Case_Study__4_Lungs_08 - Gas Exchange in the Lungs Case...

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Gas Exchange in the Lungs Case Study #4 BEE 331 Bio-Fluid Mechanics Background The alveolar gas transfer model shown in class is dependent on only a single physiology based constant, the ventilation-perfusion ratio, expressed as Q V r A = V A is ventilation of fresh air entering in and leaving an alveolus and Q is the perfusion, or blood flow to the alveolus. Respiration along the alveolar walls can also be thought as a dual convection system with the ventilation-perfusion ratio (V A /Q) as the ratio of its two convection constants. V A Pulmonary Capillary Alveolus Figure: Alveolus schematic and scanning electron microscope images 1 . The V A /Q ratio for the entire lung in humans is approximately 0.8. This global ratio can be higher (a condition known as dead space, an impairment of the alveolar capillaries) or lower (right to left shunt, a ventilation impairment) in patients due to a variety of clinical conditions. The global ratio, a property of the whole lung, is actually the mean of the individual ratios for each alveolus. n r = i r where, n is the number of alveolus in the lungs, 3x10 8 1 SCIENCES/LUNG-RESEARCH/index/96703330.html , accessed on 4/24/07
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Ventilation and perfusion are not constant in the alveoli due to physiological features (blood and air obstructions), hydrostatic pressure differences, and the gravitational effect. For example, blood doesn’t reach upper alveoli as well as it does lower ones, leading to a general gradient in perfusion. However, for optimal gas transfer efficiency, it is the matching of ventilation and perfusion that is critical, not their individual values. The lungs develop in such a way so that ventilation and perfusion are closely matched for each alveolus, meaning the variation in ventilation or perfusion values is far greater than the variation in their ratio. Alveoli that have a high blood flow are also generally well ventilated and vice a versa. However, the lungs are not ideally designed (no biological system is) and variation does exist in V A /Q. This is known as ventilation-perfusion heterogeneity and it is one of the primary causes of inefficiency in gas exchange in the lungs. Experimental Approach:
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This note was uploaded on 03/29/2009 for the course BEE 3310 taught by Professor Gebremedhin,k. during the Summer '07 term at Cornell.

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Case_Study__4_Lungs_08 - Gas Exchange in the Lungs Case...

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