Question

The diagram below represents one of many microscopic air sacs in a human lung. The alveolus (air sac) is the place where oxygen (O2) and carbon dioxide (CO2) move into or out of the blood, as represented in the diagram. Which statement best explains why these gases are able to move in the directions shown in the diagram?

O The CO2 moves out of the capillary and into the alveolus to make more room for the blood to carry O2.

O The O2 is needed by the cells, so it is actively transported into the blood. The CO2, which is not needed, is actively transported out of the blood.

O The blood coming to the lungs is low in CO2 and high in O2, so the gases each diffuse from a lower to a higher concentration in this area.

O The blood coming to the lungs is high in CO2 and low in O2, so the gases each diffuse from a higher to a lower concentration in this area.

Answer 1

Based on the image and information provided, the best statement explaining the movement of gases is:

C) The blood coming to the lungs is low in CO2 and high in O2, so the gases each diffuse from a lower to a higher concentration in this area.

Here's why:

*The diagram shows arrows indicating the movement of oxygen (O2) from the alveolus into the blood capillary and carbon dioxide (CO2) from the capillary into the alveolus.

Gas diffusion occurs passively, driven by **concentration gradients**. Gases move from areas of higher concentration to areas of lower concentration until equilibrium is reached.

The blood coming to the lungs has just circulated through the body, delivering O2 to the cells and picking up CO2 as a waste product. Therefore, it is **low in O2 and high in CO2** compared to the air in the alveolus.

The alveolus, on the other hand, is filled with fresh air rich in O2 and low in CO2.

This creates a concentration gradient: O2 is higher in the alveolus than in the blood, and CO2 is higher in the blood than in the alveolus.

As a result, O2 diffuses from the alveolus into the blood, and CO2 diffuses from the blood into the alveolus, as shown in the diagram.

The other statements are incorrect because:

A) This statement focuses on making room for O2 in the capillary but doesn't explain the driving force for CO2 movement.

B) This statement introduces active transport, which is not the primary mechanism for gas exchange in the lungs. Diffusion is the main driver.

D) This statement incorrectly assumes the blood coming to the lungs is high in CO2 and low in O2, which would not create the necessary concentration gradient for diffusion.

Therefore, based on the principles of gas diffusion and concentration gradients, C) is the most accurate explanation for the movement of gases in the lung alveolus.