- A pneumothorax is a condition where air enters the pleural cavity, the space between the chest wall and the lung. This leads to a collapsed lung because:
Answer: the negative intrapleural pressure that holds the lung open is lost as it equilibrates with atmospheric pressure.
The lungs are held open against their natural elastic recoil by the negative pressure (sub-atmospheric pressure) within the pleural cavity. If this cavity is breached and air enters, the intrapleural pressure equalizes with the atmospheric pressure. The loss of this negative pressure 'suction' allows the lung's inherent elasticity to cause it to collapse. Distractor A is incorrect; the pressure equalizes, it doesn't become highly positive. Distractor B is a contributing factor but the loss of the pressure gradient is the primary mechanism. Distractor D is incorrect; the diaphragm can still contract, but its movement will not inflate the collapsed lung.
- Which statement correctly distinguishes between the processes of gas exchange and cellular respiration?
Answer: Gas exchange is the passive diffusion of gases across a respiratory surface, while cellular respiration is the series of metabolic reactions that generate ATP.
This question addresses a common point of confusion. Gas exchange is a physical process involving the movement of gases (O₂, CO₂) between an organism and its environment across a specialized surface (e.g., alveoli, gills), driven by partial pressure gradients (diffusion). Cellular respiration is a chemical process that occurs within cells (cytoplasm and mitochondria) to break down organic molecules and produce ATP, often using the oxygen obtained via gas exchange. Distractor A reverses the definitions. Distractor C is incorrect as gas exchange occurs in many organisms without lungs, and cellular respiration does not occur in all living cells (e.g., some obligate anaerobes). Distractor D is incorrect as gas exchange is passive and cellular respiration involves many active steps.
- The air inhaled at sea level has a partial pressure of oxygen (pO₂) of approximately 21 kPa. However, the pO₂ in the alveoli is only about 14 kPa. Which factor is the most significant contributor to this reduction?
Answer: The mixing of fresh inspired air with the residual volume of air that remains in the lungs after exhalation.
The air in the lungs is never fully replaced with each breath. Fresh air (high pO₂) mixes with the functional residual capacity of air that remained from the previous cycle (lower pO₂, higher pCO₂). This mixing, along with the saturation of the air with water vapour, lowers the pO₂ of the alveolar gas mixture compared to the inhaled atmospheric air. Distractor A is the reason pO₂ in the capillaries rises, but it is the consequence, not the cause, of the alveolar pO₂. Distractor C is incorrect as gas exchange is passive. Distractor D is incorrect as the partial pressure of nitrogen actually decreases slightly in the alveoli.
- The primary stimulus for changes in the rate and depth of breathing in a healthy person at rest is detected by chemoreceptors. Which event provides the most powerful and direct stimulus to the respiratory centre in the medulla?
Answer: A slight increase in blood pCO₂, leading to a decrease in the pH of the cerebrospinal fluid.
The respiratory centre in the medulla is most sensitive to changes in the pH of the cerebrospinal fluid (CSF). CO₂ from the blood diffuses into the CSF, where it forms carbonic acid and lowers the pH. This provides the primary drive for breathing. Distractor A is incorrect because the response to low pO₂ is much less sensitive and only becomes a major factor at very low oxygen levels (e.g., high altitude). Distractor C is incorrect because lactate and H⁺ ions in the blood do not easily cross the blood-brain barrier to affect the central chemoreceptors directly, though they can stimulate peripheral ones. Distractor D refers to conscious control, not the primary homeostatic mechanism.
- A spirometer records a person's vital capacity as 4.8 L, tidal volume as 0.5 L, and breathing rate as 15 breaths per minute. During maximal exercise, their breathing rate increases to 45 breaths per minute and their tidal volume increases to 3.5 L. What is the approximate increase in their minute ventilation from rest to maximal exercise?
Answer: 150.0 L·min⁻¹
Minute ventilation = Tidal Volume × Breathing Rate. First, calculate resting minute ventilation: 0.5 L/breath × 15 breaths/min = 7.5 L·min⁻¹. Next, calculate exercise minute ventilation: 3.5 L/breath × 45 breaths/min = 157.5 L·min⁻¹. The question asks for the increase, so subtract: 157.5 L·min⁻¹ - 7.5 L·min⁻¹ = 150.0 L·min⁻¹. The vital capacity information is not needed for this calculation.
- In large, active insects, the tracheal system is ventilated by rhythmic body movements. If this ventilation process is experimentally inhibited, what is the first consequence for gas exchange at the tissues?
Answer: The concentration gradient for oxygen between the tracheoles and the muscle cells will decrease.
