Question

Which one has the smallest diameter? (a) Right bronchus (b) Left bronchus (c) Secondary bronchiole (d) Respiratory bronchiole

Short answer

The respiratory bronchiole has the smallest diameter.

Step-by-step solution

Understand the Respiratory System

Primary bronchi branch off from the trachea. The right bronchus is slightly wider and straighter than the left. As they get deeper into the lungs, these bronchi subdivide into secondary bronchi which further divide into even smaller bronchi and eventually bronchioles. The bronchioles then lead to the smallest branches in the system, the respiratory bronchioles.

Compare the Diameters

Given this hierarchy, it is understood that each subsequent branching leads to a smaller structure. Therefore, the respiratory bronchiole, being at the end of the branching sequence, should have the smallest diameter. This is based on the principle that as the airways branch, their collective cross-sectional area increases, while the diameter of each individual branch decreases.

Key concepts

Bronchus

The bronchus is a key structure within the respiratory system. It serves as one of the main passageways through which air travels from the trachea into the lungs. When a person inhales, the air moves down the trachea and enters the bronchus. There are two primary bronchi: the right bronchus and the left bronchus.
The right bronchus is wider and shorter compared to the left bronchus. This is because the right lung has three lobes, while the left lung has only two lobes. The asymmetry accommodates room for the heart on the left side of the chest.

• The right bronchus goes into the right lung.
• The left bronchus directs air into the left lung.

These bronchi further divide into smaller branches as they penetrate deeper into the lungs. The function of the bronchus is crucial for directing the flow of air toward areas of gas exchange in the lungs.

Bronchioles

Bronchioles are small airways in the lungs that branch off from the bronchi. They are smaller in diameter compared to their parent structures, the primary and secondary bronchi. These tiny tubes play an essential role as they continue the air passage deeper into the lungs, ensuring air reaches every part.
Bronchioles do not contain cartilage like the bronchi, and rely on smooth muscle for support. This allows them to constrict or dilate and regulate airflow.
On their journey deeper into the lungs, bronchioles continue to subdivide, becoming smaller and smaller. The following are major points about bronchioles:

• Lack of cartilage makes them flexible.
• Mostly consist of smooth muscle and epithelium.
• Take air deeper into the lung structures for gas exchange.

Their small size and muscular nature make them vital for fine-tuning the direction and volume of airflow reaching the alveoli.

Airways Branching

In the human respiratory system, airways branching is a critical process where the major airway, the trachea, divides repeatedly into smaller passages within the lungs. This branching is designed to maximize the surface area available for air exchange.
The branching sequence begins with the trachea dividing into the right and left bronchi. These bronchi then split into secondary and tertiary bronchi as they penetrate further into the lung lobes.
Each branching creates smaller and smaller tubes, leading eventually to the bronchioles.

• The pattern increases the total surface area for gas exchange.
• Facilitates efficient air distribution throughout the lungs.

The hierarchy of branching ensures that the airways can carry air to various parts of the lungs efficiently, despite the decreasing diameter of each branch. This arrangement enhances oxygen and carbon dioxide exchange by significantly increasing the overall cross-sectional area of airways.

Respiratory Bronchiole

Respiratory bronchioles represent the final branching in the system of airways, leading to the alveoli where gas exchange occurs. They are the smallest air passages in the lung and follow the terminal bronchioles.
The transition to respiratory bronchioles marks the shift from purely conducting airways, like bronchi and non-respiratory bronchioles, to those involved in actual gas exchange.
Here, the structure begins to allow for oxygen and carbon dioxide to pass between air and blood.

• Connects directly to alveolar ducts and sacs.
• Enables the beginning of the respiratory zone.
• Key for the exchange of gases between the air and bloodstream.

The small diameter of the respiratory bronchiole makes it ideal for maximizing contact between air and the capillaries that facilitate gas exchange, emphasizing its important role in respiration.