The narrowest and most numerous tubes of lungs are termed as bronchioles.
- Definition: Bronchioles are the smallest branches of the bronchial tree, which is the system of airways in the lungs. They are responsible for carrying air from the bronchi (larger airways) to the alveoli (air sacs) in the lungs.
- Narrowness: Bronchioles are narrower in diameter compared to the bronchi, allowing for more precise regulation of airflow within the lungs. Their narrowness increases the velocity of airflow, facilitating efficient gas exchange in the alveoli.
- Number: Bronchioles are the most numerous tubes in the lungs, branching extensively to reach all areas of the lung tissue. This extensive branching provides a large surface area for gas exchange to occur.
- Structure: The walls of bronchioles are composed of smooth muscle, elastic fibers, and a thin layer of epithelial cells. The smooth muscle can contract and relax, allowing for regulation of airflow and control of ventilation.
- Function: The bronchioles play a crucial role in delivering air to the alveoli, where oxygen is taken up by the bloodstream and carbon dioxide is expelled. They also help in humidifying and warming the inhaled air before it reaches the alveoli.
- Relation to other structures: Bronchioles branch off from the bronchi and further divide into even smaller tubes called terminal bronchioles. These terminal bronchioles then connect to the respiratory bronchioles, which lead to the alveoli.
In summary, bronchioles are the narrowest and most numerous tubes in the lungs. They are responsible for delivering air to the alveoli and play a vital role in the process of respiration.

Bronchioles
Bronchioles are the narrowest and most numerous tubes in the lungs. They are branching subdivisions of the bronchi and are responsible for carrying air to the alveoli.

Function
- Bronchioles help in the distribution of air within the lungs.
- They regulate airflow and control air resistance in the respiratory system.

Structure
- Bronchioles are smaller in diameter compared to bronchi.
- They lack cartilage in their walls and are mainly made up of smooth muscle.
- The walls of bronchioles contain elastic fibers that allow them to stretch and recoil during breathing.

Importance
- The bronchioles play a crucial role in the process of gas exchange in the alveoli.
- They help in maintaining the proper functioning of the respiratory system by regulating airflow and ensuring efficient oxygen exchange.
In conclusion, bronchioles are essential structures in the respiratory system that facilitate the passage of air to the alveoli for gas exchange. Their narrow diameter and numerous branches make them crucial for proper lung function.