TY - CHAP M1 - Book, Section TI - Disturbances of Respiratory Function A1 - Naureckas, Edward T. A1 - Solway, Julian A2 - Kasper, Dennis A2 - Fauci, Anthony A2 - Hauser, Stephen A2 - Longo, Dan A2 - Jameson, J. Larry A2 - Loscalzo, Joseph PY - 2014 T2 - Harrison's Principles of Internal Medicine, 19e AB - The primary functions of the respiratory system—to oxygenate blood and eliminate carbon dioxide—require virtual contact between blood and fresh air, which facilitates diffusion of respiratory gases between blood and gas. This process occurs in the lung alveoli, where blood flowing through alveolar wall capillaries is separated from alveolar gas by an extremely thin membrane of flattened endothelial and epithelial cells, across which respiratory gases diffuse and equilibrate. Blood flow through the lung is unidirectional via a continuous vascular path, along which venous blood absorbs oxygen from and loses CO2 to inspired gas. The path for airflow, in contrast, reaches a dead end at the alveolar walls; thus the alveolar space must be ventilated tidally, with inflow of fresh gas and outflow of alveolar gas alternating periodically at the respiratory rate (RR). To provide an enormous alveolar surface area (typically 70 m2) for blood-gas diffusion within the modest volume of a thoracic cavity (typically 7 L), nature has distributed both blood flow and ventilation among millions of tiny alveoli through multigenerational branching of both pulmonary arteries and bronchial airways. As a consequence of variations in tube lengths and calibers along these pathways as well as the effects of gravity, tidal pressure fluctuations, and anatomic constraints from the chest wall, the alveoli vary in their relative ventilations and perfusions. Not surprisingly, for the lung to be most efficient in exchanging gas, the fresh gas ventilation of a given alveolus must be matched to its perfusion. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2022/05/18 UR - accessmedicine.mhmedical.com/content.aspx?aid=1120807419 ER -