**Question:** Which of the following is not a function of lung?

A. Gas exchange
B. Protection of the respiratory system
C. Regulation of pH and oxygenation
D. Production of mucus

**Core Concept:** The lungs perform various essential functions to facilitate gas exchange and maintain respiratory homeostasis. These functions include:

1. Gas exchange: The primary function of the lungs is to facilitate the exchange of oxygen (O2) and carbon dioxide (CO2) between the air we breathe and our bloodstream.
2. Protection of the respiratory system: The lungs protect the respiratory system by preventing entry of foreign particles, pathogens, and irritants.
3. Regulation of pH and oxygenation: The lungs help in maintaining the blood pH and oxygen levels by removing excess CO2 and supplying oxygen to the bloodstream.
4. Production of mucus: Although the lungs produce mucus, this is not listed as a primary function in the question options.

**Why the Correct Answer is Right:** Production of mucus is an essential function of the respiratory system, specifically involving the respiratory tract (trachea, bronchi, and bronchioles). However, the question specifically asks for a function of the lung itself, not the respiratory tract. Therefore, production of mucus is not considered a primary function of the lung.

**Why Each Wrong Option is Incorrect:**

A. Gas exchange: While not a primary function, gas exchange is a critical process involving the lung.
B. Protection of the respiratory system: Although not specifically a lung function, this is achieved through the respiratory tract and not the lung itself.
C. Regulation of pH and oxygenation: Regulation of pH and oxygenation are essential processes that occur as a result of gas exchange within the lung.
D. Production of mucus: As mentioned above, this is an essential function of the respiratory tract, not the lung.

**Clinical Pearl:** The understanding of the primary functions of the lung helps to appreciate its importance in maintaining respiratory homeostasis and overall health. Knowing the roles of the respiratory tract and the circulatory system in gas exchange further enhances comprehension of respiratory physiology.