Many problems regarding structure-function relationships have remained unsolved in the field of respiratory physiology. In the present review, we highlighted these uncertain issues from a variety of anatomical and physiological viewpoints. Model A of Weibel in which dichotomously branching airways are incorporated should be used for analyzing gas mixing in conducting and acinar airways. Acinus of Loeschcke is taken as an anatomical gas-exchange unit. Although it is difficult to define functional gas-exchange unit in a way entirely consistent with anatomical structures, acinus of Aschoff may serve as a functional gas-exchange unit in a first approximation. Based on anatomical and physiological perspectives, the multiple inert-gas elimination technique is thought to be highly effective for predicting ventilation-perfusion heterogeneity between acini of Aschoff under steady-state condition. Changes in effective alveolar P_O2, the most important parameter in classical gas-exchange theory, are coherent with those in mixed alveolar P_O2 decided from the multiple inert-gas elimination technique. Therefore, effective alveolar-arterial P_O2 difference is considered useful for assessing gas-exchange abnormalities in lung periphery. However, one should be aware that although alveolar-arterial P_O2 difference sensitively detects moderately low ventilation-perfusion regions causing hypoxemia, it is insensitive to abnormal gas exchange evoked by very low and high ventilation-perfusion regions. Pulmonary diffusing capacity for CO (D_LCO) and the value corrected for alveolar volume (V_AV), i.e., D_LCO/V_AV (K_CO), are thought to be crucial for diagnosing alveolar-wall damages. D_LCO-related parameters have higher sensitivity to detecting abnormalities in pulmonary microcirculation than those in the alveolocapillary membrane. We would like to recommend four categories derived from combining behaviors of D_LCO with those of K_CO for differential diagnosis on anatomically morbid states in alveolar walls: type-1 abnormality defined by decrease in both D_LCO and K_CO; type-2 abnormality by decrease in D_LCO but increase in K_CO; type-3 abnormality by decrease in D_LCO but restricted rise in K_CO; and type-4 abnormality by increase in both D_LCO and K_CO.

• Secondary lobule of miller
• Acinus of Loeschcke
• Acinus of Aschoff
• Convection
• Gas-phase diffusion (stratification)
• Aqueous-phase diffusion
• Ventilation-perfusion heterogeneity

• ANTIHISTAMINICS

• Anti-Allergic Agents
• Histamine H1 Antagonists
• Humans
• Physical Phenomena