The Respiratory System: Structure and Function

The Respiratory System

Overview of the Respiratory System and Respiratory Tract

The respiratory system is responsible for the exchange of gases between the atmosphere and the blood, as well as for sound production and regulation of blood pH, volume, and pressure. It is divided into upper and lower respiratory tracts, each with distinct anatomical structures and functions.

• Upper respiratory system: Nose, nasal cavity, paranasal sinuses, and pharynx

• Lower respiratory system: Larynx, trachea, bronchi, bronchioles, and alveoli

Functions of the Respiratory System

• Gas exchange: Provides a surface for oxygen and carbon dioxide exchange between air and blood

• Sound production: Involved in verbal communication

• Regulation: Assists in controlling blood volume, blood pressure, and body fluid pH

The Respiratory Tract

The Respiratory Epithelium

The respiratory tract is lined with specialized epithelium that protects and cleanses the airway.

• Pseudostratified ciliated columnar cells: Move mucus and trapped debris upward toward the pharynx (mucociliary escalator)

• Stratified squamous cells: Provide protection against abrasion, especially in areas exposed to food and air

• Mucus-producing cells: Trap inhaled particles to prevent them from entering the lungs

The Upper Respiratory System

The Nose and Nasal Cavity

The nose and nasal cavity filter, warm, and humidify incoming air. They also house olfactory receptors and contribute to resonance in speech.

• Dorsum and apex: The bridge and tip of the nose

• Nasal bones and cartilages: Form the structure of the nose

• Nasal septum: Divides the nasal cavity into right and left portions (formed by the vomer and perpendicular plate of the ethmoid)

• Hard and soft palate: Separate the nasal and oral cavities

• Nasal conchae: Increase surface area and turbulence for air filtration

The Pharynx

The pharynx is a muscular tube that serves as a common passageway for air and food, connecting the nasal and oral cavities to the larynx and esophagus.

• Nasopharynx: Posterior to the nasal cavity; contains the pharyngeal opening of the auditory tube

• Oropharynx: Posterior to the oral cavity; contains the pharyngeal arch and uvula

• Laryngopharynx: Inferior region; entrance to the trachea and esophagus

The Lower Respiratory System

The Larynx

The larynx is a cartilaginous structure that connects the pharynx to the trachea and houses the vocal cords, playing a key role in sound production and airway protection.

• Unpaired cartilages: Thyroid (with laryngeal prominence), cricoid, and epiglottis

• Paired cartilages: Arytenoid, corniculate, and cuneiform (control opening of the glottis)

• Epiglottis: Closes over the glottis during swallowing to prevent food entry into the airway

Vocal Folds and Sound Production

• Vestibular folds (false vocal cords): Inelastic, do not produce sound

• Vocal folds (true vocal cords): Elastic, vibrate to produce sound

The Trachea

The trachea is a flexible tube that conducts air from the larynx to the bronchi. It is supported by C-shaped rings of hyaline cartilage and bifurcates at the carina into the right and left main bronchi.

• Length: Approximately 11 cm; Diameter: 2.5 cm

• Cartilage rings: 15–20 C-shaped rings prevent collapse

• Trachealis muscle: Allows for constriction and dilation of the trachea

The Main Bronchi and Lungs

The trachea divides into the right and left main bronchi, each entering the lung at the hilum. The lungs are divided into lobes and have distinct surfaces for anatomical orientation.

• Right lung: Three lobes (superior, middle, inferior); horizontal and oblique fissures

• Left lung: Two lobes (superior, inferior); oblique fissure and cardiac notch

• Lung surfaces: Costal, mediastinal, diaphragmatic

Bronchial Tree and Bronchioles

The bronchial tree consists of branching airways that progressively decrease in size, ending in the alveoli where gas exchange occurs.

• Main bronchi: Divide into lobar (secondary) bronchi

• Lobar bronchi: Right lung has three, left lung has two

• Segmental (tertiary) bronchi: Supply bronchopulmonary segments

• Bronchioles: Branch into terminal and respiratory bronchioles

Alveolar Ducts and Alveoli

Alveoli are the primary sites of gas exchange in the lungs. Each alveolus is surrounded by capillaries and elastic fibers, and the alveolar wall forms the blood air barrier.

• Type I alveolar cells: Simple squamous cells for gas exchange

• Type II alveolar cells: Secrete surfactant to reduce surface tension and prevent collapse

• Alveolar macrophages: Phagocytize debris and pathogens

• Blood air barrier: Formed by alveolar epithelium, fused basement membrane, and capillary endothelium

Pleural Cavities and Pleural Membranes

Each lung is enclosed by a double-layered serous membrane called the pleura. The pleural cavity contains pleural fluid, which reduces friction during breathing movements.

• Visceral pleura: Covers the lung surface

• Parietal pleura: Lines the thoracic wall, mediastinum, and diaphragm

• Pleural cavity: Space between the layers, filled with pleural fluid

Respiratory Muscles and Pulmonary Ventilation

Pulmonary Ventilation

Pulmonary ventilation is the process of moving air into and out of the lungs, driven by changes in thoracic volume and pressure.

• Primary muscles of inhalation: Diaphragm and external intercostals

• Normal exhalation: Occurs by relaxation of the diaphragm and external intercostals

Accessory Respiratory Muscles

• Muscles aiding inhalation: Serratus anterior, scalenes, pectoralis minor, sternocleidomastoid

• Muscles aiding exhalation: Internal intercostals, transversus thoracis, external oblique, internal oblique, rectus abdominis

Summary Table: Major Structures and Functions of the Respiratory System

Structure	Main Function
Nose/Nasal Cavity	Filters, warms, humidifies air; olfaction
Pharynx	Passageway for air and food
Larynx	Sound production; airway protection
Trachea	Conducts air to bronchi; cleanses air
Bronchi/Bronchioles	Distribute air to lungs; further cleanse air
Alveoli	Gas exchange
Pleura	Reduces friction; compartmentalizes lungs