Chapter 22: The Respiratory System – Structure and Function

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Chapter 22: The Respiratory System

Overview of Respiration

The respiratory system is responsible for the exchange of gases between the atmosphere and the body’s cells, supporting cellular metabolism and ATP production. It involves ventilation, gas exchange, and cellular respiration.

Ventilation: Movement of air into and out of the lungs.
Gas Exchange: Oxygen (O2) moves from air to blood to tissues; carbon dioxide (CO2) moves from tissues to blood to lungs for exhalation.
Cellular Respiration: Cells use O2 to produce ATP, generating CO2 as a waste product.

Diagram of the respiratory system showing upper and lower tracts and major organs

Organs of the Respiratory System

The respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and lungs. It is divided into upper and lower respiratory tracts. The diaphragm forms the floor of the thoracic cavity and is essential for breathing.

Upper Respiratory Tract

Nose: Structure and Functions

The nose is the primary entryway for air and serves several important functions:

Provides an airway for respiration
Moistens and warms inhaled air
Filters and cleans inhaled air
Detects odors
Acts as a resonating chamber for the voice

Anatomy of the external nose

Anatomy of the Nasal Region

The nose is supported by both bone and cartilage. The superior half is formed by the nasal bones and maxillae, while the inferior half is shaped by various cartilages and dense connective tissue, which forms the lateral wall of each nostril.

Bony and cartilaginous supports of the nose

Nasal Cavity

The nasal cavity extends from the nostrils to the posterior nares and is divided by the nasal septum into right and left chambers (nasal fossae). Key features include:

Vestibule: Chamber inside nostrils lined with stratified squamous epithelium and vibrissae (guard hairs).
Nasal Conchae: Superior, middle, and inferior projections that increase surface area and help warm, moisten, and filter air.

Sagittal section of the nasal cavity showing conchae and vestibule

Nasal Cavity Mucosa

The nasal cavity is lined by two types of mucosa:

Olfactory Mucosa: Located in the roof of the nasal fossa; detects odors.
Respiratory Mucosa: Lines the rest of the nasal cavity; consists of ciliated pseudostratified epithelium. Goblet cells produce mucus to trap particles, and lysozyme destroys bacteria.

Microscopic view of cilia and goblet cells in respiratory mucosa Electron micrograph of respiratory epithelium with cilia

Cilia and Erectile Tissue in the Nasal Cavity

Cilia in the respiratory epithelium sweep mucus and trapped debris toward the pharynx to be swallowed. The erectile tissue of the inferior concha contains a venous plexus that periodically engorges with blood, shifting airflow and preventing drying of the mucosa. Trauma to this area can cause spontaneous epistaxis (nosebleed).

Pharynx

Regions of the Pharynx

The pharynx is a muscular tube that serves as a passageway for air, food, and drink. It is divided into three regions:

Nasopharynx: Receives auditory tubes and contains the pharyngeal tonsil; passageway for air only.
Oropharynx: Contains palatine and lingual tonsils; passageway for air, food, and drink.
Laryngopharynx: Located at the level of the hyoid bone; passageway for air, food, and drink.

Regions of the pharynx: nasopharynx, oropharynx, laryngopharynx

Larynx (Voice Box)

Structure and Function

The larynx is a 4 cm cartilaginous chamber whose primary function is to keep food and drink out of the airway. It also plays a role in sound production (phonation).

Epiglottis: Flap of tissue that guards the glottis and directs food and drink to the esophagus.
Glottis: Opening to the trachea, including the vocal cords.
Composed of 9 cartilages and muscular walls.
Infant larynx is positioned higher, allowing simultaneous breathing and swallowing; descends with age.

Anterior, posterior, and median views of the larynx Endoscopic view of the larynx showing epiglottis, glottis, and vocal cords

Action of Vocal Cords

The vocal cords can be adducted (tightened for high-pitched sounds) or abducted (loosened for low-pitched sounds). Sound is produced by vibration of the cords, but speech requires additional modification by the pharynx, oral cavity, tongue, and lips.

