Ventilation worksheet: moving air in & out Note to instructors: This worksheet represents a way that I have taught this material, which incorporates figures/tables created by others. I have cited my sources, but I have not obtained formal permission to use the figures/ tables. As far as Im concerned, youre welcome to use this worksheet as is or modify it. If you do the latter, please continue to cite the sources and be aware that their inclusion here may or may not be permissible under fair use doctrine. --Greg Crowther, Everett Community College ([email protected]) 1
Worksheet: moving air in and out A simple description of breathing is this: The contraction of your diaphragm and external intercostal muscles expands your thorax. Since your lungs are tethered to the thorax walls, they expand too, and additional air enters the nowenlarged lungs. Then your inspiratory muscles relax and the process reverses. A limitation of this simple description is that it doesnt explain scenarios such as pneumothorax, when a hole in the chest wall impairs breathing even if the lungs themselves are still intact. The thorax can still expand and shrink so why dont the lungs follow along any more? We will arrive at a clearer understanding of this scenario by progressing through the following topics: (A) Introduction: The pressure-volume relationship (B) Lung volume: pressure gradient vs. elastic recoil (C) How muscles change pressures, and thus lung volumes (D) Drawing the respiratory cycle (E) Pneumothorax
2 (A) Introduction: the pressure-volume relationship Boyles Law, a fundamental principle of chemistry, states that for a given quantity of gas molecules, the pressure and volume are inversely proportional, i.e., as one goes up the other goes down. This idea can be illustrated in figures like the one at right.
A1. Blood is not a gas, and thus is not strictly governed by Boyles law. But does the Boyles-law relationship between volume and pressure also hold true for blood in the cardiovascular system? Give an example involving blood pressure. Martini et al. (2015), Figure 23.12 3 A2. Label the following structures: Diaphragm Parietal pleura Visceral pleura Plural cavity Thoracic wall The pleural cavity is normally a closed cavity containing a fixed
number of molecules, so it obeys Boyles Law. A3. If the pleural cavity expands in volume, what happens to the pressure inside it? Marieb & Hoehn (2016), Figure 22.13 4 Atmospheric pressure at sea level is 760 mm Hg. Intrapleural pressure is generally a bit lower -- say, 756 mm Hg. Sometimes pressures are presented on a scale in which 760 mm Hg has been subtracted from all pressures, so that sealevel atmospheric pressure is 0. A4. What is an intrapleural pressure of 756 mm Hg, according to this scale?
A5. In the figure, assume that the atmosphere and lungs have the same air pressure, and at the intrapleural cavity has a pressure that is 4 mm Hg lower. Label the atmospheric, pulmonary (inside-the-lungs), and intrapleural (in-the-pleuralcavity) pressures according to both scales. Marieb & Hoehn (2016), Figure 22.13 5 (B) Lung volume: pressure gradient vs. elastic recoil The pressure inside the lungs (intrapulmonary pressure) is very close to the atmospheric pressure,
while pressure in the pleural cavity (intrapleural pressure) is a bit lower. This transpulmonary pressure difference drives the higher-pressure lungs to expand into the lowerpressure area surrounding them. When the lungs are driven to expand by lower pressure surrounding them, we call that negative-pressure ventilation. The alternative is to force air under positive pressure into the lungs. Both strategies can be used by clinical devices (see figure). slideshare.net/SharathKrishnaswami1/mechanical-ventilation-Sharath (original source unknown)
6 The pressure gradient promoting lung expansion is opposed by the lungs elastic recoil, which promotes lung shrinkage, like an inflated balloon driven to shrink back to its resting volume (see figure). At any given moment, the lungs volume reflects a balance of these two opposing forces. B1. Say that the thoracic cavity expands, causing the the pleural cavity to expand too. Will this pleural cavity expansion affect the lungs? Why or why
not? Hint: Boyles law! slideshare.net/noureldenelnaggar/1-lung-mechanics (original source unknown) 7 (C) How muscles change pressures, and thus lung volumes Lets now consider how respiratory muscles like the diaphragm and external intercostal muscles both of which are used for inspiration (breathing in) affect the lungs. C1. When these inspiratory muscles contract, they get shorter. But do they increase or decrease the volume of the thoracic cavity?
C2. How will this change in the thoracic cavity specifically affect the pleural cavity? Amerman (2016), Figure 21.14 8 C3. How will these changes in the pleural cavity affect the lungs themselves? C4. Do your answers to C1 through C3 agree with your answer to B1? (D) Drawing the respiratory cycle Lets draw a complete cycle of inspiration and expiration. For simplicity, we will break changes occurring simultaneously into separate steps, and we will exaggerate the size of the pleural cavity.
Lets start at the very start of inspiration. Use your drawings to show changes in volumes and pressures. You can use arrows to indicate the expansion/contraction of volumes, and/or the movement of air into/out of the lungs. 9 10 11 D8. During inspiration, why dont the lungs expand even further in step D3 so that the intrapulmonary pressure becomes equal to the intrapleural pressure? D9. Lungs are often modeled as a balloon in a jar. How is
actual human ventilation similar to and different from this model? SIMILARITIES (at least 2): DIFFERENCES (at least 2): Freeman et al. (2016), Figure 44.12 12 (E) Pneumothorax E1. What does the root pneumo mean? E2. How does that relate to
the definition of pneumothorax? E3. A knife blade goes through the skin into the thoracic cavity, but does not touch the lungs. Is this a pneumothorax situation? youtube.com/watch?v=0vZ9gVyWreo 13 E4. Explain why pneumothorax leads to lung collapse. (Think about the 2 factors governing lung volume.) E5. Does a pneumothorax typically occur bilaterally (on both sides) or unilaterally (on one side only)? Explain.
E6. If someone has a collapsed lung due to a puncture wound, just sealing the hole will not cause the lung to reinflate. Why not? 14
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