Respiratory System Notes

The Respiratory System

Functions of the Respiratory System

The respiratory system facilitates gas exchange, supplying oxygen and eliminating carbon dioxide. This process occurs in the lungs within air-filled chambers called alveoli.

Five Basic Functions:

1. Gas Exchange Area: Provides a large surface area for gas exchange between air and circulating blood.

2. Air Movement: Moves air to and from the gas exchange surfaces.

3. Protection: Protects respiratory surfaces from dehydration, temperature changes, and pathogens.

4. Sound Production: Produces sounds for speech and non-verbal communication.

5. Olfactory Sensations: Provides olfactory sensations to the central nervous system.

Organization of the Respiratory System

Major Anatomical Structures:

• Nose (including nasal cavity and paranasal sinuses)

• Pharynx (throat)

• Larynx (voice box)

• Trachea (windpipe)

• Bronchi

• Lungs (containing bronchioles and alveoli)

Respiratory Tract Divisions:

• Conducting Portion: From the nasal cavity entrance through the pharynx, larynx, trachea, bronchi, and larger bronchioles. Functions to filter, warm and humidify air.

• Respiratory Portion: Includes the smallest bronchioles and alveoli, where gas exchange occurs.

The Nose

Air enters through external nares (nostrils) into the nasal cavity.

Key Features:

• Nasal Vestibule: Space within the flexible tissues of the nose.

• Nasal Hairs: Guard the nasal cavity from large airborne particles.

• Nasal Septum: Divides the nasal cavity into left and right sides, consisting of hyaline cartilage anteriorly and the vomer and ethmoid bone posteriorly.

• Hard Palate: Formed by palatine and maxillary bones, separates the oral and nasal cavities.

• Internal Nares: The nasal cavity opens into the nasopharynx

Respiratory Mucosa

The respiratory tract is lined by a mucous membrane consisting of:

• Respiratory Epithelium: Ciliated columnar epithelium with goblet cells.

• Lamina Propria: Underlying loose connective tissue layer containing mucous glands.

Mucus produced by goblet cells and mucous glands traps debris and microorganisms. Cilia sweep the mucus toward the pharynx to be swallowed. Paranasal sinuses and the nasolacrimal duct also flush the nasal cavity with mucus and tears, respectively. Exposure to irritants increases mucus production, causing a runny nose.

Nasal Conchae

Superior, middle, and inferior nasal conchae project into the nasal cavity, creating narrow grooves for airflow. This turbulent flow promotes filtration, warming, and humidification of incoming air.

The Pharynx

The pharynx (throat) is shared by the digestive and respiratory systems with three subdivisions:

1. Nasopharynx: Connected to the nasal cavity via internal nares; contains the pharyngeal tonsil and entrances to the auditory tubes; lined by respiratory epithelium.

2. Oropharynx: Extends from the soft palate to the base of the tongue; contains palatine tonsils; lined by stratified squamous epithelium.

3. Laryngopharynx: Extends from the hyoid bone to the entrance of the esophagus; lined by stratified squamous epithelium.

The Larynx

Air enters the larynx via the glottis.

Cartilages:

The larynx consists of nine cartilages including:

• Epiglottis: Shoehorn-shaped, folds over the glottis during swallowing to prevent food from entering the respiratory tract.

• Thyroid Cartilage: Forms the anterior and lateral surfaces of the larynx; the "Adam's apple".

• Cricoid Cartilage: Provides posterior support to the larynx.

• Arytenoid, Corniculate, and Cuneiform Cartilages: Smaller cartilages supported by the cricoid cartilage.

Vocal Cords:

• False Vocal Cords: Prevent foreign objects from entering the glottis.

• True Vocal Cords: Contain elastic ligaments that vibrate to produce sound, altered by muscles that change tension in the ligaments.

Cough Reflex:

Food or liquids touching the vocal cords trigger the cough reflex, which expels material blocking the glottis.

