Properties of special circulations

• Why does the coronary circulation have high oxygen demand?

  • The heart is highly metabolically active, requiring a continuous oxygen supply.
  • It extracts 70-80% of oxygen from coronary blood at rest (compared to ~25% in systemic circulation).
  • It relies almost entirely on aerobic metabolism (oxidative phosphorylation).
• What are the key special requirements of coronary circulation?

  1. High oxygen delivery due to high demand.
  2. Constant blood flow to match metabolic needs.
  3. Efficient autoregulation to maintain perfusion despite changes in pressure.
  4. Rapid oxygen extraction (~70-80% at rest).
  5. Adaptability to increased workload (e.g., during exercise).
• How does coronary circulation maintain blood flow despite being compressed during systole?

  • Most coronary blood flow occurs in diastole, not systole.
  • The heart compensates with:
  • Diastolic perfusion pressure (aortic pressure in diastole drives flow).
  • Collateral circulation (alternative pathways for blood supply).
  • Metabolic vasodilation (increased adenosine & nitric oxide cause vasodilation).
• How does coronary circulation adapt to increased oxygen demand?

  • Coronary vasodilation via:
  • Adenosine release (a powerful vasodilator).
  • Nitric oxide (NO) production (relaxes vessels).
  • Increased coronary blood flow (~5x increase during exercise).
  • Increased heart rate & contractility lead to enhanced metabolic signals for vasodilation.
• What happens in coronary artery disease (CAD)?

  • Atherosclerosis leads to narrowing of coronary arteries.
  • Reduces oxygen delivery, causing ischemia.
  • Can lead to angina (chest pain) or myocardial infarction (heart attack)
• What are the key special requirements of cutaneous circulation?

  1. Thermoregulation (primary function).
  2. Protection from extreme temperatures (vasoconstriction in cold, vasodilation in heat).
  3. Maintaining skin oxygenation & nutrition.
  4. Ability to rapidly alter blood flow based on environmental changes.
• How does cutaneous circulation regulate body temperature?

  • Heat dissipation (vasodilation):
  • Increased blood flow to the skin.
  • Arteriovenous anastomoses (AVAs) open, allowing heat loss.
  • Controlled by sympathetic cholinergic fibers (release ACh, stimulating sweat glands).
  • Heat conservation (vasoconstriction):
  • Reduced blood flow to the skin.
  • AVAs close to prevent heat loss.
  • Mediated by sympathetic noradrenergic fibers (release norepinephrine).
• What role do arteriovenous anastomoses (AVAs) play in cutaneous circulation?

  • AVAs are direct connections between arterioles & venules, bypassing capillaries.
  • Located in fingertips, toes, ears, nose (areas sensitive to cold).
  • Open in heat → increased skin blood flow → heat loss.
  • Close in cold → reduced skin blood flow → heat retention.
• What is paradoxical cold vasodilation?

  • Initial vasoconstriction in extreme cold conserves heat.
  • If exposure continues, vasodilation occurs (to prevent frostbite).
  • This is due to sympathetic nerve fatigue or local mediator effects (e.g., NO release).
• How does emotional stress affect cutaneous circulation?

  • Blushing: Emotional stimuli cause facial vasodilation via sympathetic withdrawal.
  • Pallor: Fear or stress triggers vasoconstriction via sympathetic activation.
• What are the key structural features of cardiac circulation?

  • Coronary arteries arise from the aortic sinuses.
  • High capillary density (~3000-4000/mm²) to support oxygen demand.
  • Extensive collateral circulation to provide alternative blood supply.
  • Endothelial cells produce nitric oxide (NO) to regulate vasodilation.
  • Low vascular resistance and autoregulation through metabolic demand.
• What is the primary functional adaptation of the coronary circulation?

  • High oxygen extraction (~70-80%) at rest, requiring increased flow for higher demand.
  • Flow primarily occurs during diastole due to systolic compression of coronary vessels.
  • Autoregulation via metabolic (adenosine, CO2, H+, NO) and myogenic mechanisms.
  • Sympathetic stimulation indirectly increases coronary blood flow via metabolic demand.
• Why does myocardial ischemia occur during increased heart rate?

  • Shorter diastolic phase reduces time for coronary perfusion.
  • Increased myocardial oxygen demand with limited supply.
  • Fixed stenosis in coronary arteries (e.g., atherosclerosis) further restricts flow.
• How does coronary blood flow change during exercise?

  • Increased metabolic demand triggers vasodilation (via adenosine, NO).
  • Coronary flow can increase up to 4-5 times resting levels.
  • Sympathetic stimulation increases heart rate and contractility, raising oxygen demand.
• What are the structural adaptations of cutaneous circulation?

