Respiratory Deposition

:: Section 3

Retention and Clearance

The retention of deposited particles varies significantly depending on their physicochemical properties, deposition location and clearance mechanism involved. In the head airways and lung airways, the surface is covered with a layer of mucus. This layer is slowly propelled by ciliary action to the pharynx and is then subconsciously swallowed to the gastrointestinal tract. Generally, this mucociliary transport can get the deposited particles out of the respiratory system in a matter of hours. If the inhaled air contains low doses of irritating gases or aerosols, this clearance mechanism can be accelerated. However, it can be slowed down if doses are high.

The alveolar region, however, does not have the same mucociliary mechanism because BY NATURE it's designed for gas exchange. Insoluble particles deposited in this region may be engulfed by alveolar macrophages and transported to lymph nodes although generally it can take months or years to clear them. Fibrogenic dusts, such as silica, asbestos and coal dust, interfere with this mechanism resulting in fibrosis of this region. Due to its long retention time and subsequently continuous radiation, insoluble radioactive material deposited in this region causes significant damages to the local region. Soluble materials can pass through the thin alveolar membrane and be transported to other parts of our body. Hence, it is the region where viruses love to invade and it is also the target region for therapeutic aerosol delivery.