Rest and Digest vs Fight or Flight

Tiger Tiger

Our bodies, when you think about it, have to deal with very different sets of challenges at different times: fending off tigers (or opposing players on the hockey team) is a very different activity than recuperating after a tiger-attack or hockey-blow. So throughout the day (or hour, or minute), our bodies move on a spectrum between two key states: “rest and digest” and “fight or flight”

In the “rest and digest” state, our bodies are being kept well-fed and grounded. The walls of the esophagus, stomach, and intestines to keep on churning. Glands that aid in digestion, like the gallbladder, pancreas, and salivary glands become more active.

Since we’re at rest, we don’t really need to work too hard. Our heart muscles start beating slower and less forcefully. We don’t need to breath so hard, so the pathways leading to our lungs constrict.

In the “fight or flight” state, we are on our toes and ready for battle. At a moments notice, we’re ready to pounce - or quickly retreat. When we say a stressful/high pressured activity gives us a “rush of adrenaline” - we’re actually being literal since adrenaline is a family of chemical signals that can stimulate a flight-or-fight response.

We don’t need digestion so all that gets shut off. The digestive system comes to a stop, the glands decrease in activity. Our heart muscles start beating faster and more forcefully. We need a lot of oxygen so those pathways into the lung dilate. We need to make sure that every organ has enough blood and oxygen. To do this, we clamp down on those low-yield arteries of our periphery, keeping most of the blood in the higher-yield central ones. We also open up the arteries leading to important organs (i.e. brain, heart, lungs) and also leading to our skeletal muscles (so that we can actually move with haste).

And in a funny twist of nature, control over sexual function is split between the two systems. Evolution must have thought it both both exciting and restful.

How does our body “make” us go into the right state of being? Basically, the body has two keys systems, and which of them is relatively more active at any given time determines which state we are in. When the Parasympathetic Nervous System (PNS) is relatively more active we will be in the “rest and digest” state; when the Sympathetic Nervous System (SNS) is relatively more active we will be in “fight or flight”. The PNS and SNS tend to affect our bodies via something called receptors, proteins on cells that look out for external signals (like nerves coming from our brain) and activate various functions in different parts of our bodies.

Since, well, we usually aren’t being chased by tigers, our “rest and digest” phase is usually the dominant system. It’s what keeps us functional, well-energized creatures that can carry out routine functions. But it does not take much for “fight or flight” to take over the reigns from “rest and digest.” In other words, you don’t need a tiger on your trail - you could just be stressed out about an exam, or a first-date, or well, stressed about anything.

How does this relate to various common medical conditions? Let’s find out.

Heart Disease

In heart disease, there is damage (or the potential for damage) to the muscles of the heart. These muscles are responsible for pumping blood to the entire body - including themselves. However, sometimes the arteries that feed the heart muscles become blocked and not enough oxygen reaches them. This can lead to bad things like chest pain, heart attacks, and arrhythmias.

To prevent these things from happening, we make the heart stop working so hard, reducing the amount of oxygen it actually needs. In other words, we want the “rest and digest” phase to be more dominant in our heart - we want to block the effects of the “fight or flight” state. To do this, we need to block the receptors that are responsible for the “fight or flight” response in the heart.

One well-known class of drugs that can block the “fight or flight” response are Beta Blockers. They block a receptor called β1, which are found pretty specifically on heart muscles. When β1 receptors activated, they increase its rate of and the force behind the muscles’ contractions.

When we use beta blockers to block β1 we thereby reduce heart rate and force of contraction. Now, the patient has a lower risk of having an adverse cardiac event.


In asthma, some environmental or stress trigger causes acute inflammation and constriction of the bronchi of the lungs (the tubes that bring in outside air). If you’ve ever seen someone having an asthma attack, it’s pretty frightening since they literally can’t breathe.

This case is almost opposite to heart disease. Here, we want the “fight or flight” state to be more dominant in the lungs and bronchi. When we are being chased by tigers or playing hockey, our bronchi become large and wide open so that our lung can get a lot of air in them. To do this, we need to trigger the receptors that are responsible for the “fight or flight” response in the bronchi and lungs.

If you’ve ever seen someone having an asthma attack, you’ve also probably seen them pull out an inhaler loaded with a life-saving dose of Albuterol and the symptoms vanish. What is this miracle drug? It activates a particular receptor which is found on blood vessels and lung bronchi and dilates them when activated. Therefore, by triggering this receptor, the drug can get the bronchi to rapidly open.

Nasal decongestion

Allergies and respiratory infections suck. The worst symptom for many is the nasal congestion. The blood vessels around the nose dilate and the nearby sinuses fill with fluid.

Luckily, we can reverse this effect. How? This case is similar to Asthma. We want the “fight or flight” state to be more dominant in the blood vessels around our nose. When we are being chased by a tiger, we don’t want all this blood being wasted on a trivial thing like our nose - we want that blood flowing to our vital organs and muscles.

To combat allergies and respiratory infections, we activate a particular receptor found on blood vessels which narrow them them when activated. Therefore, by activating the receptor we can get those vessels back to normal diameter. The relief is almost instantaneous.

Urinary Incontinence

One of the most discomforting disorders for patients is urinary incontinence - the frequent and unstoppable urge to go to the bathroom. It’s often caused by an overactive PNS system, incorrectly telling the bladder that it is full and needs to empty. Therefore, to solve the problem, we want the “rest and digest” state to be less active in the bladder. To make this happen, we block the receptors that are responsible for the “rest and digest” response in our bladder.

But again, thankfully, we can try and block this effect. Medications like Detrol and Vesicare block a certain receptor which is found on the bladder and stimulates emptying. By blocking the function of the receptor, it blocks the end signal of the PNS on the bladder.


I hope that after reading this, you have an appreciation for the dynamic, balanced systems that keep us going day in, day out. And with this understanding, we can also act to modulate specific body systems to solve clinical problems and provide a lot of value to patients with these conditions and others.

Vinayak Venkataraman is a rising second-year medical student at Duke University School of Medicine. He graduated from Princeton in 2011 with a degree in Electrical Engineering and certificate in Bioengineering. After graduating, he worked with Dr. Ray Dorsey at Johns Hopkins, researching the feasibility and value of using web-based videoconferencing (i.e. telemedicine) to expand access to care for patients with Parkinson disease. Outside of medicine, Vinayak is the author of an awesome novella and loves Indian cooking, tennis, and coffee.

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