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((Sympathetic Activation:, Renal Response:, CNS Ischemic Control of BP*,…
- Ischemia activates the sympathetic nervous system (SNS).
- Increases release of norepinephrine.
- Reduced renal blood flow triggers renin release.
- Angiotensin II raises BP through vasoconstriction and aldosterone release.
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- Occurs when cerebral blood flow is reduced (hypoperfusion).
- Medullary Vasomotor Center:
- Located in the medulla oblongata, crucial for BP regulation.
- Increased Heart Rate & Contractility:
- SNS activation also increases heart rate (chronotropy) and contractility (inotropy).
- These combined effects increase systemic vascular resistance and cardiac output, raising BP to restore cerebral perfusion.
- BP rise reduces ischemia, providing relief to the brain’s blood supply.
- Location: Carotid sinus, aortic arch
- Stimulus: Changes in blood pressure (stretch of arterial walls)
- Increased BP: Baroreceptors increase firing → Activation of parasympathetic nervous system (PNS) → Decreased heart rate (via vagus nerve) → Vasodilation
- Decreased BP: Baroreceptors decrease firing → Activation of sympathetic nervous system (SNS) → Increased heart rate, contractility, and vasoconstriction (via norepinephrine)
- Effect: Restores BP to normal levels via HR, SV, and vascular tone adjustments.
- Location: Carotid bodies, aortic bodies
- Stimulus: Low O₂, high CO₂, low pH
- Hypoxia, Hypercapnia, or Acidosis: Chemoreceptors signal to medulla → Increase sympathetic output → Vasoconstriction → Increased BP
- Effect: Ensures adequate perfusion of vital organs (especially brain and heart) during oxygen deprivation.
- Stimulus: Stress, pain, sight of blood, or standing for long periods
- Overactivation of parasympathetic response or withdrawal of sympathetic tone
- Effect: Vasodilation, bradycardia → Sudden drop in BP → Decreased cerebral perfusion → Fainting
- Outcome: Protective mechanism to restore upright posture and circulation to the brain.
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- Stimulus: Increased venous return (e.g., exercise, fluid infusion)
- Stretch of atrial walls → Activation of stretch receptors → Increased heart rate (via increased sympathetic activity)
- Effect: Increases heart rate to accommodate increased blood volume and prevent congestion.
- Atrial Blood Flow (Frank-Starling Mechanism)
- Stimulus: Increased blood volume or venous return
- Increased atrial stretch → Increased force of ventricular contraction (due to length-tension relationship of muscle fibers)
- Effect: Ensures that the amount of blood entering the heart matches the amount leaving, stabilizing BP.
- Norepinephrine induces vasoconstriction of peripheral blood vessels.
- Increases total peripheral resistance (TPR).