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Lecture 10

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Biological Sciences
Stephen Reid

1    Lecture 10: Lung Mechanics 2 1. The Ideal Gas Law The Ideal Gas Law is as follows: PV = nRT Where P is gas pressure, V is the volume in which the gas is contained, n is the number of moles of gas, R is the universal gas constant, and T is temperature in degrees Kelvin. Pressure can be calculated by the equation P = nRT/V. Boyle's Law, which states that for a given quantity of gas in a chamber, the gas pressure is inversely proportional to the volume of the chamber. There are three main pressures associated with pulmonary mechanics as well as two important pressure differentials. Atmospheric pressure, inter-alveolar pressure, and inter-pleural pressure are the three pressure values, and the important pressure differentials are between the former two (which is the driving force for moving air in and out of the lungs) and between the latter two (called transpulmonary pressure, which is the driving force for lung expansion). Air flow in and out of the lungs is caused by differences between atmospheric pressure and pressure inside the lungs (inter-alveolar pressure). If the former pressure is higher than the latter, air will flow into the lungs, and vice versa. We can calculate air flow via this equation: Air Flow = (atmospheric pressure – inter-alveolar pressure) / Resistance We consider (in this case) atmospheric pressure to be constant (atmospheric pressure changes are primarily due to changes in altitude). 2    2. Pressure and Volume Changes during Inspiration and Expiration As the lung expands, inter-alveolar pressure will decrease. As inter-alveolar pressure falls lower relative to atmospheric pressure, the lungs will begin to fill with molecules of air. As the lungs fill, pressure begins to increase again, and when it reaches equilibrium with the pressure of air outside the lungs, air flow will cease. Even though P alvrelative to P atm goes down and up during inspiration, breath volume increases during this entire time period until equilibrium is reached (i.e., alv relative to Patm is zero). During expiration, as the lungs recoil to the “resting state”, inter-alveolar pressure increases relative to atmospheric pressure forcing air out of the lungs. When enough air has been exhaled, pressure will begin to decrease again, until it reaches equilibrium and the process begins again. If one were to graph inter-alveolar pressure relative to atmospheric pressure during the process of a breath, it would resemble a sine wave: a shallow dip down and a shallow curve up, and back again. 3    3. Pressure and Volume Changes during Inspiration – A More Detailed Look Motor activity in the phrenic nerve causes the diaphragm to contract and move downward causing the chest wall to expand. This causes a pull on the interpleural fluid leading to a decrease in intrapleural pressure. As intrapleural pressure decreases (with no immediate change in intra- alveolar pressure), transpulmonary pressure increases. This causes the lungs to expand and therefore lung volume increases. As lung volume increases, inter-alveolar pressure decreases leading to an increase in the atmospheric pressure – inter-alveolar pressure difference. This causes air to flow into the lungs. 4. Surface Tension and Pulmonary Surfactant Recall that compliance is a measure of how easy it is to expand, in this case, the lungs. The inner walls of the alveoli have a thin fluid layer on their surface. As such, when the lungs expand, work is required to expand both the lungs (alveoli) and this fluid layer. The fluid layer exerts a surface tension due to its interaction with the lung tissue and other fluid molecules. This surface tension
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