Respiratory System Overview

Organization and Divisions of the Respiratory Tract

The respiratory system is divided into anatomical and functional regions that facilitate the movement of air and gas exchange. Understanding these divisions is essential for grasping the mechanisms of respiration and the roles of different structures.

• Conducting Zone: Includes the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. This zone transports air but does not participate in gas exchange.

• Respiratory Zone: Comprises respiratory bronchioles, alveolar ducts, and alveoli. This is where gas exchange occurs.

• Upper Respiratory Tract: Nose, nasal cavity, pharynx, and larynx.

• Lower Respiratory Tract: Trachea, bronchi, bronchioles, and alveoli.

Example: Air enters through the nose, passes through the pharynx and larynx, travels down the trachea, and reaches the bronchi and alveoli for gas exchange.

Structural Components of the Respiratory System

Nasal Cavity and Nasal Conchae

The nasal cavity is lined with specialized epithelium and contains structures called nasal conchae (turbinates) that enhance airflow and filtration.

• Epithelium Types: Pseudostratified ciliated columnar epithelium with goblet cells (respiratory mucosa), stratified squamous epithelium (vestibule and oropharynx).

• Nasal Conchae: Increase surface area, create turbulence, and enhance filtration and humidification of inspired air.

Example: The superior, middle, and inferior nasal conchae help trap particles and moisten air before it enters the lungs.

Pharynx

The pharynx is a muscular tube that serves both respiratory and digestive functions. It is divided into three regions:

• Nasopharynx: Lined with pseudostratified ciliated columnar epithelium.

• Oropharynx: Lined with stratified squamous epithelium.

• Laryngopharynx: Lined with stratified squamous epithelium.

Example: The nasopharynx connects the nasal cavity to the oropharynx and contains the pharyngeal tonsil.

Larynx

The larynx is a cartilaginous structure that houses the vocal cords and functions in sound production and airway protection.

• Epithelium: Superior portion lined with stratified squamous epithelium; inferior portion lined with pseudostratified ciliated columnar epithelium.

• Vocal Cords: True vocal cords produce sound; false vocal cords (vestibular folds) do not.

• Cartilages: Thyroid, cricoid, arytenoid, and epiglottis.

Example: The epiglottis prevents food from entering the trachea during swallowing.

Trachea and Bronchi

The trachea is a tube supported by C-shaped cartilaginous rings that prevent collapse. It branches into the right and left primary bronchi, which further divide into secondary and tertiary bronchi.

• Epithelium: Pseudostratified ciliated columnar epithelium with goblet cells.

• Bronchial Tree: Primary bronchi → secondary (lobar) bronchi → tertiary (segmental) bronchi → bronchioles.

• Bronchioles: Lack cartilage, have smooth muscle, and are lined with simple cuboidal epithelium.

Example: The right main bronchus is wider, shorter, and more vertical than the left, making it more likely for aspirated objects to enter.

Alveoli

Alveoli are the primary sites of gas exchange in the lungs. They are surrounded by capillaries and have specialized cells for gas diffusion and surfactant production.

• Type I Alveolar Cells: Simple squamous cells for gas exchange.

• Type II Alveolar Cells: Secrete surfactant to reduce surface tension and prevent alveolar collapse.

• Alveolar Macrophages: Remove debris and pathogens.

Example: Surfactant is essential for keeping alveoli open during exhalation.

Respiratory Mucosa and Defense Mechanisms

Respiratory Mucosa

The respiratory mucosa lines most of the respiratory tract and consists of pseudostratified ciliated columnar epithelium with goblet cells. It traps particles and moves them toward the pharynx.

• Mucus: Produced by goblet cells and submucosal glands; traps dust and microbes.

• Cilia: Move mucus toward the pharynx for swallowing or expectoration.

Example: Sneezing and coughing help clear mucus and particles from the respiratory tract.

Pulmonary Ventilation and Pressures

Phases of Pulmonary Ventilation

Pulmonary ventilation consists of two phases: inhalation and exhalation. These processes depend on pressure changes within the thoracic cavity.

• Inhalation: Occurs when alveolar pressure () is lower than atmospheric pressure ().

• Exhalation: Occurs when alveolar pressure () is higher than atmospheric pressure ().

Equation:

= atmospheric pressure = alveolar pressure = intrapleural pressure

Pressure Relationships

Several pressures are important in pulmonary ventilation:

• Intrapleural Pressure (): Pressure within the pleural cavity, usually lower than alveolar pressure.

• Transpulmonary Pressure (): Difference between alveolar and intrapleural pressure.

Example: A pneumothorax occurs when air enters the pleural cavity, causing to equal and leading to lung collapse.

Muscles of Respiration

Respiratory muscles facilitate changes in thoracic volume:

• Primary Muscles of Inhalation: Diaphragm and external intercostals.

• Accessory Muscles of Inhalation: Sternocleidomastoid, scalenes, pectoralis minor.

• Muscles of Exhalation: Internal intercostals, abdominal muscles (active exhalation).

Example: During forced inhalation, accessory muscles increase thoracic volume more than during quiet breathing.

Airway Resistance and Bronchoconstriction

Airway Resistance

Airway resistance is affected by the diameter of the airways and the presence of smooth muscle. Resistance is greatest in medium-sized bronchi.

• Bronchodilation: Relaxation of smooth muscle increases airway diameter and decreases resistance.

• Bronchoconstriction: Contraction of smooth muscle decreases airway diameter and increases resistance.

• Factors Affecting Resistance: Parasympathetic stimulation, histamine, and irritants increase resistance; sympathetic stimulation decreases resistance.

Example: Asthma attacks involve bronchoconstriction and increased airway resistance.

Alveolar Surface Tension and Surfactant

Surfactant and Alveolar Stability

Surfactant is a substance secreted by type II alveolar cells that reduces surface tension, preventing alveolar collapse (atelectasis).

• Surface Tension: The force exerted by liquid molecules at the air-liquid interface in alveoli.

• Surfactant: Disrupts hydrogen bonding between water molecules, reducing surface tension.

• Atelectasis: Collapse of alveoli due to insufficient surfactant or airway obstruction.

Example: Premature infants may develop respiratory distress syndrome due to lack of surfactant.

Lung Volumes and Capacities

Measurement of Lung Volumes

Lung volumes and capacities are measured using spirometry. These values are important for assessing respiratory health.

• Tidal Volume (TV): Volume of air inhaled or exhaled during each respiratory cycle (about 500 mL).

• Vital Capacity (VC): Maximum amount of air that can be exhaled after a maximal inhalation.

• Total Lung Capacity (TLC): Total volume of air in the lungs after maximal inspiration.

• Residual Volume (RV): Volume of air remaining in the lungs after maximal exhalation.

Example: VC is typically greater in males than females due to larger thoracic cavity size.

Table: Major Lung Volumes and Capacities

Summary of Key Terms

• Alveolus: Air sac in the lung where gas exchange occurs.

• Surfactant: Substance that reduces surface tension in alveoli.

• Bronchoconstriction: Narrowing of airways due to smooth muscle contraction.

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

• Spirometry: Measurement of lung volumes and capacities.

Additional info: Some explanations and definitions have been expanded for clarity and completeness. Table values are standard averages and may vary by source.