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QUANTITATIVE (COMPUTATIONAL TASKS) 1. At the end of a normal expiration, a person’s lung volume is 2 L, his alveolar pressure is 0 mmHg, and his intrapleural pressure is 24 mmHg. He then inhales 800 mL of air. At the end of inspiration, the alveolar pressure is 0 mmHg and the intrapleural pressure is 28 mmHg. Calculate this person’s lung compliance. 1. 200 mL/mmHg. Lung compliance = Δ lung volume/Δ (Palv - Pip) = 800 mL/ [0 - (-8)] mmHg - [0 - (-4)] mmHg = 800 mL/4 mmHg = 200 mL/mmHg 2. A patient is unable to produce surfactant. To inhale a normal tidal volume, will her intrapleural pressure have to be more or less subatmospheric during inspiration, relative to a healthy person? 2. More subatmospheric than normal. A decreased surfactant level causes the lungs to be less compliant (i.e., more difficult to expand). Therefore, a greater transpulmonary pressure (Palv - Pip) is required to expand them a given amount. 3. A 70 kg adult patient is artificially ventilated by a machine during surgery at a rate of 20 breaths/min and a tidal volume of 250 mL/breath. Assuming a normal anatomical dead space of 150 mL, is this patient receiving an adequate alveolar ventilation? 3 . No. Alveolar ventilation = (Tidal volume - Dead space) × Breathing rate = ( 250 mL - 150 mL)/breath × 20 breaths/min = 2000 mL/min Normal alveolar ventilation is approximately 4000 mL/min in a 70 kg adult. 4. Why must a person floating on the surface of the water and breathing through a snorkel increase his tidal volume and/or breathing frequency if alveolar ventilation is to remain normal? 4. The volume of the snorkel constitutes an additional dead space, so total pulmonary ventilation must be increased if alveolar ventilation is to remain constant. 5. A healthy person breathing room air voluntarily increases alveolar ventilation two fold and continues to do so until reaching new steady-state alveolar gas pressures for oxygen and carbon dioxide. Are the new values higher or lower than normal? 5. The alveolar pO 2 will be higher than normal, and the alveolar pCO 2 will be lower. 6. A person breathing room air has an alveolar pO 2 of 105 mmHg and an arterial pO 2 of 80 mmHg. Could hypoventilation due to, say, respiratory muscle weakness produce these values? 6. No. Hypoventilation reduces arterial pO 2 but only because it reduces alveolar pO 2 . That is, in hypoventilation, both alveolar and arterial pO 2 are decreased to essentially the same degree. In this problem, alveolar pO 2 is normal, and so the person is not hypoventilating. The low arterial pO 2 must therefore represent a defect that causes a discrepancy between alveolar pO 2 and arterial pO 2 . Possibilities include impaired diffusion, a shunting of blood from the right side of the heart to the left through a hole in the heart wall, and a mismatch between airflow and blood flow in the alveoli.
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  • Spring '10
  • Pedigo
  • mm Hg, pulmonary disease, lung diseases, hypoxemia

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