Increase in heart rate when standing up from supine position is feedback or feed forward mechanism
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heart rate increase standing up orthostatic response feedback feedforward mechanism
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cardiovascular response posture change supine to standing autonomic nervous system
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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
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cutaneous vasoconstriction skin cooling thermoregulation mechanism
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feedforward feedback thermoregulation cold temperature autonomic response
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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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