Adrenaline and Noradrenaline: Regulation of Acute Stress Physiology
Adrenaline (epinephrine) and noradrenaline (norepinephrine) are the primary catecholamines responsible for the body’s acute stress response. Released from both the adrenal medulla and sympathetic nerve terminals, these hormones allow the body to react within seconds to physical threat, emotional distress or sudden exertion. They increase cardiovascular output, elevate glucose availability and enhance alertness, producing a coordinated response commonly referred to as “fight-or-flight.” Although related in structure, adrenaline and noradrenaline exert distinct physiological actions that complement one another and together ensure the body can respond effectively to rapidly changing conditions.
What You Need to Know
Adrenaline (epinephrine) and noradrenaline (norepinephrine) are catecholamines that function as the body’s primary rapid-response hormones during acute stress. Both are synthesised from the amino acid tyrosine and stored in vesicles until released by sympathetic stimulation. Although they are closely related, their physiological roles are not identical, and their effects depend on receptor distribution and tissue sensitivity.
Adrenaline is produced predominantly by the adrenal medulla and is released into the circulation as a hormone. It acts mainly on β-adrenergic receptors, producing widespread metabolic and respiratory effects that support rapid energy availability and oxygen delivery. Noradrenaline is released both from sympathetic nerve endings as a neurotransmitter and from the adrenal medulla as a hormone. It has a stronger affinity for α-adrenergic receptors and plays a dominant role in regulating vascular tone and blood pressure.
Together, adrenaline and noradrenaline coordinate the acute stress response by producing complementary effects across multiple organ systems:
cardiovascular system, increasing cardiac output and maintaining arterial pressure
respiratory system, promoting bronchodilation and increased ventilation
metabolism, mobilising glucose and fatty acids for immediate energy use
circulation, redistributing blood flow toward skeletal muscle, heart, and brain
These actions occur within seconds of sympathetic activation and are tightly regulated to ensure that the response is rapid but reversible. Once the stressor resolves, catecholamine levels fall quickly, allowing physiological systems to return to baseline. Through this finely tuned regulation, adrenaline and noradrenaline provide the body with short-term metabolic readiness and cardiovascular stability during physical, emotional, or metabolic stress.
Beyond the Basics
Catecholamine synthesis
The biosynthesis of adrenaline and noradrenaline follows a defined enzymatic pathway beginning with the amino acid tyrosine. Tyrosine is converted to L-DOPA, then to dopamine, which is subsequently converted to noradrenaline within chromaffin cell vesicles (specialised secretory organelles found in the adrenal medulla). In the adrenal medulla, noradrenaline undergoes further conversion to adrenaline via the enzyme phenylethanolamine N-methyltransferase (PNMT).
PNMT expression is strongly upregulated by cortisol, which diffuses from the surrounding adrenal cortex into the medullary tissue. This anatomical and functional coupling ensures that adrenal medullary catecholamine output is integrated with hypothalamic–pituitary–adrenal axis activity, linking rapid sympathetic responses to longer-term glucocorticoid-mediated stress regulation.
Mechanisms of release
Catecholamine release is initiated by direct neural stimulation rather than hormonal signalling (meaning the adrenal medulla is activated directly by the nervous system, rather than hormones circulating in the bloodstream). Preganglionic sympathetic neurons release acetylcholine onto chromaffin cells, causing membrane depolarisation and opening of voltage-gated calcium channels. The resulting calcium influx triggers exocytosis of catecholamine-containing granules, releasing adrenaline and noradrenaline directly into the circulation.
Because catecholamines are released as hormones rather than neurotransmitters in this context, their actions are systemic rather than localised. When a neurotransmitter is released from a neuron, it is released into a synaptic cleft and acts on receptors located very close to the release site. Its effects are therefore localised, specific, and short-lived.
In contrast, when the adrenal medulla releases adrenaline and noradrenaline during the sympathetic stress response, these catecholamines are secreted directly into the bloodstream. The circulation carries them throughout the body, allowing them to bind to adrenergic receptors in multiple organs simultaneously, including the heart, blood vessels, lungs, liver, and skeletal muscle. Their effects are therefore systemic rather than localised. This allows a single neural signal to produce coordinated activation of cardiovascular, respiratory, and metabolic systems, with effects that persist longer than those mediated by synaptic neurotransmission alone.
Distinct actions of adrenaline and noradrenaline
Although adrenaline and noradrenaline are often released together, their physiological effects differ due to variations in receptor affinity and tissue distribution. Adrenaline preferentially activates β-adrenergic receptors, producing increases in heart rate and myocardial contractility, bronchodilation, and rapid mobilisation of glucose and fatty acids. These effects support increased oxygen delivery and energy availability during acute stress.
Noradrenaline has a stronger affinity for α-adrenergic receptors and exerts more pronounced effects on vascular tone. By inducing vasoconstriction, it elevates systemic blood pressure and helps maintain perfusion of vital organs such as the heart and brain. The combined actions of both catecholamines generate a balanced stress response that enhances energy supply while preserving circulatory stability.
