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Integrated Cardiovascular Responses: Exercise, Posture, Hemorrhage & Valsalva

Name: Integrated Cardiovascular Responses: Exercise, Posture, Hemorrhage & Valsalva
Creator: Veli Bakalov, MD

How reflexes, hormones, and venous pumps defend blood pressure under stress

Cardiovascular·Physiology·6 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Baroreflex: a fall in BP reduces carotid sinus (CN IX) and aortic arch (CN X) firing to the nucleus tractus solitarius, raising sympathetic tone (HR, contractility, TPR, venous tone) and lowering vagal tone.
Dynamic exercise raises HR, SV, CO, SBP, pulse pressure and the a–vO2 difference; TPR falls from metabolic vasodilation, so DBP stays about the same.
Standing pools ~500–700 mL of blood in the legs; orthostatic hypotension is a fall of ≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes.
Orthostatic hypotension without a heart-rate rise points to autonomic failure (diabetes, Parkinson disease, MSA); volume depletion produces brisk tachycardia.
Hemorrhage: tachycardia comes first and pulse pressure narrows; hypotension often waits until ~30% volume loss.
After hemorrhage, RAAS, ADH and transcapillary refill restore volume; hematocrit is normal initially and falls over hours.
Valsalva strain lowers preload and BP (reflex tachycardia; HOCM murmur louder); release causes a BP overshoot and reflex bradycardia.
Cushing triad of hypertension, bradycardia and irregular respirations signals rising ICP; the Bezold–Jarisch reflex causes vasovagal syncope.
Carotid sinus massage raises vagal tone to terminate AVNRT or unmask flutter; hypersensitivity is a ≥3 s pause or ≥50 mmHg SBP fall.

The reflex toolkit

Arterial baroreceptors. Stretch receptors in the carotid sinus and aortic arch fire faster as arterial pressure rises. Carotid sinus signals travel in CN IX and track both rises and falls in BP; aortic arch signals travel in CN X and mainly report rises. Both project to the nucleus tractus solitarius in the medulla. A fall in BP reduces firing, which raises sympathetic outflow (heart rate, contractility, arteriolar constriction, venoconstriction) and withdraws vagal tone. It acts within seconds and is the main short-term defense of BP.

Sensors that drive cardiovascular reflexes

Sensor	Location / nerve	Stimulus	Main role
Carotid sinus baroreceptors	Carotid bifurcation / CN IX	Rises and falls in BP	Beat-to-beat BP control; target of carotid massage
Aortic arch baroreceptors	Aortic arch / CN X	Mainly rises in BP	Buffer pressure surges
Peripheral chemoreceptors	Carotid and aortic bodies	Low PO2, high PCO2, low pH, poor perfusion	Add sympathetic drive in severe hypotension
Low-pressure receptors	Atria and great veins	Central blood volume	Low volume raises sympathetic tone and ADH
Ventricular mechanoreceptors	LV wall / vagal C fibers	Forceful contraction of an underfilled LV	Bezold–Jarisch reflex (vasovagal syncope)

Hormones act more slowly (minutes to days): renin–angiotensin–aldosterone, ADH, and natriuretic peptides.

Dynamic exercise

Cardiac output. Central command and signals from muscle mechanoreceptors and chemoreceptors raise sympathetic outflow and withdraw vagal tone. Heart rate, contractility and stroke volume all rise, and CO increases several-fold, reaching roughly 20 L/min or more in a fit young adult. At high workloads stroke volume levels off, and the continuing rise in heart rate supplies most of the extra output.

Resistance and pressure. Local metabolites dilate arterioles in working muscle, so TPR falls even though splanchnic and renal beds constrict. Because CO rises, SBP and pulse pressure increase. Because TPR falls, DBP stays about the same or dips slightly, and MAP rises only modestly. Isometric (static) exercise differs: sustained contraction compresses muscle vessels, so DBP and MAP rise much more.

Venous return and O2 extraction. The skeletal muscle pump squeezes veins against their valves. Deeper breaths lower intrathoracic pressure (the respiratory pump), and venoconstriction mobilizes stored blood, so venous return keeps up with output. Working muscle extracts more O2: heat, CO2 and H+ shift the O2–hemoglobin curve right. The arteriovenous O2 difference widens from about 5 mL O2/dL at rest to as much as about 15, and mixed venous O2 content falls. Because O2 consumption = CO × a–vO2 difference (Fick), the two gains multiply.

Standing and orthostatic hypotension

Normal response. On standing, about 500–700 mL of blood pools in the leg and splanchnic veins. Venous return, stroke volume and MAP fall briefly. Within seconds the unloaded baroreceptors raise heart rate, contractility, TPR and venous tone, and the leg muscle pump helps. Normally SBP barely changes, DBP rises slightly and heart rate rises modestly. In postural orthostatic tachycardia syndrome (POTS), heart rate rises by at least 30 bpm within 10 minutes of standing without a fall in BP.

