Question

Choose the options having all the correct statements with respect to the exchange of gases:
I. p$O_2$ is higher in the alveoli than that in the blood.
II. p${\mathrm{CO}}_2$ is lower in tissues than that in the blood.
III. The diffusion membrane is made up of two layers.
IV. The exchange of gases occur by facilitated diffusion.

• A. Only II and III
• B. Only I
• C. Only II, III and IV
• D. Only I and II

Answer 1

The correct option is B Only I

In human beings, the primary sites of exchange of gases are alveoli, which are vascularised sac-like structures.

The exchange of gases happens via simple diffusion based on the concentration/pressure gradient or partial pressure of the individual gases. Partial pressure of a gas is the pressure contributed by it in a mixture of gases.

The two gases that are exchanged in alveaoli are carbon dioxide and oxygen. Also, the sites of diffusion of these gases are alveoli in lungs and tissues in the body.
I. p$O_2$ is higher in the alveoli (104 mmHg) than that in the blood (40 mmHg).

II. p${\mathrm{CO}}_2$ is higher in tissues (45 mmHg) than that in the blood (40 mmHg).

III. The diffusion membrane is made up of three layers- the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance. All these three layers cumulatively form a 0.22 mm thick membrane.

IV. The exchange of gases occur by diffusion as the gases don't require any membrane proteins to facilitate their transport. During facilitated diffusion, the molecules pass through membrane proteins along the concentration gradient.