Ch4 -notes-ch 4

Ch4 -notes-ch 4 - Blood Flow and Metabolism Blood flow in...

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Blood Flow and Metabolism Blood flow in the lungs is different than in the rest of the body because the alveoli can have pressures up to 30 cm H2O (22 mm Hg) which can exceed the average pressure in the pulmonary artery. Given the ultrathin separation of alveolar gas and capillaries it is not surprising that blood flow is highly sensitive to surrounding pressures due to alveolar pressure as well as hydrostatic pressures due to liquid in the pleural space. The lungs can be considered to be floating in a continuous liquid column even though the total amount of fluid is only a few mi. The pressures in the different lung zones are shown below. In the upright posture there is about a 17 cm height difference between the lung apex(top) and base(bottom). In the liquid pleural space this means that pressure at the base is 23 mm Hg higher than the apex. This pressure gradient will be present for arteries and veins in the different zones as well. Alveolar pressure will be a constant with a level of 6 mm Hg representing an average level during breathing. The pressure differences promote a higher flow in the base compared to the apex zone. From an engineering standpoint, zone 2 is the most interesting. The pressure-flow relationships for all 3 zones have been studied using a fluid mechanical arrangement as shown in the picture below (Conrad, IEEE Trans BME, 16:284-295, 1969).
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Such an adjustable collapsible tube resistance is called a Starling resistor after the physiologist who first used it in isolated heart experiments. The interesting characteristic is shown in the pressure-flow relationship from the same reference for what can be considered zone 2 behavior. Note the region where the pressure-flow curve has a negative slope, or “negative resistance”. What happens in this region is that it is highly unstable and easily starts to oscillate. Closer inspection of the non-linear curve shows a high resistance region at low flow and a low resistance region at high flow. During oscillation there is a cycling between the two extremes. Does this happen in the lungs?
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Ch4 -notes-ch 4 - Blood Flow and Metabolism Blood flow in...

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