Orthostatic hypotension. This is defined as a fall in SBP of at least 20 mmHg or DBP of at least 10 mmHg within 3 minutes of standing. Causes fall into four groups:

Volume depletion: diuretics, vomiting and diarrhea, bleeding.
Autonomic failure: diabetic autonomic neuropathy, Parkinson disease, multiple system atrophy, pure autonomic failure, amyloidosis.
Drugs: α1-blockers such as prazosin and terazosin (first-dose syncope), tricyclic antidepressants, nitrates.
Deconditioning and age: prolonged bed rest and aging blunt baroreflex sensitivity.

Reading the heart rate. With an intact reflex, as in volume depletion, BP falls and heart rate climbs briskly. In neurogenic orthostatic hypotension the efferent sympathetic limb is damaged, so heart rate barely rises. Treatment includes salt and fluids, compression stockings, fludrocortisone, midodrine (an α1 agonist) and droxidopa (a norepinephrine prodrug), all of which risk supine hypertension.

Hemorrhage

Reflex phase. Blood loss lowers mean systemic filling pressure, venous return, CO and MAP. Baroreceptor unloading, reinforced by chemoreceptor stimulation from poor perfusion and acidosis, triggers a sympathetic surge. Tachycardia is the earliest sign. Contractility increases, veins constrict, and arterioles constrict in skin, gut, kidney and resting muscle, while flow to the brain and heart is preserved. Diastolic pressure is held up by vasoconstriction, so pulse pressure narrows, and systolic pressure often holds until about 30% of blood volume is lost.

Hormonal phase and refill. Low renal perfusion and β1 stimulation release renin. The angiotensin II that follows constricts vessels and stimulates aldosterone and thirst. ADH causes V1 vasoconstriction and V2 water retention. Arteriolar constriction also lowers capillary hydrostatic pressure, so interstitial fluid moves into the plasma (transcapillary refill).

SecondsBaroreflex tachycardia, arteriolar constriction and venoconstriction
MinutesRenin, angiotensin II and ADH released
HoursTranscapillary refill and renal Na+ and water retention restore plasma volume
Days to weeksEPO-driven reticulocytosis rebuilds red-cell mass

Hematocrit is normal right after an acute bleed because whole blood was lost; it falls over hours as refill and IV fluids dilute the red cells.

Valsalva and other named reflexes

Valsalva maneuver. Forced expiration against a closed glottis raises intrathoracic pressure. Phase I: BP rises briefly as the aorta is squeezed. Phase II: venous return and preload fall, so stroke volume and BP drop and baroreflex tachycardia and vasoconstriction partly restore BP. The lower preload makes the murmur of hypertrophic obstructive cardiomyopathy louder and moves the mitral valve prolapse click earlier. Phase III: on release, BP dips for a few beats. Phase IV: venous return surges into still-constricted arterioles, BP overshoots baseline, and reflex bradycardia follows; this vagal surge can terminate SVT. A missing overshoot and heart-rate swing indicates autonomic failure. In heart failure, BP stays high throughout the strain (square-wave response) because high filling pressures maintain stroke volume.

Cushing reflex. Rising intracranial pressure compresses cerebral vessels, and the resulting brainstem ischemia triggers massive sympathetic discharge and hypertension, often with a wide pulse pressure. Baroreceptors then produce reflex bradycardia, and brainstem compression causes irregular respirations. Hypertension, bradycardia and irregular breathing together signal impending herniation.

Vasovagal syncope. This is the most common cause of fainting. Triggers include prolonged standing, pain, fear and the sight of blood. An underfilled LV contracting vigorously stimulates ventricular mechanoreceptors (vagal C fibers). The resulting Bezold–Jarisch reflex paradoxically increases vagal output (bradycardia) and withdraws sympathetic tone (vasodilation), so BP falls and the patient faints. Nausea, pallor, sweating and warmth come first, and recovery is rapid once supine. The same reflex explains bradycardia and hypotension in inferior MI. Tilt-table testing reproduces it.

Carotid sinus massage and hypersensitivity. Pressure on the carotid sinus mimics high BP. CN IX firing increases, and a vagal surge slows the SA node and prolongs AV nodal refractoriness. This can terminate AV node–dependent reentry (AVNRT, AVRT) or slow the ventricular rate enough to reveal flutter waves.

Avoid carotid massage in patients with a carotid bruit or a recent stroke or TIA.

In carotid sinus hypersensitivity, usually in older men, turning the head, shaving or a tight collar causes syncope. Massage reproduces it with asystole of at least 3 seconds (cardioinhibitory) and/or an SBP fall of at least 50 mmHg (vasodepressor). The cardioinhibitory form is treated with a pacemaker.