Ch1-notes - Systems Physiology Systems Physiology is the...

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Systems Physiology Systems Physiology is the counter approach to the emphasis of biology on determining structure first and inferring function. In engineering a systems engineer is criticized ( like a biomedical enginer)as –a jack of all trades but master of none. However, the opposing criticism is that specialization produces an iso-idiot. A systems approach starts with function and uses structure as is useful. It is looking first at the forest before the trees. The utility of this view can be argued from the new current view that to preserve the long term survival of forests fires are useful. They eliminate insect infestations and promote new growth despite the destruction of some trees. Our approach beginning with respiration then starts with function. Function: Questions : 1. How does barometric pressure changes affect breathing? 2. What is the simplest (cheapest) way to detect breathing of a patient using a transducer? 3. How can you use tank pressure of a scuba diver to measure subject airflow? Normal air in the atmosphere contains about 21% oxygen, close to 0% carbon dioxide , 78% nitrogen , and 1% trace gases such as argon. All mammals consume oxygen and produce carbon dioxide during metabolism. For an adult human at rest 250 ml/min of oxygen must be provided and about the same flow of carbon dioxide must be eliminated by the lungs. During exercise these gas fluxes can easily increase 10 fold. Diffusion is how oxygen and carbon dioxide moves from gas to blood which then carries oxygen to and removes carbon dioxide from tissues of the body. Diffusion of a gas occurs because of a difference in partial pressure P which for a dry gas is: P =F*B Where P=partial pressure in mm Hg, F=gas fraction (%/100), and B=barometric pressure in mm Hg. (Appendix A of West lists some useful equations) At sea level B=760 mm Hg. B decreases with altitude(253 mm Hg at 29,028 feet-Everest) and increases at depths(increases by 760 mm Hg every 33 feet sea water). Note that gas fraction or percentage remains the same at any depth or altitude attainable by commercial aircraft. Since partial pressure is the main driving force for diffusion, it represents a major determinant of lung gas exchange. Since diffusion requires a difference in partial pressure between atmospheric air and blood the first question that can be asked is what is the partial pressure in blood? The normal partial pressures of arterial blood is 100 mm Hg for oxygen and 40 mm Hg for carbon dioxide. For a normal person these values don’t change in going from rest to exercise. Arterial blood partial pressures is what is supplied to the body and so results from gas exchange by the lungs. The body venous blood is the input to the lungs and will have a lower oxygen and higher carbon dioxide partial pressure which varies as a function of metabolic level . Since the fraction of carbon dioxide in atmospheric air is virtually zero, the partial pressure is also zero and does not change with altitude or depth. This is not the case with oxygen.
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This note was uploaded on 02/15/2012 for the course BME 403 taught by Professor Yamashiro during the Spring '07 term at USC.

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Ch1-notes - Systems Physiology Systems Physiology is the...

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