Pulmonary Circulation Physiology

Pulmonary Circulation Physiology is the study of blood flow through the vessels connecting the right side of the heart to the lungs and back to the left atrium, a circuit specialized for gas exchange that operates at markedly lower pressure and resistance than the systemic circulation while still accommodating the entire cardiac output with each heartbeat.

Structure and Basic Function

The pulmonary circuit

Deoxygenated blood leaves the right ventricle through the pulmonary artery, which divides into branches supplying each lung; within the lungs, these vessels progressively narrow into pulmonary capillaries that surround the alveoli, where gas exchange occurs, before oxygenated blood returns via the pulmonary veins to the left atrium.

A low-pressure, low-resistance circuit

Despite receiving the entire output of the right ventricle — the same volume of blood per minute as the systemic circulation receives from the left ventricle — the pulmonary circulation operates at roughly one-sixth the pressure of the systemic circulation, owing to pulmonary vessels being shorter, wider, and more distensible, and possessing substantially thinner smooth muscle walls than their systemic counterparts.

Pulmonary vascular resistance (PVR) is calculated from the pressure drop between the pulmonary artery (PA) and left atrium (LA) divided by pulmonary blood flow (Q); its low value reflects the specialized, low-resistance design of the pulmonary vasculature.

Recruitment and Distension

Increasing flow without raising pressure

When cardiac output rises, such as during exercise, the pulmonary circulation accommodates the increased flow largely by recruiting previously closed or underperfused capillaries and by distending already-open vessels, rather than by a proportional rise in pressure; this capacity to lower resistance as flow increases is a distinctive feature of the pulmonary vascular bed.

Effect of lung volume on vascular resistance

Pulmonary vascular resistance depends on lung volume in a U-shaped manner: at very low lung volumes, extra-alveolar vessels are compressed by lack of surrounding tissue tension, while at very high lung volumes, capillaries running through alveolar walls are stretched and narrowed by alveolar distension; total pulmonary vascular resistance is generally lowest near the functional residual capacity, the resting lung volume between breaths.

Regional Distribution of Blood Flow

Gravity-dependent distribution

Because pulmonary arterial pressure is low, the effect of gravity on blood flow distribution within the lung is proportionally larger than in the systemic circulation, resulting in greater perfusion of lower lung regions relative to upper regions in an upright posture, a pattern classically described through zones defined by the relationship between alveolar, arterial, and venous pressures at different lung heights.

Hypoxic pulmonary vasoconstriction

Unlike most systemic vascular beds, which dilate in response to local hypoxia, pulmonary arterioles constrict when the alveoli they serve are poorly ventilated and hypoxic; this hypoxic pulmonary vasoconstriction diverts blood flow away from poorly ventilated alveoli toward better-ventilated regions, improving the overall matching of ventilation to perfusion within the lung.

Ventilation-Perfusion Matching

The importance of matching

Effective gas exchange requires that ventilated alveoli also receive adequate blood flow; mismatches between ventilation and perfusion — whether from blocked airways, obstructed vessels, or gravitational and postural effects — reduce the efficiency of oxygen uptake and carbon dioxide elimination, even when total ventilation and total perfusion are each individually adequate.

Physiological consequences of poor matching

Regions with ventilation but no perfusion contribute to alveolar dead space, while regions with perfusion but no ventilation act as a right-to-left shunt, allowing poorly oxygenated blood to reach the systemic circulation; the lung's regional flow control mechanisms, particularly hypoxic vasoconstriction, work continuously to minimize such mismatches.

Why Pulmonary Circulation Physiology Matters

Right ventricular workload

Because the right ventricle is adapted to pump against the low resistance of the healthy pulmonary circulation, conditions that chronically raise pulmonary vascular resistance — such as pulmonary hypertension — impose a workload the right ventricle is poorly suited to sustain, often leading to right ventricular failure if the underlying cause is not addressed.

Clinical assessment of gas exchange

Understanding pulmonary vascular physiology, including regional flow distribution and hypoxic vasoconstriction, underlies the clinical interpretation of blood gas abnormalities and imaging findings in diseases affecting the lung's blood supply, ventilation, or both.