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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Lecture 10 Study Notes • Lung compliance: a measure of the ease of expansion of the lungs and thorax o Compliance = Lung Volume / ( P alv– P ip OR Lung Volume/TRANSPULMONARY PRESSURE Transpulmonary pressure: refers to the driving force behind lung expansion • Not the same as driving force for air flow Driving force for air flow isP atm – P alv o Low compliance: small change in lung volume with given pressure change o High compliance: large change in lung volume with given pressure change • Recoil pressure: pressure exerted when chest wall relaxes and lung volume decreases o Recoil pressure is what causes the Pip to be negative. Look back at previous lecture • Emphysema: lungs are overcompliant o Alveolar walls break down, causing lungs to become loose and floppy • Fibrosis: lungs are rigid and less compliant • Compliance is influenced by surface tension o A liquid layer adheres to the surface of alveoli o Hydrogen bonding • So during inspiration, we expand: o Lung tissue (via transpulmonary pressure changes) o Surface tension to allow gas exchange • Laplace’s Law o P = 2T/r o Laplace’s law = pressure required to prevent alveolar collapse at rest o The smaller the alveoli (radius), the more pressure needed to prevent collapse • Pulmonary Surfactant: detergent like substance that reduces surface tension o Produced by type II cells in the lungs o Interfered with H bonding that holds liquid in surface tension together • Pulmonary surfactant is more in small alveoli than in larger alveoli o Alveolar pressure within the lungs is equal throughout, and as noted before, greater distending pressure is needed to prevent alveolar collapse in small lungs o The larger quantity of pulmonary surfactant levels the playing field, as there is also higher surface tension on small alveoli • Infant distress syndrome: can result from failure of type II cells in the lungs to produce pulmonary surfactant o Abnormal serotonin levels • Air Flow = (P – P / R atmospheric alveolar) • Alveolar Pressure/Lung Pressure is influenced by two things: o Moles of air in lungs o Volume of the lung • Three main factors affect airway resistance: o Passive forces (short term) Change in transpulmonary pressure i.e pressure that drives lung expansion • P alv – P ip
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