Functional Anatomy of the Lung

Basic Structural Organization

The lung has 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, 2
The lung connects a small entrance to a massive alveolar gas exchange surface through sequential, irregular, dichotomous branching patterns 3
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 3
The gas exchange apparatus forms a sleeve of alveoli on the surface of approximately eight generations of the most distal airways 3

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 3
Pulmonary arteries follow airways in a similar branching pattern but include additional "supernumerary" branches at nearly all levels to perfuse nearby parenchyma 3
Pulmonary arteries branch over approximately five more generations than airways before reaching capillaries 3
Pulmonary veins course independently of airways in intermediate positions related to interlobular septa, converging on the left atrium in four main stems 3

The air-blood barrier consists of alveolar epithelium, capillary endothelium, and their shared basement membrane, with the harmonic mean barrier thickness being a critical measure of diffusion resistance 4, 5
Alveolar O₂ uptake occurs in two major steps: diffusion across the membrane barrier and binding to capillary hemoglobin, with each step imposing specific resistances 4, 5
The lung diffusing capacity for oxygen (DLO₂) is determined by the joint contribution of alveolar capillary blood volume, intra-acinar alveolar and capillary surfaces, and the harmonic mean air-blood barrier thickness 4, 5
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 4, 5