In the context of the human respiratory tract, what happens to the particles trapped by the cilia?

• A. They are digested by enzymes secreted by the cilia.
• B. They are absorbed directly into the bloodstream.
• C. They remain in the lungs and accumulate over time.
• D. They are moved upward toward the throat to be swallowed or expelled.

Answer: (D)

Verified step by step guidance

1. Understand the role of cilia in the human respiratory tract: Cilia are tiny hair-like structures lining the respiratory tract that help in moving mucus and trapped particles.
2. Recognize that particles such as dust, microbes, and other debris get trapped in the mucus layer covering the respiratory epithelium.
3. Know that the coordinated beating of cilia moves this mucus layer along with the trapped particles in a specific direction—toward the throat.
4. Realize that once the mucus and particles reach the throat, they can either be swallowed and destroyed by stomach acid or expelled by coughing or sneezing.
5. Conclude that the particles are not digested by enzymes secreted by cilia, nor absorbed directly into the bloodstream, nor do they accumulate indefinitely in the lungs.