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 the capillaries.
• Thin-walled arterial system
(relatively little smooth muscle)
• Anatomical considerations
• Vessels are surrounded by bags of gas and flexible interstitial fluidfilled regions
• No true arterioles??
• Alveolar and extra-alveolar vessels respond to conditions differently
Influences on resistance
•
•
Pulmonary
Circulation
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
• recruitment (primary)
• distension
R
1/P
Pulmonary Vascular Resistance
(cmH2O/L•min-1)
300
Perfusion
200
Increasing
Arterial Pressure
100
Increasing
Venous Pressure
0
10
20
30
Arterial or Venous Pressure
(cm H2O)
40
Influences on resistance
•
Pulmonary
Circulation
Neural/Humoral: all the same players as in the periphery (but
less effective) and a few with more specific actions
•
• Serotonin: Aretrial side vasoconstriction
• Histamine Venular 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
• Lung volume
(cmH2O/L•min-1)
Vascular Resistance
120
100
80
60
50
100
150
Lung Volume (ml)
200
Influences on resistance
Pulmonary
Circulation
• 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.
• PO2:
Local Control Decreased PAO2 causes
vasoconstriction. Advantage: blood is shunted away from
poorly ventilated alveoli.
• Surrounding Pressure
• Alveolar
• Interstitial (<0)
The Starling Resistor
Concept
Pulmonary
Circulation
Arterial (Pa)
Pulmonary
Circulation
Zone 1
PA>Pa>Pv
Alveolar (PA)
Zone 2
Pa>PA>Pv
Distance
Venous (Pv)
Zone 3
Pa>Pv>PA
Blood Flow
Pulmonary Artery
Pulmonary
Circulation
60
mmHg
50
Pulmonary Arterial
Catheter Tracing
40
30
20
10
0
Left Atrial Pressure Tracing
Pulmonary Capillaries
Left
Atrium
Pulmonary Vein