MCAT Biology Review
Chapter 6: The Respiratory System
1. All of the following facilitate gas exchange in the lungs EXCEPT:
1. thin alveolar walls.
2. multiple subdivisions of the respiratory tree.
3. differences in the partial pressures of O2 and CO2.
4. active transporters in alveolar cells.
2. Which of the following associations correctly pairs a stage of respiration with the muscle actions occurring during that stage?
1. Inhalation—diaphragm relaxes
2. Inhalation—internal intercostal muscles contract
3. Exhalation—diaphragm contracts
4. Exhalation—external intercostal muscles relax
3. Total lung capacity is equal to the vital capacity plus the:
1. tidal volume.
2. expiratory reserve volume.
3. residual volume.
4. inspiratory reserve volume.
4. The intrapleural pressure is necessarily lower than the atmospheric pressure during:
1. inhalation, because the expansion of the chest cavity causes compression of the intrapleural space, decreasing its pressure.
2. inhalation, because the expansion of the chest cavity causes expansion of the intrapleural space, decreasing its pressure.
3. exhalation, because the compression of the chest cavity causes compression of the intrapleural space, decreasing its pressure.
4. exhalation, because the compression of the chest cavity causes expansion of the intrapleural space, decreasing its pressure.
5. A patient presents to the emergency room with a stab wound to the left side of the chest. On a chest x-ray, blood is noted to be collecting in the chest cavity, causing collapse of both lobes of the left lung. The blood is most likely located between:
1. the parietal pleura and the chest wall.
2. the parietal pleura and the visceral pleura.
3. the visceral pleura and the lung.
4. the alveolar walls and the lung surface.
6. Each of the following statements regarding the anatomy of the respiratory system is true EXCEPT:
1. the epiglottis covers the glottis during swallowing to ensure that food does not enter the trachea.
2. the trachea and bronchi are lined by ciliated epithelial cells.
3. the pharynx contains two vocal cords, which are controlled by skeletal muscle and cartilage.
4. the nares are lined with vibrissae, which help filter out particulate matter from inhaled air.
7. Which of the following is a correct sequence of passageways through which air travels during inhalation?
1. Pharynx → trachea → bronchioles → bronchi → alveoli
2. Pharynx → trachea → larynx → bronchi → alveoli
3. Larynx → pharynx → trachea → bronchi → alveoli
4. Pharynx → larynx → trachea → bronchi → alveoli
8. Idiopathic pulmonary fibrosis (IPF) is a disease in which scar tissue forms in the alveolar walls, making the lung tissue significantly more stiff. Which of the following findings would likely be detected through spirometry in a patient with IPF?
1. Decreased total lung capacity
2. Decreased inspiratory reserve volume
3. Increased residual volume
1. I only
2. II only
3. I and II only
4. I, II, and III
9. Studies have indicated that premature babies are often deficient in lysozyme. What is a possible consequence of this deficiency?
1. Respiratory distress and alveolar collapse shortly after birth
2. Increased susceptibility to certain infections
3. Inability to humidify air as it passes through the nasal cavity
4. Slowing of the respiratory rate in response to acidemia
10.Some forms of pneumonia cause an excess of fluids such as mucus or pus to build up within an entire lobe of the lung. How will this affect the diffusion of gases within the affected area?
1. Carbon dioxide can diffuse out, but oxygen will not be able to enter the blood.
2. Oxygen can diffuse into the blood, but carbon dioxide cannot diffuse out.
3. No change in diffusion will occur.
4. No diffusion will occur in the affected area.
11.Some people with anxiety disorders respond to stress by hyperventilating. It is recommended that these people breathe into a paper bag and then rebreathe this air. Why is this treatment appropriate?
1. Hyperventilation causes an increase in blood carbon dioxide, and breathing the air in the bag helps to readjust blood levels of carbon dioxide.
2. Hyperventilation causes a decrease in blood carbon dioxide, and breathing the air in the bag helps to readjust blood levels of carbon dioxide.
