2. Pulmonary Circulation Characteristics Low Resistance /Pressure The specific structure(s) primarily responsible for the majority of control of resistance in the resting state is controversial. Resistance more evenly distributed, but majority of resistance is normally on the arterial side (upstream) of the capillaries. Thin-walled arterial system (relatively little smooth muscle) Anatomical considerations Vessels are surrounded by bags of gas and flexible interstitial fluid- filled regions No true arterioles?? Alveolar and extra-alveolar vessels respond to conditions differently

3. Influences on resistance Neural/Humoral: all the same players as in the periphery (but less effective) and a few with more specific actions Serotonin:Arterial side vasoconstriction Histamine:Venular side vaso(veno)constriction ***venous side can become dominant resistance Intravascular Pressure R 1/P Pulmonary Circulation recruitment (primary) distension

4. Pulmonary Vascular Resistance (cmH 2 O/L min-1) 300 0 100 200 10403020 Arterial or Venous Pressure (cm H 2 O) Increasing Arterial Pressure Increasing Venous Pressure Perfusion

5. Influences on resistance Lung volume Neural/Humoral: all the same players as in the periphery (but less effective) and a few with more specific actions Serotonin:Aretrial side vasoconstriction HistamineVenular side vaso(veno)constriction Intravascular Pressure R 1/P recruitment (primary) Distension ***Thus, cardiac output is considered to be the primary determinant of pulmonary vascular resistance Pulmonary Circulation

6. 120 Vascular Resistance (cmH 2 O/Lmin -1 ) 60 100 80 Lung Volume (ml) 50 200 100150

7. Influences on resistance Lung volume Low Volume: extra-alveolar vessels are smaller, capillaries more open High Volume: extra-alveolar vessels are pulled open; capillaries become defromed by stretch (more oval; higher resistance) * Lung volume effects on extra-alveolar vessels is lost with positive pressure ventilation. PO 2 : Local Control Decreased PAO2 PAO2 causes vasoconstriction. Advantage: blood is shunted away from poorly ventilated alveoli. Surrounding Pressure Alveolar Interstitial (<0) Pulmonary Circulation

8. The Starling Resistor Concept Pulmonary Circulation

9. Arterial (P a ) Venous (P v ) Alveolar (P A ) Zone 1 P A >P a >P v Zone 2 P a >P A >P v Zone 3 P a >P v >P A Blood Flow Distance Pulmonary Circulation

10. Pulmonary Artery Pulmonary Capillaries Pulmonary Vein Left Atrium 0 10 20 30 40 50 60 mmHg Pulmonary Arterial Catheter Tracing Left Atrial Pressure Tracing Pulmonary Circulation