Physiological Pharmaceutics

224 Physiological PharmaceuticsFigure 10.2 The tree structure of the lungexchange. Although the decrease in diameter minimizes the deadspace, it is associated witha large increase in resistance to flow. For example, a 10% reduction in diameter is associatedwith an increase in airway resistance of more than 50%. Thus the airways should be as wideas possible to minimize resistance, but this will of course increase deadspace. In the bodythese opposing factors are balanced by finely tuned physiological control which is easilydisturbed by lung pathologies such as asthma and bronchitis.The respiratory gases diffuse from air to blood, and vice versa, through the 140 squaremetres of internal surface area of the tissue compartment. The gas-exchange tissue properis called the pulmonary parenchyma, while the supplying structures, conductive airways,lymphatics, and non-capillary blood vessels belong to the non-parenchyma. Theparenchyma of the lung consists of approximately 130,000 lobules, each with a diameterof about 3.5 mm and containing around 2,200 alveoli. It is believed that each lobule issupplied by a single pulmonary arteriole. The terminal bronchioles branch intoapproximately 14 respiratory bronchioles, each of which branches further into the alveolarducts. The ducts carry 3 or 4 spherical atria that lead to the alveolar sacs supplying 15–20alveoli (Figure 10.3). Additional alveoli arise directly from the walls of the alveolar ducts,and these are responsible for approximately 35% of the total gas exchange. It has beenestimated that there are 300 million alveoli in an adult human lung. The volume of analveolus changes with the degree of inflation, but assuming 75% lung inflation, the diameterof an alveolus is thought to be between 250 and 290 µm. It is estimated that each alveolushas a volume of 1.05×10 - 5 ml, with an air-tissue interface of 27×10 -4 cm 2 . For thesecalculations, it was assumed that the lung had a total air volume of 4.8 L, a total respiratoryzone volume of 3.15 L and a total alveolar air-tissue interface of 81 m 2 .