Free Gaseous Exchange in Lungs MCQs with Answers

1. Which feature of the alveoli is the most important adaptation for efficient gaseous exchange?

• A. Their supporting rings of cartilage
• B. Their walls, one cell thick, giving an enormous surface area with a dense capillary network
• C. The cilia lining their inner surface
• D. The mucus that coats them

Explanation: Around 300 million alveoli give a surface of roughly 70 square metres, and a wall a single cell thick lying against an equally thin capillary means oxygen diffuses across a distance well under a micrometre. Continual blood flow and ventilation keep the diffusion gradient steep. Alveoli contain no cartilage or cilia, and a coat of mucus would in fact lengthen the diffusion path.

2. Most of the carbon dioxide carried in the blood travels as

• A. hydrogen carbonate ions dissolved in the plasma
• B. carbaminohaemoglobin
• C. carbon dioxide dissolved in the plasma
• D. carbon monoxide

Explanation: About seventy per cent of carbon dioxide enters the red cells, where carbonic anhydrase converts it with water into carbonic acid, which dissociates into hydrogen and hydrogen carbonate ions, and the latter diffuse out into the plasma. Roughly twenty three per cent is carried on haemoglobin as carbaminohaemoglobin and only about seven per cent stays simply dissolved. The reactions reverse in the lungs.

3. One molecule of haemoglobin can combine with

• A. one molecule of oxygen
• B. four molecules of oxygen
• C. two molecules of oxygen
• D. an unlimited number of oxygen molecules

Explanation: Haemoglobin has four polypeptide chains, each with an iron containing haem group that binds one oxygen molecule, giving a maximum of four. Binding is cooperative, so the attachment of the first oxygen makes the next ones easier, and this is what gives the oxygen dissociation curve its S shape. Full saturation is reached in the lung capillaries within a fraction of a second.

4. An increase in carbon dioxide concentration makes haemoglobin release oxygen more readily. This is known as

• A. the chloride shift
• B. the countercurrent effect
• C. the Bohr effect
• D. the induced fit effect

Explanation: Carbon dioxide lowers the pH of the blood, which alters the shape of haemoglobin and reduces its affinity for oxygen, shifting the dissociation curve to the right. The benefit is that the most active tissues, which produce the most carbon dioxide, automatically receive the most oxygen. The chloride shift is the separate movement of chloride ions into the red cell to balance the loss of hydrogen carbonate.

5. The rate and depth of breathing are regulated mainly by the level of

• A. oxygen in the alveoli
• B. glucose in the blood
• C. nitrogen in the blood
• D. carbon dioxide in the blood, monitored by the medulla oblongata

Explanation: Chemoreceptors in the medulla and in the great arteries respond to the fall in pH that a rise in carbon dioxide produces, and the respiratory centre then increases the rate and depth of breathing until the excess is blown off. Oxygen level acts only as a weak backup stimulus and matters mainly at high altitude. This is why holding the breath becomes unbearable from the build up of carbon dioxide, not from the lack of oxygen.

6. The chloride shift refers to the movement of chloride ions

• A. out of the red cell as oxygen enters
• B. into the red cell to balance the hydrogen carbonate ions leaving it
• C. from the plasma into the alveolar air
• D. into the plasma from the kidney tubules

Explanation: As hydrogen carbonate ions formed inside the red cell diffuse out into the plasma, the cell would be left with a net positive charge, so chloride ions move in to keep it electrically neutral. This exchange lets the red cell keep converting carbon dioxide rapidly all the way through the tissue capillaries. In the lungs the whole sequence runs in reverse and chloride moves back out.

7. The site of gas exchange in lungs is

• A. Bronchi
• B. Bronchioles
• C. Alveoli
• D. Trachea

Explanation: Alveoli are the site of gas exchange, with thin walls and large surface area for efficient O2 and CO2 diffusion.

8. Surfactant is produced by

• A. Type I alveolar cells
• B. Type II alveolar cells
• C. Goblet cells
• D. Ciliated cells

Explanation: Type II pneumocytes (alveolar cells) produce surfactant, which reduces surface tension to prevent alveolar collapse.

9. The Bohr effect describes

• A. O2 binding to hemoglobin
• B. Effect of CO2/pH on O2 affinity
• C. CO2 transport
• D. Lung compliance

Explanation: The Bohr effect states that increased CO2/decreased pH reduces hemoglobin's affinity for O2, facilitating O2 release to tissues.

10. Most CO2 is transported as

• A. Dissolved in plasma
• B. Carbaminohemoglobin
• C. Bicarbonate ions
• D. Free CO2

Explanation: About 70% of CO2 is transported as bicarbonate (HCO3-) in plasma, formed by carbonic anhydrase in RBCs.