Respiratory_Physiology_Concepts_10 - Respiratory Physiology...

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Respiratory Physiology Concepts The fundamental function of the lungs is to provide for the exchange of gas (i.e. oxygen and carbon dioxide) between the lungs and blood. The three processes that contribute to that function are: 1. Ventilation: how air gets to the alveoli (the movement of gas by bulk flow) 2. Gas Exchange: how gases cross the blood/gas interface between the pulmonary capillaries and the alveoli 3. Blood flow: how gases are removed from the lung by the blood and transported in the blood A simplified model of ventilation depicts the respiratory system as a tube (airways) connected to a chamber (alveoli). The delivery of air to the alveoli is dependent on the mechanics of ventilation (pressure-volume relationships), the characteristics of the airways (especially resistance), and the properties of the alveoli (especially compliance and elastic recoil). The 3 main factors that influence the filling of the alveoli with air are: 1) air flow (volume); 2) resistance; and 3) the pressure gradient between the mouth and alveoli. Their relationships are depicted in the formula: Q = P / R Q = airflow or volume P = pressure gradient R = resistance Ventilation Airflow is created by the pressure gradient, as air flows from a high to lower pressure until equilibrium is attained. The mechanics of ventilation create the changes in pressure. The mechanics of ventilation are described by Boyle’s Law. P ά 1 / V P: pressure V: volume Pressure is inversely related to volume. Increases or decreases in the volume of the thoracic cavity due to muscle contraction (by the diaphragm and accessory muscles of respiration with their bucket handle and pump handle effects on the bony thoracic cage) create increases or decreases in pleural pressure, alveolar pressure, and transmural pressure (transpulmonary pressure and transairway pressure). The change in the pressures creates a gradient for air to move in or out of the lungs. The basic concept is that volume changes in the thoracic cavity create pressure changes that permit air to fill or empty from the lungs; the lungs
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are expanding before air enters, not as a result of air entering. The pleura and pleural cavity have an important role in the volume – pressure relationship because they connect the chest wall movement to the lung movement. This permits a change in the pleural pressure to be translated into a change in the alveolar pressure during inspiration and expiration. It is interesting to note that breathing at rest is active during inspiration due to contraction of the diaphragm, but passive on expiration due to the elastic recoil of the lung tissue. In order to expand a container, one must apply a greater force inside than outside. This difference in pressure is known as the Transmural pressure and is defined as the pressure inside minus the pressure outside. Transpulmonary pressure is a type of transmural pressure which is definded as: alveolar pressure minus pleural pressure. Transpulmonary pressure determines whether the lungs are inflated or collapsed. A positive transpulmonary pressure distends the lungs.
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Respiratory_Physiology_Concepts_10 - Respiratory Physiology...

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