USMLE Step 1 Quiz: Respiratory Physiology And Gas Exchange

What this quiz covers

This quiz focuses on Respiratory Physiology And Gas Exchange, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Ventilation of a non-perfused region (high V/Q)
2. Perfusion of a non-ventilated region (low V/Q) (correct answer)
3. A global decrease in lung compliance
4. A global increase in airway resistance

Explanation: In lobar pneumonia, the alveoli are filled with fluid and cellular debris, preventing ventilation (V ≈ 0). However, pulmonary blood flow (perfusion, Q) to this area continues. This creates an extreme V/Q mismatch known as an intrapulmonary shunt (V/Q ≈ 0). Deoxygenated blood passes through the pulmonary circulation without being oxygenated, leading to systemic hypoxemia.

Question 2

1. Intrapulmonary shunt
2. Anatomic dead space
3. Physiologic dead space (correct answer)
4. Airway resistance

Explanation: A pulmonary embolism obstructs blood flow to a region of the lung. This creates an area that is ventilated but not perfused (or underperfused). This is known as alveolar dead space. The sum of anatomic dead space (conducting airways) and alveolar dead space is physiologic dead space. The large increase in physiologic dead space represents wasted ventilation, leading to a severe ventilation/perfusion (V/Q) mismatch and hypoxemia.

Question 3

1. Surface area of the respiratory membrane
2. Partial pressure gradient of oxygen
3. Thickness of the respiratory membrane (correct answer)
4. Solubility of oxygen in the fluid

Explanation: Fick's law states that the rate of gas diffusion across a barrier is inversely proportional to the thickness of that barrier. In pulmonary edema, fluid accumulates in the interstitial space and alveoli, increasing the distance that oxygen must travel from the alveolar air to the red blood cells in the pulmonary capillaries. This increased diffusion distance significantly impairs the rate of gas exchange, leading to hypoxemia.

Question 4

1. Chloride ion into the red blood cell (correct answer)
2. Potassium ion out of the red blood cell
3. Sodium ion into the red blood cell
4. Hydrogen ion out of the red blood cell

Explanation: This process is known as the chloride shift or Hamburger phenomenon. To maintain electrical neutrality as the negatively charged bicarbonate ion (HCO₃⁻) exits the red blood cell, a negatively charged chloride ion (Cl⁻) moves from the plasma into the red blood cell. This exchange is mediated by the anion exchanger 1 protein (band 3 protein). The reverse process occurs in the lungs.

Question 5

1. It becomes equal to atmospheric pressure. (correct answer)
2. It becomes significantly more negative than atmospheric pressure.
3. It becomes equal to the intra-alveolar pressure.
4. It becomes more positive than atmospheric pressure.

Explanation: Normally, the intrapleural pressure is negative (subatmospheric) due to the opposing elastic recoil of the chest wall (outward) and the lungs (inward). A pneumothorax occurs when the pleural space is breached, allowing it to communicate with the atmosphere. This causes air to rush in, and the intrapleural pressure equilibrates with the atmospheric pressure, becoming approximately zero, which leads to lung collapse.

Question 6

1. Increased airway resistance
2. Decreased lung compliance (correct answer)
3. Increased physiologic dead space
4. Decreased elastic recoil of the chest wall

Explanation: Idiopathic pulmonary fibrosis is a restrictive lung disease characterized by the deposition of fibrous tissue in the lung interstitium. This makes the lungs stiff and difficult to inflate, which is defined as decreased lung compliance. A greater pressure change is required to generate a given change in volume, significantly increasing the work of breathing, particularly during inspiration.

Question 7

1. Dissolved CO₂ in plasma
2. Bound to hemoglobin as carbaminohemoglobin
3. As bicarbonate (HCO₃⁻) ions (correct answer)
4. As carbonic acid (H₂CO₃) within red blood cells

Explanation: Carbon dioxide is transported in the blood in three forms: dissolved in plasma (~7%), bound to hemoglobin as carbaminohemoglobin (~23%), and as bicarbonate (HCO₃⁻) ions (~70%). The majority of CO₂ diffuses into red blood cells, where carbonic anhydrase rapidly converts it to carbonic acid (H₂CO₃), which then dissociates into H⁺ and HCO₃⁻. The HCO₃⁻ is then transported out of the RBC into the plasma in exchange for Cl⁻ (the chloride shift).