Adduction and abduction of the vocal cords

Lower Respiratory Tract

Trachea (Windpipe)

The trachea is a rigid tube about 4.5 inches long and 2.5 cm in diameter, located anterior to the esophagus. It is supported by 16–20 C-shaped cartilaginous rings and lined with ciliated pseudostratified epithelium, functioning as a mucociliary escalator to move debris upward toward the pharynx.

Trachea and bronchial tree with carina Dissection showing the carina at the tracheal bifurcation

Tracheal Structure

The C-shaped cartilage rings keep the trachea open, while the trachealis muscle allows for adjustment of airflow. The mucosa contains goblet cells and cilia for trapping and moving particles.

Cross-section of the trachea showing trachealis muscle and cartilage ring

Tracheotomy and Tracheostomy

A tracheotomy is a surgical procedure to create an opening (tracheostomy) in the trachea to bypass upper airway obstructions. Air entering through a tracheostomy must be filtered and humidified.

Patient with a tracheostomy tube Diagram of tracheostomy procedure

Endoscopic View of the Trachea

The carina is a median ridge at the point where the trachea forks into the right and left primary bronchi. It is an important landmark in bronchoscopy.

Endoscopic view of the carina and primary bronchi

Lungs and Bronchial Tree

Lung Surface Anatomy

The lungs are asymmetrical: the right lung is shorter (due to the liver) and has three lobes, while the left lung is narrower (due to the heart) and has two lobes. The hilum is the entry/exit point for bronchi, blood vessels, and nerves.

Surface anatomy of the lungs Cross-section of the thorax showing lungs and pleura

Bronchial Tree

The bronchial tree consists of branching airways:

Primary Bronchi: Right is wider and more vertical (higher risk of aspiration).
Secondary Bronchi: Two in the left lung, three in the right lung.
Tertiary Bronchi: Ten in the right lung, eight in the left lung; each supplies a bronchopulmonary segment.
Bronchioles: Smaller branches leading to terminal bronchioles and alveolar sacs (sites of gas exchange).

Diagram of the bronchial tree Bronchial tree with color-coded branches Detailed bronchial tree

Alveoli and Gas Exchange

Alveoli are tiny air sacs where gas exchange occurs. Each alveolus is surrounded by a capillary network. There are three main cell types:

Type I (Squamous Alveolar Cells): Thin cells for gas exchange (95% of surface area).
Type II (Great Alveolar Cells): Cuboidal cells that secrete surfactant to reduce surface tension and prevent alveolar collapse.
Alveolar Macrophages (Dust Cells): Phagocytize debris and pathogens.

Alveolus structure Cell types in the alveolus

Alveolar Blood Supply

Alveoli are richly supplied with capillaries for efficient gas exchange. Oxygen diffuses from alveoli into blood, while carbon dioxide diffuses from blood into alveoli to be exhaled.

Alveolar blood supply and capillary network

Pleurae and Pleural Fluid

The lungs are enclosed by two layers of pleura:

Visceral Pleura: Covers the lungs.
Parietal Pleura: Lines the rib cage.
Pleural Cavity: Space between pleurae, lubricated with fluid to reduce friction, create a pressure gradient for lung inflation, and compartmentalize the thoracic cavity to prevent infection spread.

Visceral and parietal pleura Normal pleural lining vs. pleurisy

Pulmonary Ventilation (Breathing)

Pulmonary ventilation consists of one cycle of inspiration (inhaling) and expiration (exhaling). Quiet respiration occurs at rest, while forced respiration occurs during exercise. Airflow depends on pressure differences between the lungs and the atmosphere.

Animation of breathing mechanics Muscles involved in inspiration and expiration

Respiratory Muscles

Inspiration: Diaphragm and external intercostals contract, expanding the thoracic cavity and decreasing pressure, causing air to flow in.
Expiration: Elastic recoil of the lungs and thoracic wall during quiet breathing; forced expiration involves abdominal and internal intercostal muscles.

Neural Control of Breathing

Breathing is regulated by repetitive neural stimuli from the brain. The medulla oblongata and pons contain respiratory centers that control unconscious breathing.

Brainstem regions controlling breathing