Physiological Pharmaceutics

Pulmonary drug delivery 223toward the back and are embedded in a dense connective tissue. The dorsal wall containsa strong layer of transverse smooth muscle fibres that span the gap of the cartilage. In anadult, the trachea is 11 to 13 cm in length and 1.5 to 2.5 cm in diameter, and bifurcates toform the right and left main bronchi (Figure 10.1). The right bronchus divides into theupper, middle and lower branches whilst the left bronchus divides only into an upper anda lower branch. These lobar bronchi give rise to branches called ‘segmental bronchi’. Thebranching angle of 37° appears to be optimal to ensure smooth airflow.Structure of the tracheo-bronchial treeThe hierarchy of the dividing airways, and partly the blood vessels, in the lung largelydetermines the internal lung structure. Functionally the system can be subdivided into threezones, a proximal, purely conducting zone, a peripheral, purely gas-exchanging zone, anda transitional zone in between. Morphologically the relatively thick-walled, purely airconductingtubes can be distinguished from those branches of the airway tree structurallydesigned to permit gas exchange.Every branching of the tracheo-bronchial tree produces a new ‘generation’ of tubes,the total cross-sectional area increasing with each generation (Figure 10.2). The mainbronchi are known as the first generation, the second and third generations are the lobarand segmental bronchi respectively. The fourth to ninth generations are the small bronchiand in these segmental bronchi the diameters decrease from approximately 4 to 1 mm. Ata diameter of less than 1 mm they lie inside the lobules and are then correctly termedbronchioles. Here the function of the tissue changes from conducting airway to gasFigure 10.1 Structure of the lungs