3. Hyperventilation causes an increase in blood oxygen, and breathing the air in the bag helps to readjust blood levels of oxygen.
4. Hyperventilation causes a decrease in blood oxygen, and breathing the air in the bag helps to readjust blood levels of oxygen.
12.A patient presents to the emergency room with an asthma attack. The patient has been hyperventilating for the past hour and has a blood pH of 7.52. The patient is given treatment and does not appear to respond, but a subsequent blood pH reading is 7.41. Why might this normal blood pH NOT be a reassuring sign?
1. The patient’s kidneys may have compensated for the alkalemia.
2. The normal blood pH reading is likely inaccurate.
3. The patient may be descending into respiratory failure.
4. The patient’s blood should ideally become acidemic for some time to compensate for the alkalemia.
13.Premature infants with respiratory distress are often placed on ventilators. Often, the ventilators are set to provide positive end-expiratory pressure. Why might this setting be useful for a premature infant?
1. Premature infants lack surfactant.
2. Premature infants lack lysozyme.
3. Premature infants cannot thermoregulate.
4. Premature infants are unable to control pH.
14.In emphysema, the alveolar walls are destroyed, decreasing the recoil of the lung tissue. Which of the following changes may be seen in a patient with emphysema?
1. Increased residual volume
2. Decreased total lung capacity
3. Increased blood concentration of oxygen
4. Decreased blood concentration of carbon dioxide
15.Allergic reactions occur due to an overactive immune response to a substance. Which cells within the respiratory tract play the largest role in the generation of allergic reactions?
1. Alveolar epithelial cells
3. Mast cells
4. Ciliated epithelial cells
Answers and Explanations
Gas exchange in the lungs relies on passive diffusion of oxygen and carbon dioxide. This is accomplished easily because there is always a difference in the partial pressures of these two gases and because the subdivision of the respiratory tree creates a large surface area of interaction between the alveoli and the circulatory system. In addition, the thin alveolar walls allow for fast diffusion and gas exchange. Therefore, choices (A), (B), and (C) can be eliminated. Choice (D) is the correct answer because active transport is not used in the gas exchange process in the lungs.
The muscles involved in ventilation are the diaphragm, which separates the thoracic cavity from the abdominal cavity, and the intercostal muscles between the ribs. During inhalation, the diaphragm contracts and flattens, while the external intercostal muscles contract, pulling the rib cage up and out. These actions cause an overall increase in the volume of the thoracic cavity. During exhalation, both the diaphragm and the external intercostals relax, causing a decrease in the volume of the thoracic cavity because of recoil of these tissues. In forced exhalation, the internal intercostals and abdominal muscles may contract to force out air. Thus, the only correct association from the given answers is choice (D).
Total lung capacity is equal to the vital capacity (the maximum volume of air that can be forcibly inhaled and exhaled from the lungs) plus the residual volume (the air that always remains in the lungs, preventing the alveoli from collapsing).
During inhalation, the chest cavity expands, causing expansion of the intrapleural space. According to Boyle’s law, an increase in volume is accompanied by a decrease in pressure. When the intrapleural pressure (and, by extension, the alveolar pressure) is less than atmospheric pressure, air enters the lungs. During exhalation, these pressure gradients reverse; thus, during exhalation, intrapleural pressure is higher than atmospheric pressure, not lower.
The intrapleural space, bounded by the parietal and visceral pleurae, is a potential space. As such, it is normally collapsed and contains a small amount of fluid. However, introduction of fluid or air into the intrapleural space can fill the space, causing collapse of the lung. The other options listed are too firmly apposed to permit blood to collect in these spaces.
The pharynx, which lies behind the nasal cavity and oral cavity, is a common pathway for food entering the digestive system and air entering the respiratory system. It is the larynx that contains the vocal cords, not the pharynx.
