摘要: Following inhalation, nanoparticles must cross cellular barriers to enter the body further. In order for a substance to enter a cell’s interior, it must pass through the diffuse layer surrounding the cell and the plasma membrane which segregates the internal and external environments of a cell and regulates the entry and exit of substances into and out of the cell. The uptake of substances is accomplished via a variety of processes that can be described as active (energy dependent) or passive (energy independent). There are a number of clearly defined mechanisms for crossing the plasma membrane that include diffusion, facilitated diffusion, active transport, and endocytosis. Understanding the specific processes and physicochemical factors controlling the ability of nanoparticles to cross barriers, in particular epithelial cells, is key to understanding the intracellular fate as well as the potential for distribution of nanoparticles around the body. The respiratory system can be divided into upper (nasal cavity, pharynx, and larynx) and lower (trachea, primary bronchi, and alveoli) sections. The respiratory system is lined with a barrier of epithelial cells, whose structure and function differ between parts of the pulmonary system. Epithelial cells form a confluent barrier in healthy tissues and control the movement of substances in both directions across the epithelium. Damage to the epithelium by toxicants or disease can lead to an increase in permeability and, therefore, toxicant absorption into the body. Toxicants may also be taken up directly into the cell and then pass through the epithelial cell into the interstitium or cardiovascular system. This is known as the …