Question 8

1. 1.5 (correct answer)
2. 2.4
3. 0.9
4. 3.3

Explanation: Alveolar ventilation (Va) is the volume of fresh air that reaches the alveoli per minute and is available for gas exchange. It is calculated as: Va = (Tidal Volume - Anatomic Dead Space) × Respiratory Rate. In this case, Va = (400 mL - 150 mL) × 6 breaths/min = 250 mL/breath × 6 breaths/min = 1500 mL/min, which is equal to 1.5 L/min. This low alveolar ventilation is the cause of his respiratory acidosis.

Question 9

1. A shift to the left, increasing hemoglobin's affinity for oxygen
2. A shift to the right, facilitating the unloading of oxygen in tissues (correct answer)
3. An increase in the maximal oxygen-carrying capacity
4. A decrease in the P50 value

Explanation: Factors that shift the oxygen-hemoglobin dissociation curve to the right include increased temperature, increased PCO₂, increased 2,3-BPG, and decreased pH (acidemia). A rightward shift signifies a decreased affinity of hemoglobin for oxygen. This change facilitates the release (unloading) of oxygen from hemoglobin to the peripheral tissues, which is an adaptive response in states of high metabolic demand like sepsis.

Question 10

1. Bohr effect
2. Haldane effect (correct answer)
3. Chloride shift
4. Fick principle

Explanation: The Haldane effect describes the phenomenon where oxygenation of hemoglobin in the lungs decreases its affinity for CO₂. This occurs because oxygenated hemoglobin is a stronger acid and releases H⁺ ions, which combine with bicarbonate to form CO₂. Additionally, oxygenated hemoglobin has a lower affinity for binding CO₂ directly (as carbaminohemoglobin). Both mechanisms facilitate the unloading of CO₂ from the blood into the alveoli.

Question 11

1. Bronchodilation in adjacent lung segments
2. Increased heart rate and cardiac output
3. Vasoconstriction of pulmonary arterioles in the affected segment (correct answer)
4. A rightward shift of the oxygen-hemoglobin dissociation curve

Explanation: The primary mechanism for matching pulmonary perfusion to ventilation is hypoxic pulmonary vasoconstriction. In response to low alveolar PO₂ (hypoxia) in the obstructed segment, the surrounding pulmonary arterioles constrict. This response shunts blood away from the poorly ventilated region toward better-ventilated areas of the lung, thereby improving the overall V/Q matching and reducing the magnitude of the intrapulmonary shunt.

Question 12

1. Contraction of bronchial smooth muscle
2. Complete closure of the alveoli
3. Compression of the airways by high intrapleural pressure (correct answer)
4. A sharp increase in the elastic recoil of the lungs

Explanation: During forced expiration, the intrapleural pressure becomes highly positive. This positive pressure is transmitted to the outside of the smaller, non-cartilaginous airways. When this external pressure exceeds the pressure inside the airway, the airway is compressed, a phenomenon known as dynamic compression. This compression increases resistance and limits the maximal expiratory flow rate, regardless of how much harder the person tries to exhale.

Question 13

1. Hypoventilation
2. Residence at high altitude
3. Ventilation-perfusion (V/Q) mismatch (correct answer)
4. Severe anemia

Explanation: The A-a gradient is the difference between alveolar oxygen (PAO₂) and arterial oxygen (PaO₂). Here, the gradient is 100 mmHg - 65 mmHg = 35 mmHg. A normal A-a gradient is typically less than 15 mmHg (or estimated as [Age/4] + 4). A significantly elevated A-a gradient, as seen in this patient, indicates a problem with gas exchange within the lungs, such as a V/Q mismatch, diffusion limitation, or a shunt. Hypoventilation and breathing air with low FiO₂ (like at high altitude) cause hypoxemia with a normal A-a gradient.

Question 14

1. The affinity of hemoglobin for oxygen
2. The pH of the blood
3. Erythrocyte concentration of 2,3-bisphosphoglycerate (correct answer)
4. The partial pressure of inspired oxygen

Explanation: Acclimatization to high altitude involves several changes to improve oxygen delivery in the setting of chronic hypoxia. One key change is an increase in the synthesis of 2,3-bisphosphoglycerate (2,3-BPG) by red blood cells. 2,3-BPG binds to deoxygenated hemoglobin and decreases its affinity for oxygen. This causes a rightward shift of the oxygen-hemoglobin dissociation curve, which facilitates the unloading of oxygen to the peripheral tissues.

