Chapter 20: Pulmonary Circulation & Ventilation–Perfusion

The text elucidates the inverse relationship between pulmonary vascular resistance and cardiac output, describing how local mechanisms like capillary recruitment and distention allow the lungs to accept increased blood flow during exercise without dangerous increases in pressure. A significant portion of the discussion focuses on the factors influencing vascular resistance, including the biphasic effect of lung volume where resistance is lowest at functional residual capacity, and the critical role of alveolar oxygen tension. Unlike systemic vessels, pulmonary vessels undergo hypoxic vasoconstriction, a protective reflex that optimizes gas exchange by shunting blood away from poorly ventilated regions, though this can lead to pulmonary hypertension and right heart strain in conditions of generalized hypoxia like high altitude or chronic lung disease. The summary details fluid dynamics governed by Starling forces, surface tension, and lymphatic drainage, explaining how disruptions in hydrostatic or oncotic pressure contribute to cardiogenic and noncardiogenic pulmonary edema. Furthermore, the impact of gravity on blood flow distribution is categorized into physiological zones (Zones 1, 2, and 3), establishing that perfusion increases from the apex to the base. This gravitational gradient creates regional variations in the ventilation-perfusion ratio, resulting in physiological mismatches where the apex is relatively overventilated and the base is overperfused. The concept of venous admixture is thoroughly examined through the lens of physiologic shunts—comprising both anatomic shunts and low ventilation-perfusion regions—and is contrasted with physiologic dead space. Finally, clinical integrations illustrate the pathophysiology of pulmonary embolism, the genetic and systemic manifestations of Cystic Fibrosis, and the mechanics of pleural effusion.