Answer Key to Short Answer Questions for
Answer Key to Short Answer Questions for
“I Can’t Stop Coughing: A Case Study on the Respiratory System”
1. Describe the relationship between intrapulmonary pressure, atmospheric pressure, and air flow during normal inspiration and expiration, referring to Boyle’s law.
Intrapulmonary pressure is pressure inside the lungs, and atmospheric pressure is the pressure of gases in the environment. These pressures are usually measured in units of millimeters of mercury (mm Hg). Atmospheric pressure at sea level is 760 mm Hg. Intrapulmonary pressure is either slightly higher (i.e., 762), or slightly lower (758) than atmospheric pressure depending on the phase of the respiratory cycle. Inspiration is an active process whereby the contracting diaphragm and internal intercostals increase the dimensions of the thorax. Boyle's Law states that the volume of a gas in a container is inversely related to the pressure of the gas. So, when the volume of a gas increases, its pressure decreases, and vice versa. Here the container is the lungs: when the lung volume increases, then intrapulmonary pressure drops a couple of millimeters below atmospheric pressure, and air flows into the lungs down its pressure gradient. When the muscles of inspiration relax, the thoracic volume decreases and intrapulmonary pressure rises above atmospheric pressure. Therefore, air flows out of the lungs down its pressure gradient. Resistance is normally minimal in the airways, so just a small pressure gradient between atmospheric and intrapulmonary pressure is needed to move air.
2. Resistance varies in Mike’s conducting airways. Using your understanding of respiratory anatomy, explain where in his airway the resistance is highest and why.
Resistance is highest in the medium-sized conducting airways and lower in the large airways because of their large diameters. As the air travels into the medium-sized bronchi, it faces greater resistance due to the drop in diameter of the airway. Although the diameter of the airways continues to drop, the resistance falls dramatically as they continue to branch all the way down to the terminal bronchioles. Even though the terminal bronchioles are smaller in diameter than the medium-sized bronchi, there are many, many more of them. When added together, these smaller airways actually have a much greater cross-sectional area than the medium-sized airways. The greater the total cross-sectional area of airways, the lower the resistance through that region.
3. Several physical factors that influence the efficiency of pulmonary ventilation are compliance, alveolar surface tension, and airway resistance. Briefly describe each factor and identify the one that is affecting Mike’s efficiency of breathing.
Lung compliance is the ease with which the lungs inflate. Normally, the lungs are very distensible with just the right amount of elastic recoil to allow easy inspiration and passive expiration. However, if compliance is altered (e.g., fibrosis or emphysema), then breathing requires more energy for the same amount of air flow. Alveolar surface tension describes the attraction (to each other) of water molecules that line the alveolar sacs. This tension would tend to collapse the alveoli if this so-called "alveolar film" did not contain surfactant, which breaks the surface tension. Diminished secretion of surfactant (e.g., as seen in premature infants) allows the alveoli to collapse and greatly increases the work of breathing. Airway resistance is the drag on air flow through the respiratory passageways. Normally, the modest pressure gradient produced by inspiration and expiration is sufficient to overcome the relatively modest airway resistance. Airway resistance has increased in Mike's case. Bronchoconstriction is a narrowing of the conducting airways, which leads to an increase in airway resistance.
4. What must happen to Mike’s intrapulmonary pressure in order for him to maintain normal air flow during inhalation and exhalation when he is having one of his asthma attacks?
Air flow = pressure gradient/resistance. When airway resistance increases (as it does with the bronchoconstriction in asthma), the pressure gradient must increase in order to maintain normal air flow. The pressure gradient is the difference between intrapulmonary and atmospheric pressures, normally about 2 mmHg with quiet breathing. During inhalation, the intrapulmonary pressure must decrease more than usual to move air into the lungs against a higher resistance. During exhalation, the intrapulmonary pressure must increase more than usual to move air out against the same resistance.
5. How does Mike’s body make the necessary changes in intrapulmonary pressure to maintain normal air flow when he is experiencing cold-induced asthma?
Accessory muscles, such as the sternocleidomastoid, pectoralis minor, and scalenes, are recruited to help increase the thoracic cage dimensions on inspiration. They do so by assisting in drawing the thoracic cage upward. Expiration is normally a passive process, but with the increased resistance in asthma abdominal muscles and internal intercostals must be recruited to force air out against resistance. Contracting the abdominal muscles increase intra-abdominal pressure, pushing up on the diaphragm, and contracting the internal intercostals depresses the thoracic cage; these actions assist in decreasing thoracic volume.
6. When Mike is experiencing an asthmatic attack, his forced vital capacity (FVC) is 65%, and his FEV1 is 65%. Are these values normal? Knowing how one performs FVC tests, explain these test results in Mike’s case. (Assume that Mike and the doctor have performed an accurate test.)
Both the FVC and FEV1 should be ( 80%, so Mike's values indicate respiratory dysfunction. Asthma is classified as an obstructive disease because the reactive airways narrow and increase resistance, thus obstructing normal air flow. Despite a maximal effort, Mike cannot generate enough intrapulmonary pressure to compensate entirely for the excessive resistance, so normalized air flow on exhalation, particularly in the first second, cannot happen.
7. Albuterol is a selective beta-2 adrenergic agonist, which means it specifically activates beta-2 adrenergic receptors on smooth muscle in the airways. How does this improve Mike’s asthma?
Activation of beta-2 adrenergic receptors on bronchial smooth muscle sets into motion a second messenger system, which leads to relaxation of smooth muscle. This causes the bronchi and bronchioles to dilate. Bronchodilation decreases resistance and ameliorates the wheezing, coughing, and shortness of breath.
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