Anatomy and Physiology II Practice Test

What substance produced by the lungs helps reduce surface tension in the alveoli?

1. Sputum

2. Mucus

3. Surfactant

4. Alveolar fluid

The correct answer is: Surfactant

Surfactant is a complex mixture of lipids and proteins produced by the type II alveolar cells in the lungs. Its primary function is to reduce surface tension at the air-liquid interface within the alveoli. Each alveolus is lined with a thin layer of fluid, and the surface tension created by this liquid can cause the alveoli to collapse, particularly at low lung volumes. Surfactant disrupts the cohesive forces between water molecules, effectively lowering the surface tension and preventing the alveoli from collapsing during expiration. This not only facilitates easier lung expansion during inhalation but also helps to maintain the stability of alveoli of different sizes, ensuring effective gas exchange. While substances like sputum and mucus play significant roles in protecting the respiratory tract and trapping pathogens and particulate matter, they do not have the specific role of reducing surface tension in the alveoli. Alveolar fluid refers to the fluid lining the alveoli, which also contributes to surface tension, but it is the surfactant that specifically modifies this tension to allow for efficient respiratory mechanics.