Vocal Cords and Sound Production

Air vibrates the vocal cords producing sound waves, with pitch determined by the vocal cords' diameter, length, and tension. Children have high-pitched voices due to small larynxes and short vocal cords. Male larynxes enlarge more at puberty, resulting in thicker and longer vocal cords that produce lower tones. Tension in the vocal cords, controlled by skeletal muscles, also affects pitch. Amplification and resonance occur in the pharynx, oral cavity, nasal cavity, and paranasal sinuses.

The Trachea

Structure:

• The trachea (windpipe) is a flexible tube, approximately $2.5$ cm in diameter and $11$ cm long.

• It begins at the sixth cervical vertebra and ends at the fifth thoracic vertebra, branching into primary bronchi.

• Supported by 15-20 C-shaped tracheal cartilages.

Function:

• The trachea maintains airway patency and prevents collapse or overexpansion.

• The posterior tracheal wall contains the trachealis muscle, which adjusts the trachea's diameter under autonomic control.

• Sympathetic stimulation increases the diameter of the trachea facilitated by having the muscle trachealis.

The Bronchi

Structure:

• The trachea branches into right and left primary bronchi within the mediastinum.

• The right primary bronchus is larger and more steeply angled, making it a common site for foreign object obstruction.

Bronchial Tree:

Primary bronchi branch into:

• Secondary bronchi (entering the lobes of each lung).

• Tertiary bronchi.

• Smaller bronchi.

• Bronchioles (narrow passages, about $1$ mm without cartilages).

• Terminal bronchioles (supply air to a lobule of the lung, with diameters of $0.3-0.5$ mm).

Bronchioles

Adjustments to bronchiole diameter regulate airflow and air distribution in the lungs. This process is controlled by the autonomic nervous system:

• Bronchodilation: Sympathetic activation relaxes smooth muscles, enlarging airway diameter.

• Bronchoconstriction: Parasympathetic stimulation contracts smooth muscles, reducing airway diameter. Extreme bronchoconstriction can block airways, as seen in asthma or allergic reactions.

Lobules

A lobule is a segment of lung tissue supplied by a single bronchiole. Within a lobule, a terminal bronchiole divides into respiratory bronchioles that deliver air to the gas exchange surfaces of the lungs.

Alveolar Ducts and Alveoli

Structure:

• Respiratory bronchioles open into alveolar ducts, which lead to alveolar sacs connected to individual alveoli.

• Each lung contains around 150 million alveoli, giving the lung a spongy appearance.

Alveolar Organization

The Respiratory Membrane

Gas exchange occurs across the respiratory membrane involving three components:

1. Squamous alveolar epithelium.

2. Endothelial cells lining adjacent capillaries.

3. Fused basement membranes between alveolar and endothelial cells.

The respiratory membrane facilitates rapid diffusion due to its small distance ($0.1$ μm) and the lipid solubility of oxygen and carbon dioxide.

Alveolar Macrophages

Alveolar macrophages (dust cells) patrol the epithelium, phagocytizing dust or debris.

Septal Cells and Surfactant

• Septal cells secrete surfactant, an oily secretion that reduces surface tension on the alveolar surfaces.

• Surfactant prevents alveolar collapse.

• Inadequate surfactant levels result in respiratory distress syndrome, requiring forceful inhalations to open alveoli.

Circulation to the Respiratory Membrane

Blood Supply:

• Respiratory exchange surfaces receive blood from the pulmonary circuit, branching along the bronchi to the lobules.

• Capillaries surround each alveolus.

Pulmonary Vessels:

• Blood flows from arterioles into capillaries and then into pulmonary venules, eventually reaching the left atrium.

• Pulmonary circuit blood pressure is relatively low, making pulmonary arteries susceptible to blockages from small clots, fat masses, or air bubbles, leading to pulmonary embolism.

The Lungs

Lobes and Fissures:

• The right lung has three lobes and the left lung has two lobes, separated by deep fissures.

• The apex of each lung extends into the base of the neck, and the concave base rests on the diaphragm's superior surface.

• The costal surface follows the rib cage contours, and mediastinal surfaces bear grooves for blood vessels and the pericardium.

• The left lung has a cardiac notch.

Lung Tissue:

• The lungs have a light, spongy consistency due to air-filled passageways and alveoli.

• Elastic fibers provide the lungs with the ability to undergo large changes in volume.