Physiological considerations

Unlike the systemic circulation, the pulmonary circulation can be characterized as a 'high-flow-low-pressure system', tolerating up to three- to fourfold increases of cardiac output without significant increases in pulmonary artery pressures under physiological conditions. Vertical distribution of pulmonary blood flow is not equal but is influenced by gravitational forces as well as by the relationship between pulmonary artery pressure, pulmonary venous pressure, and alveolar pressure. Most of the blood flow is distributed to the dependent regions of the lung. Exercise redistributes flow to the more apical parts by recruiting vascular space. This homogenizes pulmonary blood flow across the lung, improving ventilation-perfusion distribution and thereby gas exchange.

The pulmonary vascular resistance (PVR) is mathematically defined by cardiac output (CO), pulmonary arterial pressure (PAP), and pulmonary artery wedge pressure (PAWP):

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