Test: Respiratory System - 1 - MCAT MCQ

Bronchodilators, which act on β-adrenergic receptors, induce smooth muscle relaxation in the airways, leading to bronchodilation. The anatomic distribution of β-adrenergic receptors is more prominent in the larger airways, such as the lobar bronchi. These bronchi are part of the conducting zone of the respiratory system, which includes the trachea, bronchi, and bronchioles. The primary function of the conducting zone is to transport air to and from the respiratory zone.

The alveoli, on the other hand, are small, thin-walled air sacs found in the respiratory zone of the lungs. They are responsible for gas exchange, where oxygen is taken up by the bloodstream and carbon dioxide is released. While bronchodilators may indirectly impact alveolar function by improving airflow, their primary effect is on the smooth muscles in the larger airways, such as the lobar bronchi.

During normal exhalation, the force that drives the process is primarily the elastic recoil of the lungs and chest wall. When we inhale, the muscles involved in breathing, such as the diaphragm and intercostal muscles, contract and expand the thoracic cavity, allowing air to enter the lungs. Once inhalation is complete, these muscles relax, and the elastic fibers in the lungs and chest wall passively recoil. This recoil generates a force that helps push air out of the lungs during exhalation.

The diaphragm and intercostal muscles are involved in active processes during inhalation. The diaphragm contracts and moves downward, while the intercostal muscles contract to lift the ribcage and increase the volume of the thoracic cavity. However, during normal exhalation, these muscles are at rest, and the process is primarily driven by the passive recoil of elastic tissues.

Therefore, option C is the correct answer. The force driving normal exhalation is the elastic force, and the process is passive.

In a situation where the respiratory bronchioles become inflamed and narrowed, such as in asthma, the most significant mechanical impairment is observed during forced expiration. During normal expiration, the process is primarily passive and relies on the elastic recoil of the lungs and chest wall to push air out of the lungs. However, when the bronchioles are narrowed due to inflammation, there is increased resistance to the flow of air, making it more difficult to exhale forcefully.

In asthma, bronchoconstriction and airway inflammation lead to increased resistance in the lower airways, making expiration more challenging. The narrowing of the bronchioles can restrict the smooth flow of air out of the lungs, particularly during forced expiration when greater respiratory effort is required.

Therefore, option D is the correct answer. The mechanical impairment of narrowed respiratory bronchioles, as seen in asthma, most significantly affects forced expiration, making it more difficult to exhale air forcefully.

The pressure of gas within the alveoli at the peak of inspiration, just before expiration, is the same as atmospheric air.

Explanation: During the peak of inspiration, just before expiration, the pressure of gas within the alveoli is equal to atmospheric pressure. This occurs when the lungs are at their maximum inhalation and the volume of the alveoli is increased.

At rest, the respiratory system operates under the principle of equalizing pressure between the alveoli and the atmosphere. The pressure within the alveoli is maintained at atmospheric pressure to ensure efficient exchange of gases between the alveoli and the pulmonary capillaries.

During inspiration, the diaphragm and other respiratory muscles contract, causing the thoracic cavity to expand. This expansion leads to an increase in lung volume, resulting in a decrease in alveolar pressure. As a result, air flows from the atmosphere, where the pressure is higher, into the alveoli to equalize the pressures.

At the peak of inspiration, when the inhalation is at its maximum, the pressure within the alveoli is the same as atmospheric pressure. This allows for effective gas exchange to occur in the lungs.

In the setting of severe interstitial lung disease (ILD), several factors can contribute to a decreased extent of oxygen passing from the air sacs of the lungs into the blood. These factors include increased lung elastic recoil, decreased lung capacity, and increased alveolar surface tension. However, decreased interstitial thickness would not contribute to this decrease in oxygen transfer.

In ILD, the pulmonary interstitium becomes thickened due to inflammation, fibrosis, or scarring. This increased interstitial thickness can impair gas exchange by increasing the diffusion distance between the alveoli and the alveolar capillaries. As a result, it becomes more difficult for oxygen to cross the thickened interstitium and reach the bloodstream.

Therefore, if the interstitial thickness is decreased in the setting of severe ILD, it would not further impede the transfer of oxygen. In fact, a decrease in interstitial thickness would potentially improve gas exchange by reducing the diffusion distance and facilitating the passage of oxygen from the air sacs of the lungs into the blood.

Hence, option A is the correct answer. Decreased interstitial thickness would not decrease the extent to which oxygen passes from the air sacs of the lungs into the blood in the setting of severe ILD.

In septic shock, there is a dysregulated immune response to a systemic bacterial infection, leading to widespread inflammation and endothelial dysfunction. This can result in increased capillary blood flow as a compensatory mechanism to improve tissue perfusion and oxygen delivery.

However, increased capillary blood flow can reduce the ability of hemoglobin in the alveolar capillaries to become fully saturated with oxygen. This is because the increased blood flow can lead to a shorter transit time of red blood cells through the alveolar capillaries. As a result, there is less time for oxygen to equilibrate between the alveolar air and the blood, leading to a decreased ability of hemoglobin to fully bind with oxygen.

Therefore, in septic shock, the increased capillary blood flow can impair the ability of hemoglobin in the alveolar capillaries to become fully saturated with oxygen, contributing to a decrease in oxygen uptake and delivery.

• Ventilation refers to air reaching a particular area of the lung; perfusion refers to the blood supply to a particular area of the lung. Though, both are important for gas exchange to occur, the question only asks about ventilation.
• Ventilation would be decreased in any setting which does not allow adequate airflow, including obstruction and structural/mechanical changes to the lung which prevent alveolar filling.
• Increased pressure within an alveolus would prevent airflow into the alveolar space.
• Gas pressure is increased with increasing temperature and decreasing container volume.
• Increased elastic recoil of the alveolar wall would increase the inward force of the wall on the gas as the wall tried to collapse, which would increase the pressure of gases within an alveolus, which would hinder airflow into the space.
• Increased alveolar elastic recoil would most greatly reduce the degree to which a particular alveolus is ventilated.

When the breath is held after a complete inspiration, the closure of the mouth and nose prevents the escape of air from the lungs. As a result, the volume of air in the lungs remains constant, but the metabolic activity of the body continues, leading to the production of carbon dioxide (CO₂) as a waste product.

As CO₂ accumulates in the alveoli, it diffuses into the pulmonary capillaries and enters the bloodstream. This results in an increase in the partial pressure of CO₂ in the alveoli. The increased partial pressure of CO₂ in the alveoli leads to a higher total pressure of gases within the alveoli, including both oxygen (O₂) and CO₂. This higher total pressure is greater than the pressure of atmospheric air.

Therefore, the pressure of gases within the alveoli, including O₂ and CO₂, is greater than the pressure of atmospheric air when the breath is held after a complete inspiration with the mouth and nose closed.

During inhalation, the diaphragm contracts and moves downward, causing it to flatten. This results in an increase in the volume of the thoracic cavity. According to Boyle's law, an increase in volume leads to a decrease in pressure. Therefore, the pressure within the thoracic cavity decreases during inhalation, creating a pressure gradient that allows air to flow into the lungs.

The epiglottis is a flap of cartilage located at the base of the tongue. During swallowing, it moves downward to cover the opening of the larynx (voice box) and prevent food and drink from entering the airway. This ensures that the food and drink are directed into the esophagus and down to the stomach, rather than going into the respiratory system. The vocal cords, trachea, and alveoli are not directly involved in the process of preventing food and drink from entering the airway during swallowing.