Brad is 45 years old and has been working as a coal cutter in a mine for the last 25 years. He likes the job because it pays well and the same mine had employed his father

Question

Like many of his colleagues, Brad has had problems with a chronic cough. He has avoided his annual checkups for fear that he will be told he has "black lung" or coal worker's pneumoconiosis. The disease causes fibrosis, decreased diffusing capacity, and permanent small airway dilation. In later stages, pulmonary capillaries, alveoli, and airways are destroyed.

How can the disease described above create a mismatch between ventilation and perfusion? Use your understanding of alveolar dead space and physiologic shunt to explain your answer.

Individuals with chronic obstructive pulmonary disease have more difficulty exhaling than inhaling. Why is this so?

In general terms, what mechanisms in lung disease can affect diffusing capacity across alveolar membranes? Use the Fick law to explain your answer.

Answer 1

Chronic lung disease can cause impairments in both pulmonary ventilation and perfusion if circulatory and alveolar tissues are destroyed. Alveolar dead space is a term that can be used to describe inflated alveoli without a local blood flow, as seen when capillaries are nonfunctional. Physiological shunt describes a blood supply to an area of the lung that is inadequately ventilated, as seen when lung tissue is damaged. In both situations, the oxygenation of blood is compromised.

Airway resistance is lower during inhalation because of the elastic properties of the lung. When inhalation occurs, elastic fibers have the effect of pulling airways open as the thoracic cage expands. During exhalation, the natural recoiling of the lung tissue results in a narrowing of the passages. Obstructive lung diseases increase airway resistance and therefore add to the resistance already present during exhalation.

The Fick law states that a volume of a gas moving across the alveolar membrane in a given amount of time is proportional to the partial pressure difference of the gas in question, the surface area of the membrane, and the diffusion coefficient. It also stipulates that diffusion is inversely proportional to membrane thickness. In lung disease, such as pneumoconiosis, the destruction of tissues and fibrotic lay down decrease the surface area and increase membrane thickness, respectively. Both events result in the decline of diffusing capacity across alveolar membranes.