Respiratory System: Structure and Function (Chapter 21) – Anatomy & Physiology Study Notes

Respiratory System Overview

Introduction to Respiration

The respiratory system is responsible for the exchange of gases between the atmosphere, blood, and body cells. This process is essential for cellular metabolism and homeostasis.

• Respiration: Exchange of oxygen (O2) and carbon dioxide (CO2) between the environment and body tissues.

• Three processes:

  • Ventilation: Movement of air into and out of the lungs.

  • External respiration: Gas exchange between lungs and blood.

  • Internal respiration: Gas exchange between blood and body cells.

• Two zones:

  • Conducting zone: Directs air to the lungs.

  • Respiratory zone: Sites of gas exchange.

Conducting Zone

Nose

The nose is the initial entry point for air and plays a role in filtering, warming, and moistening incoming air.

• External portion: Visible part, composed of cartilage and skin, lined with mucous membrane.

  • Contains external nares (nostrils).

• Internal portion: Extends from the nasal vestibule to the pharynx.

  • Lined with olfactory epithelium for smell.

  • Functions to warm, moisten, and filter incoming air.

Pharynx

The pharynx is a muscular chamber that serves both the respiratory and digestive tracts.

• Three regions:

  • Nasopharynx: Functions in respiration only.

  • Oropharynx: Functions in both respiration and digestion.

  • Laryngopharynx: Functions in both respiration and digestion.

• During swallowing, the epiglottis closes the tracheal opening to prevent aspiration.

Larynx

The larynx connects the laryngopharynx to the trachea and is involved in sound production and airway protection.

• Contains the epiglottis and multiple regions of cartilage:

  • Thyroid cartilage ("Adam's apple")

  • Cricoid cartilage

  • Three paired cartilages

• Vestibular folds (false vocal cords): Useful in holding breath against pressure in the thoracic cavity.

• Vocal folds (true vocal cords): Skeletal muscle adducts or abducts elastic ligaments; sound is produced as air movement vibrates the cords.

Trachea

The trachea is a rigid and flexible tube that connects the larynx to the bronchi, allowing air passage to the lungs.

• Located anterior to the esophagus.

• Wall contains 16–20 C-shaped "rings" of hyaline cartilage for support.

• The open region (no cartilage) faces the esophagus, allowing flexibility during swallowing.

Bronchial Tree

The bronchial tree consists of branching airways that conduct air from the trachea into the lungs.

• Trachea divides into right and left bronchi.

• Primary bronchi: Supply each lung.

• Secondary bronchi: Supply each lobe of the lung.

• Tertiary bronchi: Supply bronchopulmonary segments (~10 per lung).

• Tissue changes along the tree:

  • Decrease in cartilage, increase in smooth muscle.

  • Epithelium changes from ciliated pseudostratified to simple cuboidal.

Respiratory Zone

Bronchopulmonary Segments

Each lung is divided into bronchopulmonary segments, which are functionally and anatomically distinct regions.

• Each segment contains several lobules.

• Each lobule has its own blood supply (cardiovascular and lymphatic vessels).

Pathway of Air in the Respiratory Zone

• Respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli

Alveoli

Alveoli are the main site of gas exchange in the lungs.

• Type I alveolar cells: Numerous; responsible for gas exchange.

• Type II alveolar cells: Fewer in number; secrete surfactant to reduce surface tension and prevent alveolar collapse.

• Alveolar macrophages: Provide immune defense by removing debris and pathogens.

Lung Structure

Lobes and Membranes

The lungs are paired organs located in the thoracic cavity and are protected by pleural membranes.

• Right lung: 3 lobes separated by 2 fissures.

• Left lung: 2 lobes separated by 1 fissure; contains a cardiac notch and is about 10% smaller than the right lung.

• Pleural membrane:

  • Visceral layer: Covers the lungs.

  • Parietal layer: Lines the thoracic cavity wall.

  • Pleural space: Space between layers, filled with pleural fluid.

Summary Table: Conducting vs. Respiratory Zone

Zone	Main Structures	Function
Conducting Zone	Nose, pharynx, larynx, trachea, bronchi, bronchioles	Directs, filters, warms, and moistens air; no gas exchange
Respiratory Zone	Respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli	Site of gas exchange between air and blood

Key Terms and Definitions

• Respiration: The process of gas exchange between the atmosphere, blood, and cells.

• Ventilation: The movement of air into and out of the lungs.

• Alveoli: Tiny air sacs in the lungs where gas exchange occurs.

• Surfactant: A substance secreted by type II alveolar cells that reduces surface tension in the alveoli.

• Epiglottis: A flap of cartilage that covers the trachea during swallowing.

• Bronchopulmonary segment: A distinct region of the lung supplied by its own tertiary bronchus and blood vessels.

Example: Pathway of Air from Nose to Alveoli

1. Nose (external and internal portions)

2. Pharynx (nasopharynx, oropharynx, laryngopharynx)

3. Larynx (including vocal cords)

4. Trachea

5. Primary bronchi

6. Secondary bronchi

7. Tertiary bronchi

8. Bronchioles

9. Respiratory bronchioles

10. Alveolar ducts

11. Alveolar sacs

12. Alveoli

Additional info: These notes expand on the provided slides and handwritten content, clarifying anatomical structures and their functions for college-level Anatomy & Physiology students.