Stereology: a bridge to a better understanding of lung structure and function.

U nlike solid organs, the lung is a hollow organ that contains, under physiological conditions, an amount of air that is much larger than the total amount of its tissue. This confers to the lung a degree of heterogeneity that is much larger than in any other organ and adds difficulties to the interpretation of physiological measurements, which are in most cases related to the whole system. To overcome these difficulties and to construct suitable models for lung function, a number of studies on the relationships between structure and function have been performed over the last five decades. The major problem in relating lung function measurements to morphological data is the need to gather information on a three-dimensional (3D) structure from two-dimensional (2D) observations. Although data from simple 2D images have been widely used to describe pathological changes in lung parenchyma or airways, their relevance to physiological events occurring in 3D structures is limited.