Air enters the respiratory tract through the external nares (nostrils) and travels through the nasal cavities. It then passes through the pharynx and into the larynx. Ingested food also passes through the pharynx on its way to the esophagus; to ensure that food does not accidentally enter the larynx, the epiglottis covers the larynx during swallowing. After the larynx, air goes to the trachea, which eventually divides into two bronchi, one for each lung. The bronchi branch into smaller bronchioles, which terminate in clusters of alveoli. From the given sequences, onlychoice (D) correctly describes the sequence of the passages through which air travels.
In a patient with IPF, the increased stiffness of the lungs would likely decrease the volume of air the individual could inhale, which would decrease both the total lung capacity and inspiratory reserve volume. However, spirometry cannot measure the total lung capacity accurately because it cannot determine the residual volume—the volume of air left in the lungs when an individual has maximally exhaled. Because the residual volume makes up a portion of the total lung capacity (total lung capacity = vital capacity + residual volume), a spirometer cannot be used to determine the total lung capacity. Therefore, while Statement I is a true statement, it cannot appear in the answer choice. Finally, increased stiffness of the lungs would be expected to decrease the residual volume, not increase it; further, residual volume, as described above, cannot be measured with a spirometer.
Lysozyme is an enzyme present in the nasal cavity, saliva, and tears that degrades peptidoglycan, preventing infection by gram-positive organisms. Thus, premature infants who lack lysozyme are more likely to suffer from infections with these organisms.
If an area of the lung becomes filled with mucus and inflammatory cells, the area will not be able to participate in gas exchange. Because no air will enter or leave the area, the concentration gradient will no longer exist, and neither oxygen nor carbon dioxide will be able to diffuse across the alveolar wall.
When people hyperventilate, their respiratory rate increases. When the respiratory rate increases, more carbon dioxide is blown off. This causes a shift to the left in the bicarbonate buffer equation, and the blood becomes more alkaline. Breathing into the bag allows some of this carbon dioxide to be returned to the bloodstream in order to maintain the proper pH.
When a patient with an asthma attack does not respond to treatment and has been hyperventilating for over an hour, he or she may become fatigued and may not be able to maintain hyperventilation. In this case, the patient begins to decrease his or her breathing rate and is not receiving adequate oxygen. By extension, carbon dioxide is trapped in the blood, and the pH begins to drop. Despite the fact that this pH is normal at the moment, this patient is crashing and may start demonstrating acidemia in the near future. While the kidneys should compensate for alkalemia, this is a slow process and would not normalize the blood pH within an hour; further, adequate compensation by the kidneys would actually be a reassuring sign, eliminating choice (A). There is no evidence to believe the measurement was inaccurate, eliminatingchoice (B). Finally, after treatment, the patient should return to a normal blood pH with adequate ventilation and would not be expected to overcompensate by becoming acidemic, eliminating choice (D).
This question requires a few different levels of thinking. The question stem states that premature infants often require ventilation using positive end-expiratory pressures. While you are not expected to know ventilator settings for the MCAT, you should be able to decode what this phrase means: at the end of expiration, the ventilator will provide a higher pressure than normal, which forces extra air into the alveoli. This pressure must be used to prevent alveolar collapse, which should remind you that surfactant serves the same purpose by reducing surface tension. Thus, it makes sense that if premature babies lack surfactant, providing extra air pressure at the end of expiration would be beneficial.
The intrinsic elastic properties of the lung are important during exhalation as the passive recoil of lung tissue helps decrease lung volume. With decreased recoil, the patient will have difficulty exhaling completely, increasing the residual volume. The total lung capacity would be expected to increase in this case because there would be less recoil opposing inhalation, eliminating choice (B). With decreased alveolar surface area, one would expect decreased gas exchange, which would decrease blood concentrations of oxygen while increasing blood concentrations of carbon dioxide, eliminating choices (C) and (D).
Allergic reactions occur when a substance binds to an antibody and promotes an overactive immune response with inflammatory chemicals. The antibody is already attached to a mast cell. Thus, when the substance binds to the antibody, the mast cell can release the inflammatory mediators that cause allergic reactions.