Chapter 31: Ventilation and Perfusion of the Lungs

The chapter establishes that ventilation, defined as the movement of air into and out of the alveoli, and perfusion, the delivery of blood to the pulmonary capillaries, must be properly matched for optimal oxygen uptake and carbon dioxide elimination. Due to gravitational effects on both airflow and blood distribution, ventilation and perfusion are not uniform throughout the lungs; both are greatest in the lower regions and diminish toward the apex, creating regional variations in ventilation-perfusion ratios. The chapter introduces the concept of ideal V/Q matching, where the ratio of ventilation to perfusion remains relatively constant, allowing proportional gas exchange throughout the lung tissue. Conversely, V/Q mismatch occurs when ventilation and perfusion become uncoupled, resulting in two primary pathological scenarios: shunt conditions where blood perfuses unventilated alveoli, and dead space where ventilation occurs without corresponding perfusion. The alveolar gas equation is presented as a quantitative framework linking ventilation, pulmonary blood flow, and the partial pressures of oxygen and carbon dioxide in arterial blood. The chapter explains hypoxic pulmonary vasoconstriction, a homeostatic mechanism by which poorly ventilated lung regions experience local blood vessel constriction, redirecting perfusion toward better-ventilated areas and minimizing the impact of ventilation-perfusion inequality. Multiple clinical conditions are discussed as pathological examples of V/Q disruption, including pulmonary embolism where thromboembolism blocks perfusion, chronic obstructive pulmonary disease and pneumonia where ventilation is compromised, and acute respiratory distress syndrome where both components may be severely affected, all resulting in arterial hypoxemia and impaired gas exchange efficiency.