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. 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. 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. 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 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 a powerful 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
Excess catecholamine secretion produces striking cardiovascular and metabolic effects because adrenaline and noradrenaline act rapidly and systemically. The classic pathological cause is pheochromocytoma, a catecholamine-secreting tumour of the adrenal medulla, in which episodic hormone release leads to sudden surges in sympathetic activity rather than sustained elevation.
Clinically, catecholamine excess presents with a recognisable pattern driven by acute adrenergic stimulation:
episodic or paroxysmal hypertension due to intense vasoconstriction
palpitations, tachycardia, and arrhythmias from heightened cardiac excitability
headaches, sweating, tremor, and anxiety reflecting sympathetic overactivation
impaired glucose regulation caused by increased hepatic glucose output and insulin suppression
Outside of endocrine tumours, chronic sympathetic activation, often associated with prolonged psychological stress or chronic illness, can result in persistently elevated catecholamine tone. Over time, this contributes to hypertension, endothelial dysfunction, arrhythmias, insulin resistance, and increased cardiovascular risk, illustrating how a normally adaptive acute stress response becomes maladaptive when sustained.
Catecholamines also have critical therapeutic applications. Adrenaline is used in cardiac arrest to support coronary and cerebral perfusion, in anaphylaxis to reverse bronchoconstriction and vasodilation, and in severe airway obstruction to promote bronchodilation. Noradrenaline is a cornerstone vasopressor in septic and cardiogenic shock, where its potent α-adrenergic effects restore vascular tone and maintain blood pressure.
Understanding catecholamine physiology is therefore essential not only for recognising disorders of sympathetic overactivity and endocrine tumours, but also for the safe and effective use of these hormones in acute cardiovascular and endocrine emergencies.
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?
What features of pheochromocytoma reflect the physiological actions of catecholamines?