Regional Circulations

Coronary Circulation - Heart's Own Fuel

• Origin: Arise from aortic sinuses at aortic root.
  • Left Coronary Artery (LCA): from left posterior aortic sinus.
  • Right Coronary Artery (RCA): from anterior aortic sinus.
• Major Arteries:
  • LCA → Left Anterior Descending (LAD) & Left Circumflex (LCx).
  • RCA → Posterior Descending Artery (PDA) & marginal branches.
• Dominance: Determined by artery supplying PDA (usually RCA ~85%).
• Blood Flow: Primarily during diastole (myocardial compression ↓).
  • Left ventricle: flow mainly in diastole.
  • Right ventricle: flow in both systole & diastole.
• Oxygen Extraction: Highest in body (~70-80% at rest).
  • Coronary venous $O_2$ saturation is lowest (~25-30%).
• Regulation: Primarily by local metabolic factors.
  • Key vasodilator: Adenosine.
  • Others: $NO$, $K^+$, $H^+$, $CO_2$.
• Autoregulation: Maintains flow despite pressure changes (60-180 mmHg).

⭐ Most coronary blood flow to the left ventricle occurs during diastole. This is because during systole, the contracting myocardium compresses the coronary vessels, particularly in the subendocardial region.

• 📌 Mnemonic for RCA branches: SAMP (Sinoatrial nodal, Acute marginal, Atrioventricular nodal, Posterior descending artery).

Cerebral Circulation - Brain's Lifeline

• Normal CBF: 50-55 mL/100g/min (15% of Cardiac Output).
• Cerebral Perfusion Pressure (CPP): $CPP = MAP - ICP$. Normal: 70-90 mmHg.
  • Ischemia if CPP < 50 mmHg; Cell death if < 20 mmHg.
• Autoregulation: Maintains constant CBF between MAP 60-160 mmHg.
  • Mechanisms: Myogenic & Metabolic.
• Key Factors Influencing CBF:
  • $PCO_2$: Most potent cerebral vasodilator. Linear ↑ CBF with ↑ $PCO_2$ (range 20-80 mmHg).
  • $PO_2$: Significant vasodilation when $PO_2$ < 50 mmHg.
  • $H^+$ (pH): Acidosis (↑ $H^+$) → ↑ CBF.
• Monro-Kellie Doctrine: Intracranial volume (Brain + Blood + CSF) is constant.
• Blood-Brain Barrier (BBB): Tight junctions; restricts passage of polar substances.

⭐ Cushing Reflex: Triad of hypertension, bradycardia, and irregular respirations, often due to ↑ Intracranial Pressure (ICP).

📌 CO2, O2, PH, Pressure (COPP) are key CBF regulators (local control).

Pulmonary & Renal Circulations - Lungs & Kidneys Flow

• Pulmonary Circulation:
  • Low pressure (mean PAP ~15 mmHg), low resistance system.
  • Receives 100% of cardiac output.
  • Unique feature: Hypoxic Pulmonary Vasoconstriction (HPV) diverts blood from poorly ventilated alveoli to well-ventilated areas, optimizing V/Q matching.
  • Pulmonary capillary pressure: ~7 mmHg.
• Renal Circulation:
  • High blood flow: receives ~20-25% of cardiac output (RBF).
  • Strong autoregulation: maintains constant RBF & Glomerular Filtration Rate (GFR) over a wide range of mean arterial pressures (MAP ~80-180 mmHg).
    • Mechanisms: Myogenic response & Tubuloglomerular feedback (TGF).
  • Filtration Fraction (FF) = GFR / Renal Plasma Flow (RPF) ≈ 0.2 (20%).

⭐ High-Yield: Unlike systemic circulation where hypoxia causes vasodilation, in pulmonary circulation, hypoxia causes vasoconstriction (HPV). This is vital for matching perfusion to ventilation.

Special Circulations (Muscle, Skin, Splanchnic) - Active & Adaptive Flows

• Skeletal Muscle Circulation:
  • Dual control: Sympathetic nerves (vasoconstriction) & local metabolites (vasodilation).
  • Exercise: Metabolic override (adenosine, $K^+$, lactate, $CO_2$, ↓$O_2$) causes massive ↑ flow up to 20-25x.
  • Functional sympatholysis: Local factors blunt sympathetic vasoconstrictor effects.
  • Muscle pump significantly aids venous return.
• Cutaneous Circulation (Skin):
  • Thermoregulation is primary. Sympathetic nervous system control:
    • Vasoconstriction (α1 receptors, cold).
    • Active vasodilation (ACh → bradykinin, heat stress).
  • Arteriovenous (AV) anastomoses in acral skin aid heat exchange.
  • Triple Response of Lewis: Histamine mediated (Flush, Flare, Wheal).
• Splanchnic Circulation (Gut, Spleen, Pancreas, Liver):
  • Receives ~25% of cardiac output at rest.
  • Postprandial hyperemia: ↑ flow after meals (hormonal, neural, metabolic factors).
  • Strong sympathetic vasoconstriction (α1) diverts blood during stress/exercise.
  • Autoregulatory escape from prolonged sympathetic stimulation occurs.

  ⭐ Splanchnic circulation acts as a major blood reservoir, capable of mobilizing blood during hypovolemia.

High‑Yield Points - ⚡ Biggest Takeaways

• Coronary blood flow is maximal during diastole; adenosine is a key local regulator.
• Cerebral blood flow is tightly autoregulated; PCO2 is its most potent vasodilator.
• Skeletal muscle shows active hyperemia during exercise via local metabolites.
• Renal blood flow has strong autoregulation for GFR; highest flow per gram.
• Pulmonary circulation: low-pressure, low-resistance; unique hypoxic vasoconstriction.
• Splanchnic circulation is a major blood reservoir, modulated by sympathetic nerves.
• Cutaneous circulation is vital for thermoregulation, controlled by sympathetic activity.