Adrenaline: KS4

The hormone adrenaline helps prepare the body for action in a crisis. Its effects on the body include sharpening senses, getting muscles ready to work hard, and reducing pain. Its powerful effects make it useful as a drug, including for use in treating severe allergic reactions (called anaphylaxis).

You might sometimes see adrenaline being called epinephrine - that’s just another name for the same hormone. The hormone noradrenaline has a slightly different structure but very similar effects to adrenaline.

What is adrenaline?

Like all hormones, adrenaline is a chemical messenger that travels in the blood, carrying signals through the body. Many cells have receptors that can recognise adrenaline. When they detect it, they change their behaviour in response to its signal. But exactly how they respond will depend on what type of cell they are.

Adrenaline is made and released by the adrenal glands, which sit on top of the kidneys. By hitching a ride in the blood, the adrenaline molecules quickly spread through the body, carrying their message to cells in different organs and tissues.

A side note: the adrenaline molecule has another role as a neurotransmitter. Very small amounts of adrenaline are used to carry signals over tiny gaps between nerve cells. But this article focuses on the much larger amounts of adrenaline which are released into the blood, where adrenaline acts as a hormone. 

What does adrenaline do?

Different cell types react very differently to adrenaline’s signal, with these different responses all working together to create the body’s ‘fight or flight’ response. Hormones are excellent for coordinating this type of all-body response.

The fight or flight response is how we react to sudden stress. Imagine if a bear walked into your room right now. How would you feel? Your heart would start pounding. Your breathing would deepen. The pupils of your eyes would widen. These reactions are all due to a surge of adrenaline in your bloodstream.

Adrenaline gets your body ready for action. This includes preparing for your brain and muscles to do some hard work:

• The liver releases glucose into the blood, providing extra energy
• Breathing increases, providing extra oxygen for respiration, and removing carbon dioxide
• Heart rate increases, speeding up the delivery of resources and removal of waste
• Blood pressure increases, with more blood directed to large muscles
• Sweating increases, ready to cool off heat produced by hard-working muscles
• Changes to our metabolism also help prepare the body for action

Our pupils widen and our hearing is sharpened, helping us focus on the threat. Our brain speeds up its processing, which is why time can sometimes seem to slow down during moments of great stress. Adrenaline also activates brain areas involved in learning and memory, so we can learn from our dramatic experience.

Meanwhile our digestion slows down as the body focuses its resources on more urgent processes.

But perhaps the most extraordinary effect of adrenaline might be how it reduces our ability to feel pain. There are incredible stories of people being badly injured in war but not realising they are hurt until after they had reached safety. Not feeling pain for a short time gives people the ability to focus on escaping the immediate threat, or saving others.

As well as its primary role in the fight or flight response, a smaller amount of adrenaline is released in response to exercise. In this role too it helps muscles get the resources they need … but in a less dramatic way.  

Another hormone, cortisol (view article here), also helps with the fight or flight response. But cortisol is very different from adrenaline because it is also used for every-day signals like telling our bodies when to get ready to wake up.  

How is adrenaline controlled?

Adrenaline is made by the adrenal glands. We have two adrenal glands, one on top of each kidney. They produce and release (secrete) a variety of different hormones into the blood, of which adrenaline is just one. But how do they know when the bear walks into the room?

Many hormone-producing glands are controlled by other hormones. That works really well for hormones like cortisol where their levels need to rise and fall slowly. It takes a while for the hormonal signals to get through, and nothing changes too quickly. But it’s important that our fight or flight response be as quick as possible.

Nerves are the fastest way to send signals across the body. They can’t do adrenaline’s job of reaching cells across the whole body, but they are great for sending signals to specific places. The very first parts of the fight or flight response are set in motion by the nervous system. At the same time, the brain alerts the adrenal glands by sending rapid signals along a series of nerves.

In this way, the adrenal glands can respond to a crisis much more quickly than if the brain had used hormones to communicate with them. The adrenaline they produce takes over control of the fight or flight response, getting more cells involved and keeping things going.

When the crisis is over (“that’s not a bear, it’s Jack in a fur coat!”), the brain stops signalling down the nerves to the adrenal glands. This is another difference between hormonal and nervous signalling from the brain – nerves can stop their signal immediately, while hormones linger in the blood even after glands stop releasing them. This is helpful when stopping adrenaline secretion as adrenaline is a powerful hormone and it’s not healthy to have high levels in the blood for too long.

Emotional stress responses

It’s important for the body to react quickly when there is a life-threatening danger, but sometimes it also reacts in less-helpful situations. This includes moments of emotional stress where fight or flight are not sensible options.

We will all have experienced this type of adrenaline rush – maybe from fear of an exam, or when we are caught doing something we shouldn’t. Our hearts beat faster, we sweat... It feels terrible but it’s simply the body’s natural response preparing us for action.

But what if there is no action?

Adrenaline’s effects can make us feel dizzy or light-headed, with strange changes to our vision. A rush of adrenaline can make us feel restless and irritable.

Normally the effects wear off within an hour, and we return to our normal state. But it can be unpleasant to experience. 

Too much adrenaline

If people produce too much adrenaline too often, it can be bad for their health. High levels of adrenaline are intended only for short-term emergencies. Its effects put strain on the heart and other parts of the body.

• People who suffer from severe long-term stress can develop high blood pressure and headaches. 
• Some people with untreated obstructive sleep apnea (a condition in which breathing stops for short periods while sleeping) may experience high levels of adrenaline each night as they struggle to breathe. It is thought this might be a cause of high blood pressure in such people.
• Very rarely, an adrenal tumour (an uncontrolled growth of cells) can cause overproduction of adrenaline. In this case, surgery is required to remove the tumour. 

Adrenaline in medicine

Adrenaline is a powerful hormone and can be very useful in medical emergencies.

Adrenaline’s ability to constrict blood vessels makes it useful in situations where blood pressure is very low, and it can also be applied to wounds to stop bleeding.  

It is also sometimes used to help restart the heart if it suddenly stops beating.

But the best-known medical use of adrenaline is auto-injectors like Epi-pens. These are used to treat life-threatening allergic reactions (anaphylaxis). Adrenaline helps the situation by reducing swelling to open up the airways, and by raising blood pressure.