What is Flight or Flight
The flight or fight (FoF) mechanism is the innate survival response of an organism when presented with a harmful threat in order to maintain homeostasis.
"The greatest homeostatic disruption possible, that of physical damage or death."
This was first outlined by Cannon (1915) where he stated “conditions in wild life which are likely to involve pain and emotional excitement, i. e., the necessities of fighting or flight” (p. 211).
This report will be looking at the two extremes of the autonomic nervous system (ANS): The sympathetic nervous system (SNS) and its FoF response, and the parasympathetic nervous system (PNS) and its rest and digest response.
Under normal circumstances when an organism is not in an extreme heightened state, the PNS is primarily active. The PNS acts across the whole body keeping it in a low energy state while performing housekeeping activities (Marieb & Hoehn, 2019).
Due to the large number of systems affected by the ANS and its two divisions the focus within this report will be on the effects to the cardiovascular, respiratory, and metabolic systems, for an expanded list see Figure 1.
Rest and Digest - Parasympathetic Nervous System Control
Regarding the cardiovascular system, the PNS lowers heart rate (HR) via the vagus nerve sending inhibitory signals to the sinoatrial (SA), and atrioventricular (AV) nodes in the heart.
For respiratory, the PNS constricts the bronchioles of the lungs due to less oxygen and CO2 exchange taking place. This lowered exchange is in turn due to a lower HR and less energy being used by the body.
Additionally, because the body is using less energy and is in a low stress state, the PNS lowers the basal metabolic rate (BMR) and stimulates the liver to increase glucose uptake from the bloodstream to store as glycogen.
Fight or Flight Activation
A scenario where the FoF response is triggered and the SNS takes over, could be when an individual is out for a walk and suddenly, they see a large black shape moving rapidly in their peripherals, the first stimuli.
This is followed by loud aggressive barking, the second stimuli. These stimuli activate the FoF response which begins with the hypothalamus generating action potentials (APs) that travel down the SNS.
The SNS is designed so that this single action potential activates all targeted systems and organs simultaneously. This can be seen in Figure 1, showing the single sympathetic ganglia connecting to all systems, compared to the PNS which has multiple ganglia required to reach the same systems.
Before exploring the SNS effects on the cardiovascular, respiratory, and metabolic systems it is essential to look at the adrenal glands which are an important system that further enhances the FoF response.
The APs travel to the adrenal medulla causing it to secrete adrenaline and noradrenaline, which then enters the bloodstream.
Figure 1.
Affected systems of the PNS and SNS.
Marieb & Hoehn (11th Ed) Figure 14.3 (p. 566)
Beating Out of Your Chest - FoF Cardiovascular Response
The FoF response on the cardiovascular system begins with the APs traveling through the SNS to the heart, stimulating the SA and AV nodes as well at the heart muscle and coronary arteries directly.
This increases the HR and blood pressure (BP) by causing the heart to beat faster and contract with greater force.
This in turn, increases the rate of which the blood is moving around the body rapidly moving the adrenaline and noradrenaline that has now entered the blood to the receptor sites on cells.
Once the adrenaline and noradrenaline reach the heart it further increases the systolic (period of heart contraction and emptying) blood pressure, lowers the diastolic (period of heart relaxation and filling) BP, and increases HR (Struthers et al. 1983).
Furthermore, the skeletal muscles are now receiving an increased blood supply flooding them with the oxygen and glucose needed for increased cell respiration.
Huff and Puff - FoF Respiratory Response
When the FoF response occurs, the APs generated by the hypothalamus travel through the SNS to the lungs where they innervate the smooth muscle of the bronchioles via the release of noradrenaline.
Noradrenaline is an inhibitory adrenergic hormone that dilates the bronchioles (Hakim & Usmani, 2014) making them wider and allowing for more oxygen to enter the bloodstream, and CO2 to leave the bloodstream.
The circulating adrenaline and noradrenaline in the bloodstream further enhance this dilation and additionally increases breathing rate.
Metabolism to the Moon - FoF Metabolic Response
A number or metabolic effects occur when the FoF response activates. These metabolic effects are designed to provide fuel and increase cellular respiration.
Firstly, when the adrenaline and noradrenaline reach the liver, they bind to the cells and stimulate glycogenolysis which is the process of converting the stored glycogen to glucose.
This glucose then enters the bloodstream and is shuttled to the cells by the increased HR and BP outlined in the cardiovascular section.
Secondly, the adrenaline and noradrenaline bind to fat cells and stimulate lipolysis which is the breakdown of fat cells into fatty acids and glycerol (Frühbeck et al., 2014).
These fatty acids and glycerol then enter the bloodstream as a fuel source in addition to glucose.
Thirdly, the metabolic rate of the body’s cells increases as the adrenaline and noradrenaline bind to the adrenergic receptors on cells, stimulating energy production and operation (Goodsell, 2008).
Lastly, adrenaline and noradrenaline inhibit the release of insulin from the pancreas so that the glucose in the bloodstream is not reabsorbed into the liver before use by the cells.
The effects of the adrenaline and noradrenaline circulation, also known as an adrenaline rush are the acute response to a threatening stimulus.
Return to Normal
Once the stimulus is removed, the body’s systems will return to normal levels within an hour (WebMD, 2021).
If the stimulus is still present but the initial adrenaline rush has passed the hypothalamus, anterior pituitary, and adrenal cortex will continue to keep the body’s systems in an aroused state ready for action, prolonging the effects of the FoF response.
The FoF response is an innate reaction by an organism to protect it from the greatest homeostatic disruption possible, that of physical damage or death.
In order to achieve this, the SNS activates the body’s systems to physically stand and fight or flee.
An adrenaline rush to dial all the body’s systems up, all designed so that the cells needed to fight, flee and survive can operate at peak levels when needed. All of this happens before the person realises the threatening stimuli were from a dog.
References
Cannon, W. B. (1915). Bodily changes in pain, hunger, fear and rage (2nd ed., 1929). New York: D. Appleton & Co.
Frühbeck, G., Méndez-Giménez, L., Fernández-Formoso, J.-A., Fernández, S., & Rodríguez, A. (2014). Regulation of adipocyte lipolysis. Nutrition Research Reviews, 27(1), 63–93. http://doi.org/10.1017/S095442241400002X
Goodsell, D. S. (2008). Molecule of the Month Adrenergic Receptors. https://www.chem.uwec.edu/Chem491_W09/AdrenergicReceptors.pdf
Hakim, A., Usmani, O. S. (2014). Structure of the Lower Respiratory Tract. Reference Module in Biomedical Sciences. https://doi.org/10.1016/B978-0-12-801238-3.00215-4
Marieb, E.N. & Hoehn, K. (2019). Human Anatomy and Physiology. (11th ed.). Pearson.
Struthers, A. D., Reid, J. L., Whitesmith, R., & Rodger, J. C. (1983). Effect of intravenous adrenaline on electrocardiogram, blood pressure, and serum potassium. British heart journal, 49(1), 90–93. https://doi.org/10.1136/hrt.49.1.90
WebMD. (2021). What to Know About an Adrenaline Rush. Retrieved October 6, 2022 from https://www.webmd.com/a-to-z-guides/what-to-know-adrenaline-rush