The Respiratory System: Structure and Function

The Respiratory System

Overview of the Respiratory System

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the environment. It is divided into anatomical and functional regions, each with specialized structures and roles.

• Upper respiratory tract: Includes the nose, nasal cavity, paranasal sinuses, and pharynx.

• Lower respiratory tract: Includes the larynx, trachea, bronchi, bronchioles, and alveoli.

• Conducting zone: Passages that carry air to the sites of gas exchange (nose to terminal bronchioles).

• Respiratory zone: Sites of gas exchange (respiratory bronchioles, alveolar ducts, alveoli).

Anatomy of the Respiratory System

Upper Respiratory Tract

The upper respiratory tract filters, warms, and moistens incoming air and conducts it to the lower respiratory tract.

• Nose and Nasal Cavity: The primary entryway for air; lined with mucous membrane and hairs to filter particles.

• Paranasal Sinuses: Air-filled spaces in the skull that open into the nasal cavity, helping to warm and moisten air.

• Pharynx: A muscular tube that serves as a passageway for both air and food; divided into nasopharynx, oropharynx, and laryngopharynx.

Lower Respiratory Tract

The lower respiratory tract is involved in the conduction of air and the exchange of gases.

• Larynx: Also known as the voice box; routes air and food into the proper channels and houses the vocal cords.

• Trachea: A tube supported by C-shaped cartilage rings that conducts air to the bronchi.

• Bronchial Tree: Branching system of bronchi and bronchioles that conducts air from the trachea to the alveoli.

• Alveoli: Tiny air sacs where gas exchange occurs; surrounded by capillaries.

Structure and Function of the Nose

External Nose and Nasal Cavity

The nose is the main external opening for the respiratory system and plays several important roles in respiration.

• Provides an airway for respiration: The nasal cavity allows air to enter and exit the respiratory system.

• Moistens and warms air: The mucous membrane and blood vessels in the nasal cavity condition the air before it reaches the lungs.

• Filters inhaled air: Hairs (vibrissae) and mucus trap dust, pathogens, and other particles.

• Resonating chamber for speech: The nasal cavity contributes to the quality of voice and speech.

• Houses olfactory receptors: Specialized cells in the nasal cavity detect odors.

Structural Features of the Nose

• External nose: Formed by bone and cartilage, including the nasal bones, maxillary bones, and hvarious cartilages (septal, alar).

• Nasal septum: Divides the nasal cavity into right and left halves; composed of bone and cartilage.

• Nasal conchae: Three bony projections (superior, middle, inferior) that increase surface area and help filter, warm, and moisten air.

• Nasal meatuses: Passages beneath each concha that direct airflow and drainage of sinuses.

Histology of the Nasal Cavity

• Respiratory epithelium: Pseudostratified ciliated columnar epithelium with goblet cells; cilia move mucus and trapped particles toward the pharynx.

• Lamina propria: Loose connective tissue beneath the epithelium, containing blood vessels and glands.

Functional Zones of the Respiratory System

Conducting Zone

The conducting zone consists of all the respiratory passages that carry air to the sites of gas exchange. Its main functions are to filter, warm, and moisten air, and to conduct it into the lungs.

• Structures: Nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles.

• Histological changes: Epithelium transitions from pseudostratified ciliated columnar to simple cuboidal as airways become smaller; cartilage decreases and smooth muscle increases.

Respiratory Zone

The respiratory zone is where gas exchange occurs between air and blood.

• Structures: Respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli.

• Alveoli: Approximately 400 million in the human lungs, providing a large surface area (~140 m2) for gas exchange.

• Respiratory membrane: Thin barrier (about 0.4 μm) composed of alveolar and capillary walls and their fused basal laminae, facilitating rapid gas diffusion.

Summary Table: Major Structures of the Respiratory System

Region	Main Structures	Primary Functions
Upper Respiratory Tract	Nose, Nasal Cavity, Paranasal Sinuses, Pharynx	Filter, warm, moisten air; olfaction; resonance for speech
Lower Respiratory Tract	Larynx, Trachea, Bronchi, Bronchioles, Alveoli	Conduct air, protect airway, gas exchange
Conducting Zone	Nose to Terminal Bronchioles	Air passage, filtration, humidification
Respiratory Zone	Respiratory Bronchioles, Alveolar Ducts, Alveoli	Gas exchange

Key Terms and Definitions

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

• Bronchi: Large air passages that branch from the trachea into the lungs.

• Pharynx: Muscular tube serving as a passageway for air and food.

• Respiratory epithelium: Specialized lining of the respiratory tract, often ciliated and containing mucus-producing cells.

• Conducting zone: Airways that conduct air to the lungs but do not participate in gas exchange.

• Respiratory zone: Regions of the lung where gas exchange occurs.

Example: Airflow Pathway

Air enters through the external nose → passes through the nasal cavity (filtered, warmed, moistened) → moves through the pharynx and larynx → travels down the trachea → enters the bronchi → passes through bronchioles → reaches the alveoli for gas exchange.

Relevant Equations

• Gas Exchange (Fick's Law):

$\text{Rate of diffusion} = \frac{(\text{Surface Area}) \times (\text{Concentration Gradient})}{\text{Thickness of Membrane}}$

• Partial Pressure of Gases:

$P_{\text{gas}} = (\text{Total Pressure}) \times (\text{Fraction of gas in mixture})$

Additional info:

• The notes are suitable for an Introduction to Chemistry or introductory Anatomy & Physiology course, focusing on the structure and function of the respiratory system.