HPA Axis and Stress: Function & Definition

Instructor: Devin Kowalczyk

Devin has taught psychology and has a master's degree in clinical forensic psychology. He is working on his PhD.

In this lesson, you'll learn about the neurobiological system and mechanisms that initiate a reaction to a perceived threat. We'll discuss how adrenaline works in your body when you are threatened by something, whether it is an aggressive lioness or a fender bender.

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.


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
Hypothalamus-Pituitary Gland

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
Adrenal Glands


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.

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