Gas Exchange Quiz: Diffusion, Oxygen and Carbon Dioxide

Follow oxygen from the alveoli into blood and carbon dioxide in the opposite direction. Distinguish ventilation, perfusion and diffusion, then review how blood transports these gases.

8 questions · Estimated 6–8 minutes. Select your answer mentally, then open the explanation to check your reasoning.

1. What normally drives oxygen diffusion from alveoli into pulmonary capillary blood?

  1. A higher oxygen partial pressure in alveoli than in incoming blood
  2. A higher oxygen partial pressure in incoming blood than in alveoli
  3. Active transport of oxygen by alveolar cells
  4. Contraction of the pulmonary capillaries
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Answer: A. A higher oxygen partial pressure in alveoli than in incoming blood

Oxygen diffuses down its partial-pressure gradient, from alveolar air into the relatively oxygen-poor blood entering pulmonary capillaries. The diffusion step does not require an ATP-driven pump.

2. Why does carbon dioxide normally diffuse from pulmonary capillary blood into alveoli?

  1. Alveolar carbon dioxide partial pressure is higher
  2. Incoming blood has a higher carbon dioxide partial pressure
  3. Carbon dioxide is actively pumped through the pleura
  4. Hemoglobin carries carbon dioxide only toward the tissues
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Answer: B. Incoming blood has a higher carbon dioxide partial pressure

Blood arriving from the tissues normally has a higher carbon dioxide partial pressure than alveolar gas. Carbon dioxide therefore diffuses into the alveoli and is removed by ventilation.

3. With surface area and partial-pressure gradient unchanged, increasing the thickness of the gas-exchange barrier generally:

  1. Increases the rate of diffusion
  2. Eliminates the need for pulmonary blood flow
  3. Reverses the direction of oxygen diffusion
  4. Reduces the rate of diffusion
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Answer: D. Reduces the rate of diffusion

A thicker barrier increases the distance gases must cross and reduces diffusion rate. Diffusion direction is determined by the partial-pressure gradient, not by barrier thickness.

4. With other factors unchanged, loss of alveolar gas-exchange surface area has which effect?

  1. Increases diffusion capacity
  2. Changes oxygen into carbon dioxide
  3. Reduces diffusion capacity
  4. Increases the number of pulmonary capillaries
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Answer: C. Reduces diffusion capacity

Less available surface area reduces the lung's capacity for diffusion. This is one reason destruction of alveolar walls, as occurs in emphysema, can impair gas exchange.

5. Which statement correctly distinguishes alveolar ventilation from pulmonary perfusion?

  1. Ventilation moves air into and out of alveoli; perfusion supplies blood to pulmonary capillaries
  2. Ventilation supplies capillary blood; perfusion moves alveolar air
  3. Both terms mean gas diffusion through the respiratory membrane
  4. Both terms mean oxygen binding to hemoglobin
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Answer: A. Ventilation moves air into and out of alveoli; perfusion supplies blood to pulmonary capillaries

Ventilation renews alveolar gas, while perfusion brings blood past the alveoli. Diffusion transfers gases across the respiratory membrane. Effective gas exchange requires these processes to work together.

6. Most oxygen in normal arterial blood is transported:

  1. As bicarbonate in plasma
  2. Freely dissolved in plasma
  3. Inside platelets
  4. Bound to hemoglobin
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Answer: D. Bound to hemoglobin

Most blood oxygen is bound to hemoglobin in red blood cells; a small amount is dissolved. Oxygen partial pressure reflects the dissolved component, so partial pressure and total blood oxygen content are different measurements.

7. Most carbon dioxide is transported in blood in which form?

  1. Oxygen bound to hemoglobin
  2. Bicarbonate ions
  3. Carbon monoxide dissolved in plasma
  4. Gas bubbles inside red blood cells
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Answer: B. Bicarbonate ions

Most carbon dioxide is transported as bicarbonate. Carbonic anhydrase in red blood cells accelerates the reactions that convert carbon dioxide and water into hydrogen ions and bicarbonate. Other carbon dioxide is dissolved or bound to proteins.

8. In metabolically active tissue, increased carbon dioxide and hydrogen ion concentrations generally cause hemoglobin to:

  1. Hold oxygen more tightly
  2. Stop carrying carbon dioxide in any form
  3. Release oxygen more readily
  4. Bind oxygen irreversibly
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Answer: C. Release oxygen more readily

Increased carbon dioxide and reduced pH lower hemoglobin's affinity for oxygen, promoting oxygen unloading. This is the Bohr effect. It helps match oxygen delivery to tissue metabolism.

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