🫀 Pulmonary Blood Flow

What are the three circulations supplying the lung?

• Pulmonary circulation: RV → alveoli → LA; low pressure, low resistance, highly compliant; gas exchange.

• Systemic circulation: LV → body → RA; high pressure, nutrient delivery.

• Bronchial circulation: from aorta → nourishes conducting airways, pleura, lymph nodes, pulmonary vessels; drains into pulmonary veins → venous admixture.

Compare pressures in pulmonary vs systemic circulation.

• Pulmonary arteries: 24/9 mmHg (mean ≈ 14)

• Pulmonary capillaries: ≈10.5 mmHg

• Pulmonary veins: ≈9 mmHg

• Systemic arteries: 120/80 mmHg (mean ≈ 90)

• Systemic capillaries: ≈20 mmHg

• Systemic veins: ≈10 mmHg

How is alveolar fluid balance maintained?

• Thin alveolar-capillary membrane (~0.5 µm)

• Tight junctions (Type I & II cells)

• Surfactant ↓ surface tension

• Negative lymphatic pressure + efficient drainage

• Normal filtration ≈ 30 mL/hr into interstitium, not alveoli

Causes of pulmonary edema?

• ↑ permeability (ARDS, toxins)

• ↑ hydrostatic pressure (LV failure, mitral stenosis, IV fluids)

• ↓ interstitial pressure (rapid evacuation of pneumothorax)

• ↓ plasma oncotic pressure (protein loss, nephrotic syndrome)

• Impaired lymph drainage (tumors, ILD)

• Other: high altitude, neurogenic edema, overdose

What are passive adaptive mechanisms of pulmonary circulation?

• Recruitment: open previously unperfused capillaries (exercise).

• Distension: dilate existing vessels.

• Compliance: thin, elastic vessels buffer pressure changes.

Effect of lung volume on resistance?

• ↑ Lung volume (above FRC): alveolar vessel compression → ↑ resistance.

• ↓ Lung volume (below FRC): extra-alveolar compression → ↑ resistance.

• Minimum resistance at FRC.

How do O₂ and CO₂ affect pulmonary vessels vs airways?

• ↓ O₂: pulmonary vasoconstriction (HPV); bronchodilation (minor).

• ↑ O₂: pulmonary vasodilation; bronchoconstriction (minor).

• ↓ CO₂: pulmonary vasoconstriction (minor); bronchoconstriction (strong).

• ↑ CO₂: pulmonary vasodilation (minor, via acidosis); bronchodilation (strong).

Neurohumoral regulation?

• Sympathetic NE: α₁ → vasoconstriction; β₂ → vasodilation (less); β₂ → bronchodilation.

• Parasympathetic ACh: vasodilation via NO (minor); bronchoconstriction via M₃.

• Nitric Oxide: potent vasodilation (cGMP); mild bronchodilation.

What is HPV?

• Local hypoxia → pulmonary vasoconstriction (via K⁺ channel inhibition → depolarization → Ca²⁺ influx).

• Redirects blood away from poorly ventilated alveoli → improves V/Q matching.

• Pathology: chronic hypoxia → pulmonary hypertension, vascular remodeling.

• Contrast: systemic vessels dilate in hypoxia.

What are the lung perfusion zones (upright)

• Zone 1 (apex): PA > Pa > Pv → no flow.

• Zone 2 (mid-lung): Pa > PA > Pv → intermittent flow.

• Zone 3 (base): Pa > Pv > PA → continuous flow.

Factors shifting zones?

• Zone 1 disappears: normal tidal breathing, exercise (↑ CO).

• Zone 1 expands: positive-pressure ventilation (PEEP), hypovolemia, high lung volumes.

• Supine position: perfusion more uniform.

How does gravity affect ventilation?

• Pleural pressure more positive at base → alveoli smaller, more compliant.

• Base alveoli expand more during inspiration → ↑ ventilation.

• Perfusion ↓ more steeply than ventilation → V/Q ratio ↑ from base → apex.

• Base: low V/Q, efficient gas exchange.

• Apex: high V/Q, less efficient.

Normal ABG values: PaCO₂ 35–45 mmHg, PaO₂ 75–100 mmHg.

What happens in healthy individuals during exercise?

↑ pulmonary blood flow (not ↑ resistance).

What occurs during tidal exhalation?

Total pulmonary vascular resistance ↑ (alveolar compression + ↓ traction on extra-alveolar vessels).

Effect of ACh?

Pulmonary vasodilation (via NO); bronchoconstriction (via M₃).

At TLC upright, which is true?

No Zone 1 (Pa > PA).