106. Pulmonary Circulation and Ventilation-Perfusion Ratio – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Pulmonary circulation is a low-pressure, low-resistance vascular system that receives the entire cardiac output from the right ventricle. Its primary function is efficient gas exchange while maintaining minimal pressure changes. Pulmonary vascular resistance is about one-tenth of systemic vascular resistance.
• Functions of pulmonary circulation:
  • Transports deoxygenated blood to the lungs for oxygenation.
  • Removes carbon dioxide from blood.
  • Acts as a biological filter, trapping small emboli and thrombi.
  • Performs metabolic functions such as converting angiotensin I to angiotensin II by angiotensin-converting enzyme and inactivating bradykinin, serotonin, and prostaglandins.
  • Serves as a blood reservoir containing about 500 mL of blood.
• Special features include thin-walled, highly compliant pulmonary vessels that accommodate increased cardiac output by capillary recruitment and capillary distension, preventing excessive rises in pulmonary arterial pressure during exercise. Mean pulmonary arterial pressure is about 15 mm Hg.
• Pulmonary blood flow is influenced by:
  • Pulmonary vascular resistance
  • Gravity
  • Lung volume
  • Alveolar pressure
  • Arteriovenous pressure gradient
  • Neural, hormonal, and chemical factors.
• Hypoxia causes pulmonary vasoconstriction, a unique response that diverts blood from poorly ventilated alveoli to better-ventilated areas, improving ventilation-perfusion matching. Nitric oxide, prostacyclin, acetylcholine, and adenosine cause vasodilation, whereas endothelin, angiotensin II, serotonin, thromboxane A₂, and norepinephrine cause vasoconstriction.
• Pulmonary edema occurs when excess fluid accumulates in the interstitium and alveoli. Major causes include increased pulmonary capillary hydrostatic pressure (left ventricular failure, mitral stenosis), increased capillary permeability, decreased plasma oncotic pressure, and surfactant deficiency. It causes hypoxemia, decreased lung compliance, increased airway resistance, and respiratory distress. Treatment aims to reduce pulmonary capillary pressure using diuretics, vasodilators, and positive inotropic drugs.
• Fresh-water drowning causes absorption of hypotonic water, leading to hemodilution, hemolysis, and electrolyte disturbances. Salt-water drowning draws fluid into the alveoli, producing pulmonary edema. Severe hypoxemia is the major cause of death in both conditions.
• The ventilation-perfusion (V̇A/Q̇) ratio is the ratio of alveolar ventilation to pulmonary blood flow. Normal average value is 0.8 (alveolar ventilation ≈4 L/min; pulmonary blood flow ≈5 L/min). Efficient gas exchange requires good matching of ventilation and perfusion.
• The V̇A/Q̇ ratio varies within the lung:
  • Apex: High ratio (≈3.0) because ventilation exceeds perfusion.
  • Base: Low ratio (≈0.6) because perfusion increases more than ventilation.
  • These regional differences explain why pulmonary tuberculosis commonly affects the lung apex.
• Low V̇A/Q̇ ratio causes hypoxemia due to inadequate ventilation, whereas high V̇A/Q̇ ratio results from reduced perfusion. Shunts reduce arterial oxygenation:
  • Anatomical shunt: Blood bypasses ventilated alveoli (e.g., right-to-left cardiac shunts).
  • Physiological shunt: Normal bronchial venous blood draining into pulmonary veins (about 2% of cardiac output).
• Bronchial circulation supplies oxygenated blood to the conducting airways and supporting tissues. It accounts for about 1% of cardiac output, and part of its venous blood enters the pulmonary veins, contributing to the normal physiological shunt.