Pulmonary Circulation

Outline the anatomy of the pulmonary and bronchial circulations

Describe the physiological features of the pulmonary circulation and its resistance

Understand the differences between the pulmonary and systemic circulation

The pulmonary circulation is:

• A low-pressure, high-flow, high-pulsatility circulation
• Supplied by the pulmonary trunk (pressure 25/8 mmHg), driven by the RV (pressure 25/0 mmHg)
  • Arteries and veins run with the bronchi as far as the terminal bronchioles, dividing at the same points
  • Beyond this, they form a capillary bed so thin it is essentially sheet of flowing blood punctuated by alveoli

The bronchial circulation:

• Arises from the systemic circulation, and supplies blood to the conducting zone of the lung
• A third drains back to the systemic circulation
• The remainder drains into the pulmonary vessels - this is a physiologic shunt
  • Supply to tumours is predominantly from the bronchial circulation (rather than the pulmonary circulation) as these vessels respond to angiogenic factors.

Differences between Pulmonary and Systemic Circulations

Blood Pressure

Pulmonary arterial pressure is 25/8 mmHg (MAP 15 mmHg) compared to 120/80 mmHg (MAP 100 mmHg) in the systemic circulation. This is because the systemic circulation must:

• Regulate flow to different organs at different times
  It therefore contains resistance vessels which allow it to allocate cardiac output accordingly.
• Maintain flow to organs far above the heart

Conversely, the pulmonary circulation must:

• Accept the entirety of cardiac output, with little capacity to regulate flow (hypoxic vasoconstriction being the exception)
• Minimise extravasation of fluid
  • As per Starlings Law, fluid movement out of the capillary is given by the difference in hydrostatic gradients and oncotic gradients
  • The net oncotic gradient is small (but favours reabsorption), however the pulmonary interstitium has no hydrostatic pressure
  • Increased pulmonary capillary pressure therefore causes extravasation of large volumes of fluid

• Consequently, pulmonary vessels are thin walled and contain minimal smooth muscle
• This makes the pulmonary circulation highly compliant - the volume of blood is able to change substantially with minimal change in pressure

Pulmonary Vascular Resistance

Vascular resistance follows Ohms law, i.e.:
$Vascular \ Resistance = {P_{In} - P_{Out} \over Flow }$

• Pulmonary vascular resistance is ~1/10^th that of the systemic circulation
  • This is because the pressure drop across the pulmonary circulation is 10mmHg (MPAP - LAP), ~1/10^th that of the systemic circulation, and flow is the same

Determinants of pulmonary vascular resistance are:

• Pulmonary Artery Pressure
  Increased PAP causes a decrease in PVR. This occurs because:
  • Previously closed pulmonary capillaries are recruited when their critical opening pressure is reached
    This is more important when MPAP is low.
  • Vessels distend at higher pressures
    This is more important when MPAP is high.

• Lung volume Lung volume has a variable effect on PVR.

  • At large lung volumes:
    • Resistance in large extra-alveolar vessels decreases as the vessels are pulled opening by distension of elastic tissues
    • Resistance in small intra-alveolar vessels increases as they are compressed by the high lung volumes
  • At small lung volumes, the reverse occurs

  

• Hypoxic Pulmonary Vasoconstriction
  Low PAO₂ causes a vasoconstriction in the vessels supplying that alveolus, increasing PVR and directing blood to better ventilated alveoli.
  • Low alveolar PO₂ is the primary determinant
  • Low mixed venous PO₂ also contributes
  • Constriction begins when P_AO₂ falls below 100mmHg, and becomes dramatic below 70mmHg
  • This is important in:
    • Foetal circulation
    • Alveolar consolidation
      • Pneumonia
      • Cardiogenic pulmonary oedema
        Raised LVEDP increases pulmonary venous pressures. Basal alveoli are more affected. HPV causes constriction of basal vessels, increasing blood flow to apical alveoli and resulting in upper lobe diversion seen on chest x-ray.
    • High altitude
  • HPV is attenuated by:
    • Elevated LAP
      Greater than 25mmHg.
    • High CO

• Minor factors which affect PVR:
  • Increase PVR:
    • Hypercarbia
    • Hypothermia
    • Acidaemia
    • Pain
  • Decrease PVR:
    • Bronchodilators
    • Volatiles

Response to Substances

Oxygen:

• The pulmonary circulation constricts when PO₂ falls, whilst the systemic circulation dilates

Carbon Dioxide:

• The pulmonary circulation constrictions when PCO₂ rises, whilst the systemic circulation dilates

Distribution of Pulmonary Flow

Gravity has a significant effect on pulmonary blood flow:

• In the upright lung, flow decreases almost linearly with height
• In the supine lung, flow to posterior regions exceeds that of anterior regions
  This occurs due to the low driving pressure of the pulmonary circulation, which means gravity has a much more significant effect on pulmonary blood flow than systemic blood flow.

Due to difference in lung size:

• The right lung receives ~55% of cardiac output
• The left lung receives ~45% of cardiac output

West's Zones

The lung is divided into four zones, based on the relationship between alveolar and vascular pressures:

• West's Zone 1
  In West's Zone 1, PA > Pa > Pv.
  • This should not occur in normal conditions, because a normal pulmonary artery pressure is normally (just) sufficient
    This is because in the upright lung, the hydrostatic pressure difference will be about 30cmH₂O.
  • However, if alveolar pressure is raised (e.g. IPPV), or arterial pressure falls (shock), there may be a region where alveolar pressure exceeds arterial pressure

• West's Zone 2
  In West's Zone 2, Pa > PA > Pv.
  • Here, flow is determined by the arterial-alveolar pressure gradient rather than the arterial-venous gradient
    Alveolar pressure acts as a Starling Resistor, where flow is independent of downstream pressure.

• West's Zone 3
  Occurs when alveolar pressure falls below venous pressure, i.e. Pa > Pv > PA. Flow is dependent on the arterial-venous pressure gradient. Capillary pressure increases along their length, increasing transmural pressure and mean width.

• West's Zone 4
  Occurs at low lung volumes, as extra-alveolar vessels collapse and shunt occurs. The interstitium is acting as a Starling resistor, which can be expressed as: Pa > Pint > Pv > PA.

Hypoxic Pulmonary Vasoconstriction

As discussed above, HPV allows redirection of blood flow from poorly ventilated regions of the lung, and so improve V/Q matching. HPV is relevant in disease states, as well as specific physiologic circumstances:

• At high altitude, the PAO₂ is globally reduced, leading to high pulmonary artery pressures
• In utero, PAO₂ is negligible, and PVR is therefore very high
  This diverts blood from the pulmonary circulation into the left side of the heart via the foramen ovale. When the first breath is taken, pulmonary vessels dilate and the right-to-left shunt is reversed.

References

1. Dunn, PF. Physiology of the Lateral Decubitus Position and One-Lung Ventilation. Thoracic Anaesthesia. Volume 38(1), Winter 2000, pp 25-53.
2. West J. Respiratory Physiology: The Essentials. 9th Edition. Lippincott Williams and Wilkins. 2011.
3. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.
4. Brandis K. The Physiology Viva: Questions & Answers. 2003.