STRUCTURE AND FUNCTION
Morton Lippmann
The respiratory system extends from the breathing zone just outside of the nose and mouth through the conductive airways in the head and thorax to the alveoli, where respiratory gas exchange takes place between the alveoli and the capillary blood flowing around them. Its prime function is to deliver oxygen (O2) to the gas-exchange region of the lung, where it can diffuse to and through the walls of the alveoli to oxygenate the blood passing through the alveolar capillaries as needed over a wide range of work or activity levels. In addition, the system must also: (1) remove an equal volume of carbon dioxide entering the lungs from the alveolar capillaries; (2) maintain body temperature and water vapour saturation within the lung airways (in order to maintain the viability and functional capacities of the surface fluids and cells); (3) maintain sterility (to prevent infections and their adverse consequences); and (4) eliminate excess surface fluids and debris, such as inhaled particles and senescent phagocytic and epithelial cells. It must accomplish all of these demanding tasks continuously over a lifetime, and do so with high efficiency in terms of performance and energy utilization. The system can be abused and overwhelmed by severe insults such as high concentrations of cigarette smoke and industrial dust, or by low concentrations of specific pathogens which attack or destroy its defence mechanisms, or cause them to malfunction. Its ability to overcome or compensate for such insults as competently as it usually does is a testament to its elegant combination of structure and function.
Mass Transfer
The complex structure and numerous functions of the human respiratory tract have been summarized concisely by a Task Group of the International Commission on Radiological Protection (ICRP 1994), as shown in figure 10.1 . The conductive airways, also known as the respiratory dead space, occupy about 0.2 litres. They condition the inhaled air and distribute it, by convective (bulk) flow, to the approximately 65,000 respiratory acini leading off the terminal bronchioles. As tidal volumes increase, convective flow dominates gas exchange deeper into the respiratory bronchioles. In any case, within the respiratory acinus, the distance from the convective tidal front to alveolar surfaces is short enough so that efficient CO2-O2 exchange takes place by molecular diffusion. By contrast, airborne particles, with diffusion coefficients smaller by orders of magnitude than those for gases, tend to remain suspended in the tidal air, and can be exhaled without deposition.
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STRUCTURE AND FUNCTIONMorton LippmannThe respiratory system extends from the breathing zone just outside of the nose and mouth through the conductive airways in the head and thorax to the alveoli, where respiratory gas exchange takes place between the alveoli and the capillary blood flowing around them. Its prime function is to deliver oxygen (O2) to the gas-exchange region of the lung, where it can diffuse to and through the walls of the alveoli to oxygenate the blood passing through the alveolar capillaries as needed over a wide range of work or activity levels. In addition, the system must also: (1) remove an equal volume of carbon dioxide entering the lungs from the alveolar capillaries; (2) maintain body temperature and water vapour saturation within the lung airways (in order to maintain the viability and functional capacities of the surface fluids and cells); (3) maintain sterility (to prevent infections and their adverse consequences); and (4) eliminate excess surface fluids and debris, such as inhaled particles and senescent phagocytic and epithelial cells. It must accomplish all of these demanding tasks continuously over a lifetime, and do so with high efficiency in terms of performance and energy utilization. The system can be abused and overwhelmed by severe insults such as high concentrations of cigarette smoke and industrial dust, or by low concentrations of specific pathogens which attack or destroy its defence mechanisms, or cause them to malfunction. Its ability to overcome or compensate for such insults as competently as it usually does is a testament to its elegant combination of structure and function.Mass TransferThe complex structure and numerous functions of the human respiratory tract have been summarized concisely by a Task Group of the International Commission on Radiological Protection (ICRP 1994), as shown in figure 10.1 . The conductive airways, also known as the respiratory dead space, occupy about 0.2 litres. They condition the inhaled air and distribute it, by convective (bulk) flow, to the approximately 65,000 respiratory acini leading off the terminal bronchioles. As tidal volumes increase, convective flow dominates gas exchange deeper into the respiratory bronchioles. In any case, within the respiratory acinus, the distance from the convective tidal front to alveolar surfaces is short enough so that efficient CO2-O2 exchange takes place by molecular diffusion. By contrast, airborne particles, with diffusion coefficients smaller by orders of magnitude than those for gases, tend to remain suspended in the tidal air, and can be exhaled without deposition.)
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