Increase in heart rate when standing up from supine position is feedback or feed forward mechanism
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Searching for the physiological mechanism underlying heart rate changes on standing.
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heart rate increase standing up orthostatic response feedback feedforward mechanism
Harrison's Principles of Internal Medicine, Twenty First Edition
Page 697
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cardiovascular response posture change supine to standing autonomic nervous system
Harrison's Principles of Internal Medicine, Twenty First Edition
Page 6764
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The Answer: Feedback Mechanism
The increase in heart rate upon standing from a supine position is a feedback (negative feedback) mechanism.
Why It Is Feedback, Not Feedforward
| Feature | This Scenario |
|---|---|
| Trigger | A detectable change in the body has already occurred |
| Sensor involved | Baroreceptors (carotid sinus, aortic arch) detect the drop |
| Corrective response | Heart rate increases in response to the error signal |
| Goal | Restore blood pressure back toward the set point |
The Sequence of Events
- Standing up → gravity causes 500–1000 mL of blood to pool in the lower extremities, buttocks, and splanchnic circulation.
- Venous return decreases → reduced ventricular filling → cardiac output and blood pressure fall.
- Baroreceptors in the carotid sinus and aortic arch detect this drop in arterial pressure and reduce their firing rate.
- This reduced afferent input reaches the cardiovascular control centres (medulla) and triggers:
- Increased sympathetic outflow → tachycardia + increased contractility
- Decreased vagal (parasympathetic) tone → further heart rate rise
- The net result is increased heart rate + peripheral vasoconstriction → blood pressure is restored.
(Harrison's Principles of Internal Medicine, 21st ed., p. 697)
Key Distinction: Feedback vs. Feedforward
- Feedback: the system waits for an error (BP drop) to be detected, then corrects it. This is what happens here — the heart rate rises because BP has already fallen.
- Feedforward: the system anticipates a disturbance and acts before the error occurs (e.g., the increase in heart rate that begins at the onset of voluntary exercise — driven by central command from the motor cortex, before any metabolic changes have occurred).
Clinical Relevance
In patients with autonomic insufficiency (diabetes, Parkinson's disease), this baroreceptor-mediated feedback loop is impaired — the compensatory tachycardia does not occur, resulting in orthostatic hypotension with symptoms of lightheadedness or syncope on standing (Harrison's, p. 6764).
Then what about vasoconstriction when skin is cooled
Searching for the mechanism of vasoconstriction in response to skin cooling.
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cutaneous vasoconstriction skin cooling thermoregulation mechanism
Harrison's Principles of Internal Medicine, Twenty First Edition
Page 13170
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feedforward feedback thermoregulation cold temperature autonomic response
Harrison's Principles of Internal Medicine, Twenty First Edition
Page 13169
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Vasoconstriction on Skin Cooling: Feedforward Mechanism
This is the classic example of a feedforward (anticipatory) mechanism.
Why It Is Feedforward
| Feature | This Scenario |
|---|---|
| Trigger | Skin thermoreceptors detect cold at the periphery |
| Core temperature at this point | Still normal / unchanged |
| Response | Vasoconstriction occurs before core temperature drops |
| Goal | Prevent the error (core cooling) from ever happening |
The body does not wait for the core (deep body) temperature to fall. Instead, cutaneous cold thermoreceptors send afferent signals to the preoptic anterior hypothalamus, which pre-emptively triggers sympathetic vasoconstriction to conserve heat — all while core temperature remains at set point.
(Harrison's Principles of Internal Medicine, 21st ed., p. 13170)
The Sequence of Events
- Skin is cooled → peripheral cold thermoreceptors activated
- Signal → preoptic anterior hypothalamus (thermoregulatory centre)
- Hypothalamus triggers sympathetic noradrenergic outflow → cutaneous vasoconstriction
- Blood is shunted away from skin → heat loss is minimised
- Core temperature is preserved — the error never occurs
This is the definition of feedforward: the sensor (skin) is not the variable being regulated (core temp), and the response happens before any disturbance to the regulated variable.
Side-by-Side Comparison
| Orthostatic Tachycardia | Cutaneous Vasoconstriction to Cold | |
|---|---|---|
| Mechanism | Feedback | Feedforward |
| Error occurs first? | Yes (BP drops) | No (core temp unchanged) |
| Sensor | Baroreceptors (detect the error) | Skin thermoreceptors (detect the threat) |
| Regulated variable | Blood pressure | Core body temperature |
| Response timing | After the disturbance | Before the disturbance reaches the regulated variable |
Memory Tip
Feedback = fire alarm goes off after the fire starts. Feedforward = you close the windows when you see dark clouds, before the rain enters.
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