How Diffusion Works

Question 1

Which statement best explains why oxygen moves into the blood and carbon dioxide moves out of the blood at the alveoli?

Accepted Answer

Gases move from an area of higher concentration to an area of lower concentration, so oxygen diffuses into the blood and carbon dioxide diffuses out of the blood.

Answer

Oxygen and carbon dioxide are pulled through the alveolar membrane by electrical forces.

Answer

The heart actively pushes oxygen into the alveoli and pulls carbon dioxide into the blood.

Answer

The alveolar walls allow only oxygen to pass, while carbon dioxide is blocked.

Question 2

During one breath, the air inside the tiny air sacs has an oxygen partial pressure of 150 mmHg and a carbon‑dioxide partial pressure of 0.5 mmHg. The blood in the surrounding capillaries has an oxygen partial pressure of 40 mmHg and a carbon‑dioxide partial pressure of 45 mmHg. Which statement best explains the net movement of each gas across the sac wall?

Accepted Answer

Oxygen diffuses from the sac into the blood because its partial pressure is higher in the sac; carbon‑dioxide diffuses from the blood into the sac because its partial pressure is higher in the blood.

Answer

Both gases diffuse into the blood because the blood is cooler, causing gases to condense there.

Answer

Oxygen moves into the blood because it is heavier than carbon‑dioxide, while carbon‑dioxide moves out because it is lighter.

Answer

Oxygen enters the blood because blood flow quickly removes it, creating a low concentration, and carbon‑dioxide leaves because blood flow brings fresh carbon‑dioxide to the sac.

Question 3

At the alveoli in the lungs, oxygen moves from the air into the blood and carbon dioxide moves from the blood into the air. Which statement best explains why this direction of diffusion occurs?

Accepted Answer

Because the concentration of oxygen is higher in the alveolar air than in the blood, oxygen diffuses into the blood, while the concentration of carbon dioxide is higher in the blood than in the alveolar air, so carbon dioxide diffuses out.

Answer

Because the pressure of the blood pushes gases toward the alveoli, oxygen is forced into the blood and carbon dioxide is forced out into the air.

Answer

Because the walls of the alveoli actively transport oxygen into the blood and carbon dioxide out, using cellular energy.

Answer

Because oxygen molecules are smaller and lighter than carbon dioxide molecules, they naturally move into the blood and carbon dioxide, being heavier, moves out into the air.

Try three questions

Which statement best explains why oxygen moves into the blood and carbon dioxide moves out of the blood at the alveoli?

At the alveoli in the lungs, oxygen moves from the air into the blood and carbon dioxide moves from the blood into the air. Which statement best explains why this direction of diffusion occurs?

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