Pulmonary Circulation — Anatomy & Physiology

Anatomy

• Thin walled arteries and veins, contains little smooth muscle  → easily distorted by lung expansion
• Pulmonary arteries (deoxygenated blood) -> pulmonary capillaries (gas exchange) → pulmonary veins (oxygenated blood)

• Pulmonary capillaries are exposed to alveolar pressure and not supported by solid tissue → prone to collapse
• Contains 10% of circulating blood volume – 500mls

Physiology

• General:
  • Low pressure 25/8 MPAP 15mmHg
  • Low resistance 20-120 dynes.sec.cm-5
  • Flow: total of cardiac output 5L/min
• Pulmonary blood volume
  • ~500mls of blood – blood reservoir
  • Varies over course of the respiratory and cardiac cycle and in response to gravity
  • Varies in response to changes in intrathoracic pressure
• Pulmonary blood pressure
  • Normal PA systolic pressure = 18-25 mmHg
  • Normal PA diastolic pressure = 8-15 mmHg
  • Normal mean pulmonary arterial pressure = 9-16 mmHg
  • Capillary pressure is 8-10 mmHg
  • Venous pressure 6-12 mmHg
• Resistance
  • The resistance in the pulmonary circulation can be calculated as follows:
  • PVR = 80 × (mPAP- PAOP)/CO
  • The normal value for PVR is 100-200 dynes/sec/cm-5
  • Approximately 1/10th of systemic circulation
• Regulation of blood flow/resistance
  • West zones
    • In zone 1, PA > Pa > Pv No flow of blood
    • In zone 2, Pa > PA > Pv Resistance to flow is determined by alveolar pressure (Starling resistor effect)
    • In zone 3, Pa > Pv > PA Resistance to flow is determined by venous pressure Venous pooling causes increased distension of pulmonary capillaries
    • In zone 4 Low lung volume causes narrowing of extra-alveolar vessels
  • Hypoxia – pulmonary hypoxic vasoconstriction
  • Lung volumes

• Regional distribution
  • Distension of partially collapsed capillaries 
  • Recruitment of completely collapsed capillaries
  • Local mediators: NO, prostacycline
• Function
  • Gas exchange
  • Filtration of clots and debris
  • Immunological – pulmonary macrophages, IgA production
  • Metabolic – metabolism of drugs, removal of proteases
  • Endocrine – source of ACE

Guo 2021

Answer by Guo