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Drug deposition in the respiratory tract (demo)Structure of the airway The airway is designed to prevent the entry of particlesThe structure of the airway affects particle deposition. The upper airway consists essentially of a right-angled bend at the back of the mouth and nose with a narrowed area at the vocal cords and larynx. The fast flow of air through this region and the sudden changes in direction encourage turbulent flow of air, which increases the chance of particle deposition. Within the lungs itself the airway consists of a system of branches and tubes, which although the diameter of the tubes gets smaller, the number increases so that the cross-sectional area of the lung gets bigger the further into the lungs the particles go. Due to this increase in cross-sectional area the air speed through the airway decreases markedly, leading to more laminar flow and less particle deposition. The structure of the airway affects particle deposition:
With laminar flow particles are carried in parallel, straight lines along the airways, and there is less impaction of particles on the airway wall.
The orderly flow of gas and particles in laminar flow is broken down at high flow rates, particularly high flow rates through branched or narrowed airways, resulting in turbulent flow. In this situation there is irregular movement of particles within the airway and impaction on the airway wall is more likely to occur.
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