Adrenaline or Epinephrine, Adrenal Glands, and Stress Response
The Adrenal Glands
Our adrenal glands have a wide range of important functions. They are located at the back of our abdomen, one on top of each kidney. Each adrenal gland consists of two parts—the outer cortex and the inner medulla—and each part produces a different group of hormones.
One function of adrenal gland hormones is to control the stress response in humans. Adrenal cortex hormones influence the body's response to long term or chronic stress; adrenal medulla hormones determine the body's response to an emergency or acute stress.
Adrenaline (or adrenalin) is a hormone made by the adrenal medulla. It plays a vital role in helping our body deal with emergencies. In the United States, adrenaline is also known as epinephrine. Adrenaline, a very similar hormone called noradrenaline (or norepinephrine), and the sympathetic nervous system control the "fight or flight" response in humans. This response is a set of rapid body changes that help us to either deal with an emergency or escape from the problem.
Adrenal Glands and the Adrenal Cortex
The right adrenal gland (identified from the owner's point of view) is triangular in shape. The left adrenal gland has a semilunar shape.
Hormones Secreted by the Adrenal Glands
The Fight or Flight Response
The flight or flight response is activated when we face a physical emergency such as an earthquake or a mental emergency such as having to write a test. In situations such as these, the adrenal medulla makes a large number of adrenaline and noradrenaline molecules and releases them into the bloodstream.
Adrenaline and noradrenaline have important effects on the body. Many of these serve to increase the blood flow to the muscles and divert blood away from areas which are not so vital during an emergency. Like all cells, muscle cells used oxygen to produce energy from digested food. They get the oxygen and digested food from blood. During an emergency the muscle cells need more oxygen and nutrients so that they can produce extra energy to help a person deal with the situation.
Effects of Adrenaline (Epinephrine) on the Body
Benefit During an Emergency
Increases heart rate
Speeds up blood flow to skeletal muscles
Increases the force of the heartbeat
Increases amount of blood sent to skeletal muscles
Increases blood pressure
Increases amount of blood sent to skeletal muscles
Dilates (expands) blood vessels in the skeletal muscles
Enables muscles to receive more oxygen and nutrients
Constricts (narrows) blood vessels in the skin
Reduces blood flow to skin so that more is available for the skeletal muscles
Reduces blood flow to intestine
Allows more blood to flow to the skeletal muscles
Dilates bronchioles (air passages in the lung)
Allows more oxygen to enter the body
Increases breathing rate
Allows more oxygen to enter the body
Dilates the pupils
Allows more light to enter the eyes for a better view of the emergency
Raises blood glucose level by stimulating glycogen breakdown in the liver
Increase amount of glucose available for the muscles to use in energy production (since glycogen contains stored glucose)
What's the Difference Between Adrenaline and Noradrenaline?
About eighty percent of the hormone molecules released by the adrenal medulla are adrenaline molecules and about twenty percent are noradrenaline molecules. Adrenaline and noradrenaline have very similar molecular structures, but in one part of the molecule adrenaline has a hydrogen atom while noradrenaline has a methyl group (CH3). The two chemicals also have very similar functions when they are released from the adrenal glands. However, noradrenaline acts as a neurotransmitter as well as a hormone. A neurotransmitter is a chemical that controls the passage of nerve impulses from one neuron (nerve cell) to another.
What's the Difference Between a Hormone and a Neurotransmitter?
- A hormone is a chemical that is made by a gland and then secreted into the bloodstream, which transports it around the body. Specific parts of the body respond to the presence of the hormone. These parts are known as the target organs (or the target cells) of the hormone.
Neurotransmitters in the Space Between Neurons
- Neurons don't touch each other. There is a tiny gap between one neuron and the next. Neurotransmitters are chemicals that transmit the nerve impulse through this gap or that influence the transmission of the nerve impulse through the gap. The region where one neuron ends and another begins is called a synapse.
Neurotransmitters in the Space Between a Neuron and a Muscle or Gland
- The word "synapse" also refers to the region where the end of a neuron comes close to an effector (a muscle or a gland). When a nerve impulse reaches the synapse, neurotransmitters cross the tiny gap between the neuron and the effector and stimulate the activity of the effector.
