Which Of The Following Is Not Transferred By Alveoli

The alveoli are tiny air sacs in the lungs responsible for gas exchange between the lungs and the blood. They allow oxygen (O₂) to enter the bloodstream and carbon dioxide (CO₂) to leave the bloodstream so it can be exhaled. However, not everything can pass through the alveoli.

Substances That Are Not Transferred by Alveoli:

1. Large Molecules (e.g., Proteins)

   • Proteins, such as antibodies and enzymes, are too large to pass through the alveolar walls.
   • The alveoli have a thin membrane designed only for gas exchange, not for transporting large molecules.
   • Proteins stay in the blood and move through the circulatory system instead.

2. Viruses and Bacteria

   • Alveoli are not a direct entry point for viruses or bacteria into the bloodstream.
   • The lung’s immune defenses (like mucus and immune cells) trap and destroy most harmful pathogens before they reach deep into the alveoli.
   • Some respiratory viruses, like the flu, infect lung cells but do not pass through the alveoli into the blood.

3. Inert Gases (e.g., Nitrogen and Argon)

   • Oxygen (O₂) and carbon dioxide (CO₂) are actively exchanged in the alveoli, but gases like nitrogen (N₂) and argon (Ar) are not.
   • Even though nitrogen is the most abundant gas in the air (~78%), the body does not use it in metabolism, so it simply moves in and out of the lungs without being absorbed into the blood.

Why Can’t These Substances Pass Through the Alveoli?

• The alveoli have a selective barrier made of a thin layer of epithelial cells.
• Only small, nonpolar molecules like oxygen and carbon dioxide can easily diffuse through this barrier.
• Larger molecules, viruses, bacteria, and some gases cannot pass through because they do not fit the physical or chemical requirements for diffusion.

Conclusion

The alveoli specialize in gas exchange and are not involved in transferring large molecules (proteins), viruses, bacteria, or inert gases like nitrogen and argon. Their structure ensures that only essential gases (O₂ and CO₂) move between the air and blood, keeping the respiratory system efficient and protected.