  • Arteriovenous anastomoses (AVAs) in non-hairy skin (palms, soles, ears) allow thermoregulation.
  • Dense capillary network in superficial dermis for efficient heat exchange.
  • Dual blood supply: nutritive capillaries and thermoregulatory AV shunts.
• How does the cutaneous circulation function in thermoregulation?

  • Cold exposure: Sympathetic activation constricts AVAs, reducing heat loss.
  • Heat exposure: Sympathetic inhibition dilates AVAs, increasing blood flow for heat dissipation.
  • Sweating enhances evaporative cooling, aided by vasodilation.
• What is the role of the sympathetic nervous system in cutaneous circulation?

  • Adrenergic vasoconstriction (α1 receptors) limits blood flow to conserve heat.
  • Cholinergic vasodilation (via sweating) enhances heat loss.
  • Reflexive vasodilation in response to prolonged ischemia (reactive hyperaemia).
• What is postural vasoconstriction in cutaneous circulation?

  • Standing up triggers sympathetic vasoconstriction to prevent excessive blood pooling.
  • Helps maintain central blood pressure and prevent syncope.
• What is myocardial ischemia, and what causes it?

  • Myocardial ischemia occurs when blood flow to the heart muscle is reduced, leading to inadequate oxygen supply.
  • Causes include atherosclerosis, thrombosis, coronary artery spasm, and increased myocardial oxygen demand.
  • It can lead to angina pectoris and myocardial infarction.
• Why is the heart particularly vulnerable to ischemia?

  • High oxygen demand but limited ability to increase oxygen extraction.
  • Coronary perfusion occurs mainly during diastole; tachycardia reduces diastolic time.
  • No significant collateral circulation in some regions.
• What are the consequences of a myocardial infarction (MI)?

  • Cell death in affected cardiac tissue.
  • Impaired contractility leading to heart failure.
  • Arrhythmias due to electrical instability.
  • Risk of cardiac rupture, aneurysm, or pericarditis.
• What is a stroke?

  Stroke is a sudden loss of brain function due to disrupted cerebral blood flow
• What are the types of stroke?

  • Ischemic stroke: Due to thrombosis, embolism, or systemic hypoperfusion.
  • Haemorrhagic stroke: Due to rupture of cerebral blood vessels, often from hypertension or aneurysm.
• What is a transient ischemic attack (TIA), and why is it significant?

  • A temporary period of neurological dysfunction caused by ischemia without infarction.
  • A warning sign of potential stroke.
  • Symptoms resolve within 24 hours.
• Why is the brain particularly sensitive to hypoxia?

  • High metabolic demand with minimal energy stores.
  • Neurons rely on continuous oxygen and glucose supply.
  • Even brief ischemia can cause irreversible damage.
• What is pulmonary embolism (PE), and why is it dangerous?

  • PE occurs when a blood clot (usually from deep veins) blocks a pulmonary artery.
  • Causes hypoxia, increased pulmonary vascular resistance, and right heart strain.
  • Severe cases lead to sudden cardiac arrest.
• What is pulmonary hypertension, and what are its consequences?

  • Increased pulmonary artery pressure, leading to right ventricular hypertrophy and failure.
  • Can be primary (idiopathic) or secondary (due to lung disease, chronic hypoxia, left heart failure).
  • Symptoms: Dyspnea, fatigue, chest pain.
• What happens in renal artery stenosis?

  • Narrowing of the renal arteries reduces kidney perfusion.
  • Activates the renin-angiotensin-aldosterone system (RAAS), leading to systemic hypertension.
  • Can cause ischemic nephropathy and kidney failure.
• How does shock affect renal circulation?

  • Reduced perfusion causes acute kidney injury (AKI).
  • Prolonged ischemia leads to tubular necrosis.
  • Manifests as oliguria, electrolyte imbalances, and fluid retention.
• What happens in Raynaud’s phenomenon?

  • Exaggerated vasoconstriction of skin arterioles in response to cold or stress.
  • Leads to pallor, cyanosis, and pain in fingers and toes.
  • Can be primary (idiopathic) or secondary (associated with autoimmune diseases).
• How does chronic venous insufficiency affect skin circulation?

  • Poor venous return causes increased capillary pressure, leading to edema and venous ulcers.
  • Risk factors: Varicose veins, DVT, obesity.
• What happens in patent ductus arteriosus (PDA)?

  • The ductus arteriosus fails to close after birth, causing a left-to-right shunt.
  • Leads to pulmonary overcirculation, heart strain, and risk of heart failure.
  • Can be treated with NSAIDs (indomethacin) or surgical closure.
• What is hypoxic ischemic encephalopathy (HIE) in neonates?

  • Brain injury caused by insufficient oxygen during birth.
  • Results from placental insufficiency, cord compression, or perinatal asphyxia.
  • Can lead to cerebral palsy, seizures, and developmental delays.