adrenal medulla. The adrenal medulla is activated by nerve signals from the sympathetic nervous system to release hormones called catecholamines into the bloodstream. Catecholamines are a category of hormones that include epinephrine, norepinephrine and trace amounts of dopamine. 


If you don’t have a biology or medicine background, you may not have heard of these hormones before, but you’ve probably heard of adrenaline. Adrenaline is another name for epinephrine, which makes up 75% of the catecholamines released by the adrenal medulla. Unless your life has been utterly boring and uneventful, I’m sure you’ve experienced an adrenaline rush before, and you have some sense of how adrenaline affects the body. Specifically, adrenaline and other catecholamines increase alertness and generate energy by signaling the body to release high-energy fuels into the bloodstream, such as lactate, fatty acids, and glucose. 


Catecholamines also affect glucose metabolism. During times of relative peace and calm, the pancreas produces insulin when blood glucose levels are too high, signaling the liver, skeletal muscle and other tissue to pull glucose from the blood and pack it away. The liver also strings together long chains of glucose into the molecule glycogen for long-term storage. However, when triggered by the stress response, catecholamines reverse these processes. Glycogen is broken down into glucose, the liver generates more glucose through gluconeogenesis, and insulin secretion is inhibited, which prevents blood glucose from being stored away for later use. The important thing to remember is that lots of glucose is dumped into the bloodstream through a variety of pathways, and processes that store glucose for later use are put on hold.


At this point in our story, if you don’t have what you need to survive, it’s not your body’s fault. It’s done its job to pump you full of energy and get you ready to duke it out or exercise the better part of valor. But this isn’t the end of the story! The endocrine system has a say in what happens next, not just the nervous system. The endocrine response to stress is far-reaching and, in many ways, longer lasting than the nervous system response. We’ll cover the endocrine response to stress in part two of this series in just a few weeks.

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The Physiology of Stress: How the Body "Amps Up" in Stressful Situations

May 30, 2018

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