A Nerve Impulse (or Action Potential) at a Synapse
Sympathetic Neurons and the Adrenal Medulla in Emergencies
Sympathetic Neurons and Fight or Flight
Some sympathetic neurons can trigger the flight or fight response on their own without stimulating the adrenal gland. They release noradrenaline as a neurotransmitter. The chemical travels through the tiny gap between the neuron and the target cell. It then joins to special receptor molecules on the cell membrane of the target cell, which are called adrenergic receptors, and triggers fight or flight reactions.
The Adrenal Medulla and Fight or Flight
Other sympathetic neurons stimulate the adrenal medulla to release noradrenaline and adrenaline into the bloodstream. The hormones then travel around the body in the blood. They trigger fight or flight reactions after binding to adrenergic receptors on target cells.
Neurons in the Sympathetic Nervous System
A sympathetic neuron actually consists of two shorter neurons, one leading to the other. The long extension of each neuron is often known as a fiber. A structure called a ganglion is located between the fibers of the two neurons, as shown in the illustration below.
The fiber of the first neuron is called the preganglionic fiber. It releases a neurotransmitter called acetylcholine. This chemical causes the stimulation of the second neuron and its fiber, which is known as the postganglionic fiber. The postganglionic fiber sends the nerve impulse to an effector. The fiber stimulates the effector by releasing noradrenaline as a neurotransmitter.
The preganglionic fibers are colored orange in the above illustration. The postganglionic fibers are colored yellow. The illustration shows that the fiber going to the adrenal medulla is a preganglionic one.
Noradrenalin Is a Neurotransmitter and a Hormone
It might seem puzzling that noradrenaline can be released by both sympathetic neurons as a neurotransmitter and by the adrenal gland as a hormone. The noradrenaline has the same structure and function in each case, so to some people it may seem as though the body has developed two different ways to perform the same task.
The explanation is that the body has really developed only one way to produce noradrenaline, since adrenal medulla cells are modified neurons of the sympathetic nervous system. A sympathetic neuron that goes to the adrenal medulla contains a preganglionic fiber. This releases acetylcholine, which stimulates a postglanglionic fiber in the adrenal medulla. The adrenal medulla fiber then releases noradrenaline (and adrenaline). The noradrenaline is equivalent to the noradrenaline released by sympathetic neurons, but since it's released into the bloodstream it's called a hormone instead of a neurotransmitter.
What Is Anaphylaxis?
Adrenaline or Epinephrine and Anaphylaxis Treatment
People with a serious allergy may carry an adrenaline/epinephrine auto-injector around with them. During a severe allergic response, life-threatening changes may take place in the body very soon after the person is exposed to the allergen—even within seconds.
Symptoms of anaphylaxis may include wheezing and difficulty in breathing. Blood pressure often falls to a dangerously low level as blood vessels dilate and become "leaky", losing fluid to their surroundings. The patient may also feel faint and experience sweating, swelling of the face or throat, hives, itching, stomach cramps, nausea, vomiting, diarrhea, and an irregular heartbeat.
Some automatic injection devices are called adrenaline injectors while others are called epinephrine injectors. They both contain the same chemical and can be injected through clothing. The general procedure is to remove the cap or safety release, position the injector over the outer thigh, and then press the device into the thigh muscle. The injector must be left in place for about ten seconds. This enables a pre-measured dose of adrenaline to enter the person's body. The instructions for a specific injector may vary slightly, however. They should be read carefully before an emergency happens.
Common brands of auto-injectors are the EpiPen®, the Twinject®, and the Anapen®. It's very important that a person who has an adrenaline injector knows how to use their specific device. People who regularly come into contact with the person, such as family members and teachers, should also know how to use the device.
How to Use an Epipen
Effects of Adrenaline on Anaphylaxis
The adrenaline from an auto-injector expands constricted air passages by relaxing the muscles around them, allowing the patient to obtain oxygen. It narrows blood vessels in many parts of the body and stimulates the heart to beat more strongly, helping blood pressure to rise. It also relieves swelling and itching. Even if the injected adrenaline seems to have removed the symptoms of anaphylaxis, however, the affected person must go to hospital after the injection.
Understanding Adrenaline's Functions
The behavior of adrenaline is complex. Studying its functions can be confusing because the hormone has different effects in different parts of the body. For example, it causes most blood vessels in the body to constrict, but it causes those in the skeletal muscles to dilate. Adrenaline is an important hormone, so it's worth trying to understand its roles in the body.
© 2012 Linda Crampton