HPA Axis and Stress- Function & Definition

Coming Face-to-Face with Adrenaline

Imagine you are standing in the middle of a plain, facing down a lion. You have a wooden spear with a sharp rock on the end. That lion, with its massive claws and razor-sharp teeth, could eat you up in an instant. You feel the surge of adrenaline and lift your spear, ready to fight off the attack. Or, perhaps, you run from the lion. Either way, in the moment, you’re probably not asking yourself, Where did all this adrenaline come from? However, this is an important question that can reveal the secret to humanity’s survival.

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The Neurobiological System

Adrenaline is delivered into your neurobiological system by what is known as the hypothalamic-pituitary-adrenal axis , or the HPA axis . The HPA axis is named after three components: the hypothalamus, the pituitary gland, and the adrenal glands.

The hypothalamus is the master regulator of the body and brain and is considered the control center for the body’s autonomic (a fancy term for automatic) responses. It is located in the center of the brain. If it becomes excited, it releases corticotropin-releasing factor (CRF) to activate the pituitary gland.

The hypothalamus is the master regulator of the body and brain

The pituitary gland , often referred to as the master gland, is responsible for controlling the hormones in the body. This pea-sized part of your brain releases hormones to stimulate other areas of the body. Some of the hormones include gonadotropin (gender/sex stimulating hormone) and growth hormones. The pituitary also releases the adrenocorticotropic hormone to trigger the adrenal glands.

The adrenal glands sit on top of your kidneys, like little blobby hats. The kidneys are used to filter the blood and are in the center of the body. This makes it an ideal location for a hormone that needs to affect the entire body, because all blood passes through the kidneys. The adrenal glands release several hormones: cortisol , aldosterone , and adrenaline (also known as epinephrine ). These hormones then flow through the body, causing the body to adjust heart rate, blood pressure, and breathing rate.

Adrenal glands sit atop the kidneys

Hormones

When adrenaline courses through your body, it activates your body in many ways. It acts as a stimulant to the heart, increasing the amount it pumps. Respiration, or breathing rate, increases to allow for more oxygen. Adrenaline also acts as a vasoconstrictor, narrowing blood vessels to increase blood pressure. Lastly, the narrowing blood vessels shunt blood away from your higher brain functioning, putting the frontal lobe on standby. This causes the blood that would be in your brain to squeeze into the narrowed blood vessels in your muscles. On a side note, this is the reason adrenaline is placed in needles, called EpiPens, for treating extreme allergic reactions.

Non-necessary systems shut down when the HPA axis is activated. Your digestive tract is put on standby, because that burrito you ate earlier will not matter tomorrow if you are dead. This includes stomach digestion, nutrient absorption, and excretion. In extreme life and death situations, your body may expel the end products. Physiologically, your body does not want to exert the energy required to hold in waste, because that energy could be used elsewhere. Also, you may be unable to spit, because saliva is part of the eating processing.

What does all of this mean? Your heart rate increases and you have more blood pumping through your body since it is no longer in your brain. This means your muscles are engorged with blood, giving you resistance to damage, since the blood acts like armor. Your higher thought processes shut down, making you rely on your lower, more basic functions. This is because, when fighting a lion, you likely won’t need to know calculus or how to play chess.

Cortisol, one of the other hormones released by the adrenal gland, increases blood sugar. This gives the organs, now working faster because of the adrenaline, more available energy. The third hormone released, aldosterone, helps in balancing salt. Due to the changes in blood as well as the additional hormones and blood sugar, the salt balance may be shifted. In addition, more salt in the system can increase your blood pressure further.

Bring It All Together

You are in danger, which activates the hypothalamus. The hypothalamus releases CRF. The CRF activates the adrenal glands, positioned in the center of the body on top of the kidneys. The adrenals release adrenaline, cortisol, and aldosterone. Adrenaline, or epinephrine, increases your body’s heart rate, increases blood pressure, and takes blood from your frontal lobe. Cortisol increases your blood sugar, giving your activated organs more energy. Aldosterone helps keep your salt levels in balance. Your body is now activated, giving you more muscle power and greater endurance to fight off an attack.

Acute and Chronic Stress

This system was designed for a brief fight. You either escape from what is threatening you, or you fight it. Such temporary conditions are known as acute stress reactions . In the modern, Western world, acute stressors can include situations like a job interview, skydiving, or a minor car accident. When you live in the modern world, your body does a poor job of adapting to such acute stressors - you can’t attack a potential employer during a job interview or run from a car accident. And when such stressors happen too often and continually activate the HPA axis, they can wear out your heart and muscles and lead to problems like heart attack and mental health disorders.

Stressors that are around for a long time are considered chronic stressors . Due to the increased levels of cortisol, which specifically targets the part of the brain that makes new memories, people with chronic stress can wear down the part of the brain that makes new memories. Additionally, other parts of the brain suffer damage from the constant chemical influxes.