Active ventilation in insects functions to replenish the air in the main tracheae, maintaining a high pO₂ deep within the body. If this process is stopped, respiring cells will continue to consume O₂, lowering the pO₂ at the ends of the tracheoles. This reduces the partial pressure gradient between the air in the tracheoles and the mitochondria in the cells, slowing the rate of oxygen diffusion to the tissues. Distractor A will also happen, but the drop in the O₂ gradient is the most immediate problem for aerobic respiration. Distractor B describes a mechanism to increase diffusion, which would not happen if ventilation is inhibited. Distractor C is incorrect as most insects do not use hemolymph or pigments for O₂ transport.
- Which of the following comparisons between the gas exchange systems of a mammal and an insect is correct?
Answer: The mammal's circulatory system is integral to gas transport from the exchange surface, whereas the insect's is not.
A key difference is the role of the circulatory system. In mammals, oxygen diffuses from the alveoli into the blood, where it is transported by hemoglobin to the tissues. In insects, the tracheal system is a network of air-filled tubes that extends directly to the respiring cells, bypassing the need for the circulatory system (hemolymph) to transport gases. A is incorrect; ventilation is an active process in mammals and in active insects. C is incorrect; insects do not use hemoglobin for oxygen transport. D is incorrect; both respiratory surfaces are internal.
- Carbon monoxide poisoning is life-threatening. Its primary mechanism of toxicity involves its interaction with hemoglobin. Which statement most accurately describes this interaction and its consequence?
Answer: CO competes with oxygen for binding sites on hemoglobin, but also increases the affinity of the remaining sites for oxygen.
CO has an affinity for hemoglobin over 200 times that of O₂. It competitively inhibits O₂ binding, reducing the oxygen-carrying capacity. A more subtle and dangerous effect is that CO binding to one site on a hemoglobin molecule increases the binding affinity of the remaining sites for O₂. This shifts the oxygen dissociation curve to the left and makes it harder for the bound oxygen to be released to the tissues. Distractor A describes the action of certain inhibitors but not CO. Distractor B describes methemoglobinemia, caused by other agents. Distractor D is incorrect; CO is a systemic poison that does not cause direct lung injury.
- Pulmonary surfactant is a substance that prevents the collapse of alveoli. In cases of respiratory distress syndrome in premature infants, insufficient surfactant is produced. What is the primary biophysical reason for the resulting difficulty in breathing?
Answer: High surface tension of the fluid lining the alveoli creates a large pressure that opposes inflation.
Surfactant is a complex of lipids and proteins that reduces the surface tension of the aqueous film lining the alveoli. Without it, the high surface tension of water would cause the small alveoli to collapse and require a very large pressure to re-inflate during inspiration. Distractor A is incorrect; surfactant does not change the thickness of the epithelial cells. Distractor C is incorrect; the problem is with inflation, not the loss of elastic recoil, which is more characteristic of emphysema. Distractor D is incorrect; the composition of inspired air is unchanged.
- An athlete moves from sea level to train at high altitude. After several weeks of acclimatization, which physiological adaptation would be most effective in enhancing oxygen delivery to their muscles?
Answer: An increase in the concentration of red blood cells and hemoglobin circulating in the bloodstream.
The most significant long-term adaptation to high altitude is an increase in the oxygen-carrying capacity of the blood. The kidneys release erythropoietin (EPO), which stimulates the bone marrow to produce more red blood cells (and thus more hemoglobin). This allows the blood to carry more oxygen per unit volume, compensating for the lower pO₂. Distractor A is a short-term response; hyperventilation is not sustained indefinitely. Distractor B describes a change that may occur due to increased 2,3-BPG, but the increase in hemoglobin is the more critical adaptation. Distractor D is incorrect; the lung's surface area is anatomically fixed and does not increase during acclimatization.
- In the counter-current exchange system of fish gills, blood flows through the lamellae in the opposite direction to the water flowing over them. What would be the immediate consequence if the system were a concurrent flow (blood and water in the same direction)?
Answer: The partial pressure of oxygen in the blood would reach equilibrium with the water at approximately 50% of the water's initial oxygen concentration.
In a concurrent system, blood and water flow in the same direction. A steep diffusion gradient exists initially, but it rapidly decreases as both fluids approach equilibrium. Equilibrium would be reached when the pO₂ in both blood and water is equal, at about 50% of the initial water pO₂. This is far less efficient than counter-current flow, which maintains a gradient across the entire length of the lamella, allowing blood pO₂ to exceed 80% saturation. Distractor B is incorrect; full saturation is not possible. Distractor C is incorrect; the CO₂ gradient would also become less efficient. Distractor D is incorrect; the greatest diffusion rate would be at the beginning of the lamella where the gradient is steepest.