Question 15

1. Increased PaO₂
2. Decreased PaCO₂
3. Increased PaCO₂ (correct answer)
4. No change in PaCO₂

Explanation: Alveolar ventilation is the portion of minute ventilation that reaches the alveoli for gas exchange. With rapid, shallow breathing, a larger fraction of each tidal volume ventilates the anatomic dead space (conducting airways). This means that even if minute ventilation is constant, alveolar ventilation decreases. Reduced alveolar ventilation leads to inadequate removal of CO₂, causing the arterial PCO₂ (PaCO₂) to rise, which can result in respiratory acidosis.

Question 16

1. As carbamino compounds with plasma proteins
2. Freely dissolved as CO₂ gas in plasma
3. As carbonic acid within erythrocytes
4. As bicarbonate ions in plasma and erythrocytes (correct answer)

Explanation: Regardless of the total amount of CO₂ in the blood, the transport mechanisms remain the same. The conversion of CO₂ to bicarbonate (HCO₃⁻) via carbonic anhydrase is by far the most efficient and highest capacity system for CO₂ transport. Therefore, even in a state of severe CO₂ retention (hypercapnia), approximately 70% of the total CO₂ content in the blood is carried as bicarbonate ions.

Question 17

1. Competitive inhibition of oxygen binding sites by the anesthetic
2. Oxidation of heme iron from the ferrous (Fe²⁺) to the ferric (Fe³⁺) state (correct answer)
3. A rightward shift of the oxygen-hemoglobin curve, impairing O₂ loading
4. Irreversible binding of sulfur to the hemoglobin molecule

Explanation: Methemoglobinemia is caused by oxidizing agents that convert the iron in heme from its normal ferrous (Fe²⁺) state to the ferric (Fe³⁺) state. Ferric iron cannot bind oxygen, which reduces the effective oxygen-carrying capacity of the blood. Furthermore, the presence of Fe³⁺ in a hemoglobin tetramer increases the oxygen affinity of the remaining Fe²⁺ sites, causing a leftward shift of the dissociation curve and impairing oxygen unloading to tissues. This combination leads to functional anemia and tissue hypoxia.

Question 18

1. The flow of air in the terminal bronchioles is primarily turbulent.
2. The walls of the terminal bronchioles lack smooth muscle.
3. The terminal bronchioles have a very large total cross-sectional area. (correct answer)
4. The terminal bronchioles are supported by extensive cartilaginous rings.

Explanation: Airway resistance is inversely proportional to the total cross-sectional area of the airways at any given level. While each terminal bronchiole is very narrow, there are millions of them arranged in parallel. This massive parallel arrangement results in an enormous total cross-sectional area, far greater than that of the larger, more proximal airways. Consequently, the overall resistance to airflow in the small airways is very low. The highest resistance is found in the medium-sized bronchi.

Question 19

1. PaO₂ will increase significantly in both patients.
2. PaO₂ will show minimal improvement in both patients.
3. The patient with pneumonia will have a minimal PaO₂ increase, while the patient with the embolism will have a large increase. (correct answer)
4. The patient with the embolism will have a minimal PaO₂ increase, while the patient with pneumonia will have a large increase.

Explanation: Hypoxemia from a true shunt (as in pneumonia, where blood flows past non-ventilated alveoli) is poorly corrected by 100% O₂ because the supplemental oxygen cannot reach the shunted blood. Hypoxemia from V/Q mismatch, including high V/Q units like in a pulmonary embolism, responds well to 100% O₂. The high FiO₂ can overcome the mismatch by greatly increasing the PO₂ in the remaining normally perfused alveoli, allowing more oxygen to dissolve and saturate the blood that does pass through functional lung units.

Question 20

1. A rightward shift of the oxygen-hemoglobin dissociation curve
2. Oxidation of heme iron to the ferric (Fe³⁺) state
3. Reduced oxygen-carrying capacity and a leftward shift of the dissociation curve (correct answer)
4. A significant decrease in the partial pressure of dissolved oxygen (PaO2)

Explanation: Carbon monoxide (CO) has an affinity for hemoglobin that is ~240 times greater than that of oxygen, competitively inhibiting O2 binding and reducing the oxygen-carrying capacity of blood. Additionally, CO binding to one heme site on a hemoglobin molecule increases the affinity of the remaining sites for oxygen, causing a leftward shift of the oxygen-hemoglobin dissociation curve. This shift impairs the unloading of oxygen to the tissues, exacerbating tissue hypoxia. Standard pulse oximeters cannot differentiate carboxyhemoglobin from oxyhemoglobin, leading to a falsely high saturation reading.