Lower Respiratory Organs and Bronchial Tree: Structure and Function

Lower Respiratory Organs

Trachea

The trachea is a tubular structure that conducts air from the upper respiratory tract to the bronchi. It is lined with ciliated epithelium, which helps trap and remove debris and pathogens from inhaled air.

• Function: Conducts air to the bronchi.

• Structure: Contains ciliated epithelial cells and cartilage rings for support.

• Example: The cilia move mucus upward toward the pharynx for expulsion.

Bronchial Tree

The bronchial tree is a branching system of airways that conducts air from the trachea to the alveoli, where gas exchange occurs.

• Main Bronchi: The right and left primary bronchi branch from the trachea and enter each lung.

• Secondary (Lobar) Bronchi: Each supplies a lobe of the lung (three on the right, two on the left).

• Tertiary (Segmental) Bronchi: Supply bronchopulmonary segments within each lobe.

• Bronchioles: Smaller branches with smooth muscle and cuboidal epithelial cells; regulate airflow and resistance.

• Terminal Bronchioles: The last part of the conducting zone; do not participate in gas exchange.

• Respiratory Bronchioles: Begin the respiratory zone; lead to alveolar ducts and sacs.

Lungs

The lungs are the main organs of gas exchange. They contain millions of alveoli, which are tiny air sacs where oxygen and carbon dioxide are exchanged between air and blood.

• Structure: Each lung is divided into lobes and bronchopulmonary segments.

• Function: Gas exchange occurs in the alveoli.

• Example: Oxygen diffuses from alveolar air into pulmonary capillaries, while carbon dioxide diffuses out.

The Bronchial Tree: Hierarchical Structure

The bronchial tree consists of a series of branching airways that conduct air from the trachea to the alveoli. Each branch becomes progressively smaller and more specialized for gas exchange.

• Main Bronchi: First branches from the trachea.

• Secondary Bronchi: Supply each lung lobe.

• Tertiary Bronchi: Supply bronchopulmonary segments.

• Bronchioles: Smallest airways before the terminal bronchioles.

• Terminal Bronchioles: End of the conducting zone.

• Respiratory Bronchioles: Start of the respiratory zone; lead to alveolar ducts and sacs.

Conducting Zone vs. Respiratory Zone Structures

Functional Division of the Respiratory System

The respiratory system is divided into two functional zones based on their role in air transport and gas exchange.

• Conducting Zone: Structures that transport air but do not participate in gas exchange. Includes the nose, pharynx, larynx, trachea, bronchi, and terminal bronchioles.

• Respiratory Zone: Structures where gas exchange occurs. Includes respiratory bronchioles, alveolar ducts, and alveoli.

Zone	Main Structures	Function
Conducting Zone	Nose, pharynx, larynx, trachea, bronchi, terminal bronchioles	Air transport, filtration, humidification
Respiratory Zone	Respiratory bronchioles, alveolar ducts, alveoli	Gas exchange

Example: Air passes through the conducting zone before reaching the respiratory zone, where oxygen enters the blood and carbon dioxide is removed.

Additional info: The transition from conducting to respiratory zone is marked by the presence of alveoli in the airway walls, allowing for gas exchange to begin.