The Respiratory System: Structure, Function, and Regulation

The Respiratory System

Introduction

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is closely integrated with the circulatory system and is divided into conducting and respiratory zones, each with specialized structures and functions.

Functions of the Respiratory System

• Gas Transport and Exchange: Supplies oxygen and removes carbon dioxide from the body.

• Blood pH Regulation: Maintains acid-base balance by regulating CO2 levels.

• Sensory Functions: Contains receptors for the sense of smell.

• Filtration: Filters inhaled air to remove particles and pathogens.

• Sound Production: Produces sounds for speech.

• Excretion: Excretes small amounts of water and heat in exhaled air.

• Platelet Production: Over 50% of platelets are produced in the lungs from megakaryocytes.

Processes of Respiration

1. Pulmonary Ventilation: Movement of air into and out of the lungs (breathing).

2. External Respiration: Exchange of gases between alveoli and pulmonary capillaries.

3. Transport of Respiratory Gases: Movement of O2 and CO2 in the blood between lungs and tissues.

4. Internal Respiration: Exchange of gases between systemic capillaries and tissue cells.

Gross Anatomy of the Respiratory System

• Nose, Nasal Cavity, Paranasal Sinuses: Warm, moisten, and filter incoming air.

• Pharynx: Muscular tube (throat) serving as a passageway for air and food; divided into nasopharynx, oropharynx, and laryngopharynx.

• Larynx: Voice box; connects pharynx to trachea, contains vocal cords.

• Trachea: Windpipe; conducts air to bronchi, supported by C-shaped cartilage rings.

• Bronchi and Bronchial Tree: Branching airways leading to each lung.

• Lungs and Alveoli: Main organs of respiration; alveoli are the site of gas exchange.

Conducting vs. Respiratory Zones

• Conducting Zone: Includes all respiratory passageways that convey air (nose to terminal bronchioles). Functions to filter, warm, and moisten air.

• Respiratory Zone: Site of gas exchange; includes respiratory bronchioles, alveolar ducts, and alveoli.

Histology of the Respiratory System

Structure	Epithelium	Cilia	Goblet Cells	Special Features
Nose	Pseudostratified ciliated columnar	Yes	Yes	Filters, warms, moistens air
Pharynx	Stratified squamous (oropharynx, laryngopharynx)	No	No	Passage for food and air
Larynx	Stratified squamous (superior); pseudostratified ciliated columnar (inferior)	Yes (inferior)	Yes (inferior)	Voice production
Trachea	Pseudostratified ciliated columnar	Yes	Yes	C-shaped cartilage rings
Bronchi	Pseudostratified ciliated columnar (primary); simple columnar/cuboidal (smaller)	Yes	Yes	Cartilage plates, smooth muscle
Lungs (Alveoli)	Simple squamous	No	No	Site of gas exchange

Oro- and Laryngopharynx

• Oropharynx: Extends from soft palate to epiglottis; common passageway for food and air; lined with stratified squamous epithelium.

• Laryngopharynx: Extends from epiglottis to cricoid cartilage; ends as esophagus; also lined with stratified squamous epithelium.

Larynx

• Inferiorly continuous with the trachea.

• Constructed of 3 single and 3 paired cartilages.

• Functions as the voice box and protects the airway during swallowing.

• Extends from the 4th to 6th cervical vertebrae.

Epiglottis and Glottis During Swallowing

• During swallowing, the larynx elevates, causing the epiglottis to close over the glottis, preventing food and liquids from entering the airway (rima glottidis).

Trachea

• Windpipe; tubular passageway for air.

• Extends from the larynx to the superior border of T5, then divides into right and left primary bronchi.

• Supported by C-shaped cartilage rings to prevent collapse.

• Carina marks the division into two primary bronchi.

Tissue Composition of the Tracheal Wall

• Mucosa: Pseudostratified ciliated columnar epithelium.

• Submucosa: Connective tissue with seromucous glands.

• Media: Hyaline cartilage.

• Adventitia: Outermost connective tissue layer.