Metabolic effects
Catecholamines play a central role in acute metabolic regulation. They stimulate hepatic glycogenolysis (breaking down glycogen into glucose-1-phosphate, which is then converted into glucose) to rapidly increase blood glucose levels and promote lipolysis in adipose tissue, releasing free fatty acids as an alternative energy source. At the same time, they suppress insulin secretion and reduce peripheral glucose uptake, ensuring that glucose remains available for tissues with high and immediate metabolic demand, particularly the brain and myocardium.
These metabolic changes are transient and tightly regulated, allowing rapid adaptation to stress without long-term disruption of energy balance once catecholamine levels fall.
Integration within the stress response
The adrenal medulla functions as an amplifier of sympathetic nervous system activity. During acute stress, catecholamines coordinate simultaneous activation of multiple organ systems by increasing cardiac output, enhancing respiratory capacity, redistributing blood flow toward skeletal muscle, and heightening alertness and cognitive focus.
This integrated neural–endocrine response allows the body to respond rapidly and cohesively to threats, emergencies, or sudden physiological challenges. Once the stressor resolves, rapid catecholamine clearance permits prompt return to baseline, preventing prolonged sympathetic overactivation.
Clinical Connections
Although adrenaline and noradrenaline are naturally released during activation of the sympathetic nervous system, they are also widely used as medications in emergency and critical care settings. Their ability to rapidly influence heart rate, blood pressure, cardiac output, and tissue perfusion makes them valuable in the management of life-threatening conditions. Understanding their physiological effects helps explain both their therapeutic uses and the risks associated with their administration.
Adrenaline (Epinephrine)
Adrenaline acts on both α- and β-adrenergic receptors, producing widespread cardiovascular and respiratory effects. These include increased heart rate, increased cardiac contractility, bronchodilation, and peripheral vasoconstriction. Collectively, these responses improve oxygen delivery to vital organs and help the body respond to acute physiological stress.
Common clinical uses include:
Cardiac arrest – Supports coronary and cerebral perfusion during cardiopulmonary resuscitation
Anaphylaxis – Reverses bronchoconstriction, vasodilation, and airway oedema
Severe asthma or airway obstruction – Promotes bronchodilation and improves airflow
Refractory shock states – Occasionally used when additional cardiovascular support is required
Local vasoconstriction during procedures– In some surgical, dental, and procedural settings, adrenaline may be used locally to constrict blood vessels, helping to minimise bleeding and improve visualisation of the operative field
Because adrenaline increases myocardial workload and oxygen demand, excessive stimulation may result in tachycardia, hypertension, arrhythmias, and myocardial ischaemia. Patients receiving adrenaline therefore require close cardiovascular monitoring.
Noradrenaline (Norepinephrine)
Noradrenaline primarily acts on α-adrenergic receptors, producing potent vasoconstriction and increasing systemic vascular resistance. Compared with adrenaline, it has a much greater effect on vascular tone and a smaller effect on heart rate. By constricting blood vessels, noradrenaline helps maintain arterial blood pressure and preserve perfusion to vital organs during states of circulatory failure.
Common clinical uses include:
Septic shock – First-line vasopressor used to maintain adequate blood pressure and organ perfusion
Cardiogenic shock – Supports blood pressure when cardiac output is compromised
Vasodilatory shock – Restores vascular tone when severe vasodilation causes hypotension
Post-operative or critical care hypotension – Used when fluid resuscitation alone is insufficient
Noradrenaline is typically administered via a central venous catheter, especially when administered at moderate to high doses. If the noradrenaline leaks out of a peripheral vein into the surrounding tissues (extravasation), it can cause intense local vasoconstriction, reducing blood flow to the affected area and potentially resulting in tissue ischaemia, necrosis, and skin injury. Noradrenaline is titrated carefully according to haemodynamic response, with dose changes required minutely at times. Patients often require continuous cardiac monitoring, frequent assessment of tissue perfusion, and invasive blood pressure monitoring through an arterial line.
Catecholamines and Chronic Stress
Catecholamines are also central to the body's physiological response to stress. During acute stress, increased adrenaline and noradrenaline release enhances alertness, increases cardiac output, mobilises glucose stores, and redirects blood flow towards vital organs and skeletal muscle. These changes are adaptive and help prepare the body to respond to immediate threats.
Problems arise when sympathetic activation becomes chronic. Persistent elevations in catecholamine activity can contribute to hypertension, endothelial dysfunction, insulin resistance, sleep disturbance, anxiety, and increased cardiovascular risk. Over time, sustained sympathetic stimulation places additional strain on the cardiovascular system, illustrating how a response designed to promote short-term survival can become detrimental when activated continuously.
Concept Check
How does cortisol exposure influence the adrenal medulla’s ability to produce adrenaline?
Why do adrenaline and noradrenaline produce different physiological effects despite being closely related?
How does catecholamine release alter glucose and fat metabolism during stress?
Why does systemic release of catecholamines produce longer-lasting effects than direct sympathetic nerve stimulation?
How do adrenaline and noradrenaline help maintain perfusion during shock?