- The oxygen dissociation curve of fetal hemoglobin is shifted to the left compared to that of adult hemoglobin. What is the physiological advantage of this property for gas exchange at the placenta?
Answer: It enables fetal hemoglobin to bind oxygen tightly at the low partial pressures present in the maternal blood of the placenta.
A left-shifted curve indicates a higher affinity for oxygen. In the placenta, the pO₂ is relatively low. Fetal hemoglobin's higher affinity allows it to effectively 'pull' oxygen away from the mother's hemoglobin and become saturated, ensuring the fetus receives adequate oxygen. Distractor A is incorrect; a higher affinity means it releases oxygen less readily at a given pO₂ (this is overcome by the very low pO₂ in fetal tissues). Distractor C is incorrect; the shift affects affinity, not the carrying capacity per gram of hemoglobin. Distractor D is incorrect; the curve relates to oxygen binding, not directly to CO₂ transport.
- An individual has a genetic disorder that results in the production of abnormally thick and sticky mucus lining their airways. How would this condition directly impede gas exchange?
Answer: The mucus layer increases the diffusion distance between the alveolar air and the pulmonary capillaries.
According to Fick's law of diffusion, the rate of diffusion is inversely proportional to the thickness of the diffusion barrier. An abnormally thick layer of mucus increases the distance that oxygen must travel to reach the blood and that carbon dioxide must travel to be exhaled. This slows the rate of gas exchange. Additionally, the mucus can block smaller airways, preventing ventilation of some alveoli. A describes a surfactant deficiency. C is biochemically incorrect as mucus does not chemically bind oxygen. D is incorrect as this condition affects mucus secretion, not neural control.
- The Bohr effect describes the rightward shift of the oxygen-hemoglobin dissociation curve in response to increased pCO₂ or decreased pH. Where in the body is this effect most physiologically significant for gas exchange?
Answer: In the systemic capillaries of exercising muscle, where high CO₂ promotes the unloading of oxygen from hemoglobin.
The Bohr effect is most significant in metabolically active tissues. During exercise, muscle cells produce large amounts of CO₂ and lactic acid, which lowers the pH of the surrounding blood. This drop in pH (and increase in pCO₂) causes hemoglobin to decrease its affinity for oxygen, shifting the curve to the right and facilitating the release of a larger amount of oxygen to the tissues that need it most. In the alveoli (A), the opposite occurs (Haldane effect), where high O₂ and low CO₂ promote oxygen loading. C and D describe locations but not the mechanism of the Bohr effect.
- Birds possess a unique respiratory system with unidirectional air flow across parabronchi, which is more efficient than the tidal flow in mammals. What feature of the avian system explains this enhanced efficiency?
Answer: It allows gas exchange to occur during both inhalation and exhalation, continuously refreshing the gas exchange surface with oxygenated air.
Due to the arrangement of air sacs, it takes two full respiratory cycles for a bolus of air to move through the avian system. This complex pathway ensures that air flowing over the gas exchange surfaces (parabronchi) is always fresh, oxygenated air moving in one direction, both during the first inhalation/exhalation and the second. This avoids the mixing of fresh and stale air that occurs in mammalian lungs, maintaining a steeper concentration gradient. B is incorrect; it's a cross-current exchange, not counter-current. C is true, but it's the flow system, not the storage volume, that is the key to efficiency. D is incorrect; dead space still exists in the trachea and primary bronchi.
- Which sequence correctly describes the mechanism of a quiet inspiration in a human?
Answer: Diaphragm and external intercostals contract, thoracic volume increases, alveolar pressure drops below atmospheric pressure, air flows in.
The correct causal sequence for inspiration is: 1) Contraction of respiratory muscles (diaphragm, external intercostals). 2) This increases the volume of the thoracic cavity. 3) The increase in volume leads to a drop in intra-alveolar pressure to below atmospheric pressure. 4) Air then flows down the pressure gradient into the lungs. All other options present an incorrect or illogical sequence of these events.
- Emphysema is a lung disease where the walls of the alveoli break down and merge. How does this structural change most significantly impair gas exchange?
Answer: It reduces the total surface area for diffusion and decreases the elastic recoil of the lungs.
The defining characteristic of emphysema is the destruction of alveolar walls. This merging of alveoli drastically reduces the total surface area available for gas diffusion. A secondary effect is the loss of elastin fibres, which reduces the lung's ability to recoil passively during expiration, making it difficult to exhale. Distractor A describes chronic bronchitis. Distractor B describes pulmonary fibrosis. Distractor D is a consequence of smoking but not the primary mechanism of impairment in emphysema itself.