Pleural Membranes & Pleural Cavity

• Visceral Pleura: Covers the lungs.

• Parietal Pleura: Lines the ribcage and diaphragm.

• Pleural Cavity: Space between the pleurae, containing lubricating fluid to reduce friction during breathing.

Trachea and Bronchial Tree

• The trachea divides into right and left primary bronchi, which further branch into secondary and tertiary bronchi, bronchioles, and terminal bronchioles.

• This branching network is known as the bronchial tree.

Microscopic Anatomy of the Lung Lobule

• Each lobule contains a terminal bronchiole, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

• Gas exchange occurs across the respiratory membrane, which consists of alveolar and capillary walls.

Cell Types of the Alveoli

• Type I Alveolar Cells: Simple squamous cells; primary site of gas exchange.

• Type II Alveolar Cells (Septal Cells): Secrete surfactant to reduce surface tension and prevent alveolar collapse; have microvilli.

• Alveolar Dust Cells: Macrophages that remove debris and pathogens.

• Both Type I and II cells have ACE-2 receptors, which are entry points for SARS-CoV-2.

Blood Supply and Innervation of the Lungs

• Pulmonary Arteries: Deliver oxygen-poor blood to the lungs.

• Pulmonary Veins: Carry oxygenated blood to the heart.

• Innervation: Sympathetic (dilates airways), parasympathetic (constricts airways), and visceral sensory fibers.

Mechanism of Breathing (Ventilation)

• Inspiration: Diaphragm and external intercostal muscles contract, increasing thoracic volume and decreasing pressure, drawing air into the lungs.

• Expiration: Usually passive; diaphragm and intercostals relax, thoracic volume decreases, pressure increases, and air is expelled.

• Forced expiration involves abdominal and internal intercostal muscles.

• Relationship between pressure, volume, and temperature is described by the ideal gas law:

$P = \frac{nRT}{V}$

External and Internal Respiration

• External Respiration: Exchange of O2 and CO2 between alveoli and blood.

• Internal Respiration: Exchange of O2 and CO2 between blood and tissue cells.

Regulation of Respiration

• Respiratory Centers: Located in the medulla oblongata and pons.

• Three groups of neurons: medullary rhythmicity, pneumotaxic, and apneustic centers.

• Medullary neurons function as a central pattern generator for rhythmic breathing.

• Pontine centers modify the rhythm generated by the medulla.

Negative Feedback Regulation of Breathing

• Increase in arterial pCO2 stimulates chemoreceptors, which activate the inspiratory center, increasing the rate and depth of breathing to restore homeostasis.

Summary Table: Structures and Functions

Structure	Main Function	Special Feature
Nose	Filters, warms, moistens air	Ciliated epithelium
Pharynx	Passage for air and food	Stratified squamous epithelium
Larynx	Voice production, airway protection	Cartilage structure
Trachea	Conducts air to bronchi	C-shaped cartilage rings
Bronchi	Air passage to lungs	Branching tree structure
Lungs/Alveoli	Gas exchange	Thin respiratory membrane

Key Equations

• Ideal Gas Law:

$P = \frac{nRT}{V}$

• Gas Exchange Equation: (for O2 and CO2 diffusion)

$\text{Rate of diffusion} = \frac{(P_1 - P_2) \times A \times D}{T}$

Where $P_1 - P_2$ is the partial pressure difference, $A$ is the surface area, $D$ is the diffusion coefficient, and $T$ is the thickness of the membrane.

Summary Points

1. Respiration involves pulmonary ventilation, external respiration, transport of gases, and internal respiration.

2. The respiratory system is divided into conducting and respiratory zones, each with specialized structures and functions.

3. Gas exchange occurs in the alveoli, which are lined by Type I and II cells and patrolled by macrophages.

4. Breathing is regulated by centers in the medulla and pons, with feedback from chemoreceptors monitoring blood gases.

Additional info: The notes also reference the role of the respiratory system in platelet production and the clinical relevance of ACE-2 receptors in viral infections such as COVID-19.