What is the functional anatomy of the lung?

Functional Anatomy of the Lung

The functional anatomy of the lung is characterized by a complex three-dimensional structure designed to optimize gas exchange through an extensive alveolar surface area, with only 10-15% of lung volume consisting of tissue and blood, while the remainder is air space. 1

Basic Structural Organization

• The lung connects a small entrance (trachea, cross-section 2.5 cm²) to a massive alveolar gas exchange surface (approximately 100 m² in humans) through sequential, irregular, dichotomous branching patterns 1
• Two functionally distinct airway regions exist: conducting airways with multilayered walls containing mucous membrane, smooth muscle, and cartilage; and acinar airways intimately associated with gas-exchanging alveoli 1
• The gas exchange apparatus forms a sleeve of alveoli on the surface of approximately eight generations of the most distal airways 1

Bronchovascular Hierarchy

• The airway tree follows a dichotomous branching pattern with the trachea as generation 0, where the number of branches in generation Z equals 2^Z 1
• Pulmonary arteries follow airways in a similar branching pattern but include additional "supernumerary" branches at nearly all levels to perfuse nearby parenchyma 1
• Pulmonary arteries branch over approximately five more generations than airways before reaching capillaries 1
• Pulmonary veins course independently of airways in intermediate positions related to interlobular septa, converging on the left atrium in four main stems 1

Alveolar Structure and Gas Exchange

• The air-blood barrier consists of alveolar epithelium, capillary endothelium, and their shared basement membrane, with the harmonic mean barrier thickness (th) being a critical measure of diffusion resistance 1
• Alveolar O₂ uptake occurs in two major steps: diffusion across the membrane barrier and binding to capillary hemoglobin, with each step imposing specific resistances 1
• The lung diffusing capacity for oxygen (DLO₂) is determined by the joint contribution of alveolar capillary blood volume (Vc), intra-acinar alveolar and capillary surfaces, and the harmonic mean air-blood barrier thickness 1
• Morphometric DLO₂ provides a meaningful estimation of the structural capacity for alveolar O₂ diffusion, which exceeds physiologic DLO₂ at rest but approaches it during peak exercise 1

Ventilation-Perfusion Relationships

• Gas exchange efficiency depends on three fundamental features: parenchymal architecture, transpulmonary pressure (prestress), and mechanical properties of the parenchyma 2
• For each gas-exchanging unit, alveolar and effluent blood partial pressures of oxygen and carbon dioxide are determined by the ratio of alveolar ventilation to blood flow (V'A/Q') 3
• Shunt and low V'A/Q' regions represent ventilation-perfusion mismatch and are the most frequent causes of hypoxemia 3
• Gas-exchanging units with little or no blood flow (high V'A/Q' regions) result in alveolar dead space and increased wasted ventilation, reducing carbon dioxide removal efficiency 3

Mechanical Properties

• The alveoli are held open by transpulmonary pressure (prestress), which is balanced by tissue forces and alveolar surface film forces 2
• The main pathway for stress transmission is through the extracellular matrix, making the mechanical properties of elastin, collagen, and proteoglycans key determinants of lung function 2
• Surface tension and the contractile state of adherent cells also influence the macroscopic mechanical properties of the lung 2

Functional Challenges and Adaptations

• The lung must accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several-fold from rest to exercise 4
• Intricate regulatory mechanisms ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin meet metabolic demands 4
• Erythrocytes should be considered an integral component of the gas exchanger, not merely passive carriers 4
• The lung demonstrates structural plasticity in response to challenges such as alveolar hypoxia or loss of lung units 4

Understanding the functional anatomy of the lung is essential for interpreting pathophysiological changes in disease states and developing effective therapeutic interventions that address the underlying structural and functional abnormalities.