Chapter 18 Notes - Gas Exchange & Transport

Chapter 18 Notes - Gas Exchange & Transport -...

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Oxygen and carbon dioxide can diffuse freely (i.e. through simple diffusion) across cell layers (between alveoli and pulmonary capillaries, or between systemic capillaries and cells) - Obeys Fick’s Law Fick’s Law: rate of diffusion α ( surface area of t h e membrane x concentration ) gradient x membrane permeability membrane t h ickness Note: Membrane permeability α lipid solubilitymolecular size Along with the three factors that influence diffusion in the lungs (according to Fick’s law), there is a fourth: diffusion distance - Diffusion is most rapid over short distances. - Diffusion distance, surface area and membrane thickness are kept constant in the body (usually maximized to facilitate diffusion) - The most important factor is thus concentration gradients that drive gas exchange . The movement of gas molecules from air into liquid is directly proportional to the pressure gradient of the gas, the solubility of the gas in the liquid, and the temperature . - Recall from general chemistry: the solubility of a gas decreases with increasing temperature (Why? think about boiling water: the liquid molecules will want to vaporize and become gas molecules: therefore increasing the temperature will decrease the solubility of a gas in a liquid) Gases flow from areas of higher pressure to areas of lower pressure. An increase in temperature causes an increase in pressure (Charles’ Law) General overview of gas exchange: oxygen exchange occurs at the alveolar-capillary interface (oxygen comes into the alveoli from the air, and carbon dioxide comes into the alveoli from the blood in order to be exhaled). Oxygen is transported via systemic circulation to the tissues, where cellular respiration occurs and produces carbon dioxide. Carbon dioxide is picked up by the venous blood, and returned to the heart. The cycle begins again when the blood enters pulmonary circulation. Carbon dioxide is much more soluble in water than is oxygen – we have evolutionarily developed to overcome this obstacle. - The low oxygen solubility in water implies that at chemical equilibrium (between oxygen molecules in air and oxygen molecules in water), the amount of oxygen per liter of fluid is more in air than it is in fluid. The normal alveolar P oxygen is roughly 100 mm Hg, and the P oxygen of systemic venous blood arriving at the lungs is 40 mm Hg Oxygen will move down its concentration gradient from the lungs into the blood. When oxygenated blood reaches the tissues, the partial pressures are switched, which
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This note was uploaded on 11/07/2010 for the course CBN 356 taught by Professor Merill during the Spring '08 term at Rutgers.

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Chapter 18 Notes - Gas Exchange & Transport -...

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