Cardiovascular Reflexes

Baroreceptor Reflex - Pressure Patrol

• Receptors: Stretch-sensitive mechanoreceptors.
  • Carotid Sinus (afferent: CN IX - Hering's nerve): Responds to ↑ & ↓ BP. More sensitive, operates at lower pressures.
  • Aortic Arch (afferent: CN X): Responds primarily to ↑ BP. Higher threshold pressure.
• Central Integration: Afferents project to Nucleus Tractus Solitarius (NTS) in the medulla.
• Efferent Response & Effect:
  • If BP ↑:
    • NTS excites vagal nuclei (↑ parasympathetic) & inhibits vasomotor center (↓ sympathetic).
    • Result: ↓ Heart Rate, ↓ Contractility, Vasodilation (↓ Total Peripheral Resistance) → ↓BP.
  • If BP ↓:
    • ↓Baroreceptor firing → NTS disinhibition of vasomotor center (↑ sympathetic) & inhibition of vagal nuclei (↓ parasympathetic).
    • Result: ↑ Heart Rate, ↑ Contractility, Vasoconstriction (↑ TPR) → ↑BP.
• Function: Rapid, moment-to-moment negative feedback regulation of arterial BP. Buffers acute BP fluctuations (e.g., posture changes).

⭐ Baroreceptors adapt (reset) to sustained changes in BP (e.g., chronic hypertension) over 1-2 days, making them less effective for long-term BP control.

📌 Mnemonic: "Pressure Patrol" - Baroreceptors Patrol blood Pressure levels, initiating rapid adjustments.

Chemoreceptor Reflex - Gas Gauge Guardians

• Function: Monitors blood chemistry ($P_{O2}$, $P_{CO2}$, pH); modulates cardiorespiratory activity to maintain homeostasis.
• Sensors (Chemoreceptors):
  • Peripheral:
    • Carotid Bodies (carotid bifurcation): Key for hypoxia. Stimuli: ↓ $P_{O2}$ (< 60 mmHg), ↑ $P_{CO2}$, ↑ $[H^+]$. Afferent: CN IX.
    • Aortic Bodies (aortic arch): Stimuli: similar. Afferent: CN X.
  • Central:
    • Medulla oblongata: Main sensor for $CO_2$. Stimuli: ↑ $P_{CO2}$ (via CSF $[H^+]$).
• Response (to Hypoxia/Hypercapnia/Acidosis):
  • ↑ Sympathetic drive → widespread vasoconstriction (except brain/heart), ↑ TPR, ↑ BP.
  • ↑ Respiration (dominant effect, aims to correct gas imbalance).
  • Initial vagal bradycardia, often overridden by lung reflexes or direct sympathetic effects.

⭐ Peripheral chemoreceptors are the only significant reflex mechanism responding to arterial hypoxemia.

Atrial & Low-Pressure Reflexes - Volume Vigilantes

• Receptors: Low-pressure (volume) receptors in atria, pulmonary vessels.
• Stimulus: ↑ Blood volume → atrial stretch.
• Afferent: Vagus (CN X).
• Goal: Regulate ECF volume.
• Responses to ↑ Volume:
  • ↓ ADH → ↑ diuresis.
  • ↑ ANP (atria):
    • Natriuresis, diuresis.
    • Vasodilation.
    • Inhibits RAAS.
  • Renal vasodilation, ↓ renal sympathetic tone → ↑ Na+/water excretion.
  • Bainbridge Reflex: ↑ Heart rate with ↑ atrial filling.

    ⭐ ANP is released from atrial myocytes in response to stretch, promoting vasodilation and excretion of sodium and water.

Special Reflexes & Maneuvers - Situational Savers

• Valsalva Maneuver: Forced expiration. 4 phases impact BP/HR. Tests autonomic function; can terminate SVTs.
• Carotid Sinus Massage (CSM): Stimulates baroreceptors → ↑vagal tone → ↓HR. For PSVT. ⚠️ Check for bruits.
• Oculocardiac Reflex (Aschner's): Eyeball pressure → ↓HR. Afferent: CN V, Efferent: CN X.
• Bainbridge (Atrial) Reflex: ↑Venous return (atrial stretch) → ↑HR.
• Bezold-Jarisch Reflex: LV receptor stimulation (e.g., inferior MI) → Hypotension, Bradycardia, Apnea.

Cushing Reflex Pathway:

⭐ Cushing reflex (Hypertension, Bradycardia, Irregular Respiration) is a late sign of severely ↑ICP, often indicating impending brain herniation.

High‑Yield Points - ⚡ Biggest Takeaways

• Baroreceptors (carotid sinus/aortic arch): Key for short-term BP control; ↑BP → ↓HR, vasodilation.
• Peripheral chemoreceptors: Respond to ↓PaO₂, ↑PaCO₂, ↓pH → ↑BP, ↑ventilation.
• Bainbridge reflex: ↑atrial stretch (↑venous return) → ↑HR.
• Cushing reflex (↑ICP): Triad of hypertension, bradycardia, irregular respiration.
• Bezold-Jarisch reflex: Coronary/ventricular stimuli → hypotension, bradycardia.
• Valsalva maneuver: Tests autonomic function. Phase II: ↓BP, ↑HR. Phase IV: ↑BP, ↓HR.
• Oculocardiac reflex: Globe pressure → bradycardia (CN V afferent, CN X efferent).