Anatomy and Physiology of the Respiratory System: Structure and Function

Respiratory System Overview

Introduction

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into the upper and lower respiratory tracts, each with specialized structures and functions. Understanding the anatomy and physiology of these components is crucial for recognizing how the system supports life and interacts with other organ systems.

• Upper respiratory tract: Nose, nasal cavity, pharynx, larynx

• Lower respiratory tract: Trachea, bronchial tree, lungs, respiratory muscles, thoracic cage

Major Functions of the Respiratory System

• Gas exchange: Oxygen uptake and carbon dioxide removal

• Speech and sound production: Air movement through the larynx produces sound

• Olfaction: Detection of odors via olfactory receptors

• Acid-base homeostasis: Regulation of blood pH by controlling CO2 levels

• Expulsion of abdominal contents: Assists in actions like coughing and sneezing

• Blood pressure regulation: Production of angiotensin-II for fluid and pressure homeostasis

Respiratory Processes

1. Pulmonary ventilation: Movement of air in and out of the lungs

2. Pulmonary gas exchange: Exchange of gases between alveoli and blood

3. Gas transport: Movement of gases through the bloodstream

4. Tissue gas exchange: Exchange of gases between blood and tissues

Anatomy of the Respiratory System

Upper Respiratory Tract

Nose and Nasal Cavity

The nose and nasal cavity serve as the primary entryway for air, warming, humidifying, and filtering it before it reaches the lungs. The external nose is composed of bone and hyaline cartilage, while the internal structures include the nasal septum and vestibule.

• External nose: Root, bridge, dorsum nasi, apex, alae, nostrils (anterior nares)

• Internal nose: Nasal septum, vestibule (with bristle-like hairs for filtration), posterior nares

Nasal Cavity

The nasal cavity contains three sets of nasal conchae (superior, middle, inferior) that create turbulent airflow, enhancing filtration and humidification. The paranasal sinuses (frontal, ethmoid, sphenoid, maxillary) are connected to the nasal cavity and help lighten the skull and enhance voice resonance.

• Nasal conchae: Bony projections lined with mucosa, increase surface area

• Sinuses: Warm, humidify, filter air; lighten skull; enhance resonance

• Epithelium: Stratified squamous in vestibule, olfactory mucosa on roof, respiratory mucosa (pseudostratified ciliated columnar) elsewhere

• Goblet cells: Secrete mucus to trap particles; cilia move debris toward pharynx

Pharynx

The pharynx (throat) is a muscular tube that serves as a passageway for both air and food. It is divided into three regions:

• Nasopharynx: Lined with pseudostratified ciliated columnar epithelium; contains pharyngeal tonsil and opening of the pharyngotympanic tube

• Oropharynx: Lined with nonkeratinized stratified squamous epithelium; passage for food and air; contains palatine and lingual tonsils

• Laryngopharynx: Lined with nonkeratinized stratified squamous epithelium; opens into larynx and esophagus

Larynx

The larynx (voice box) is located anterior to the esophagus and extends from the third to the sixth cervical vertebra. It functions to keep food and liquids out of the respiratory tract and houses the vocal cords for sound production.

• Cartilage framework: 9 pieces (epiglottis, thyroid, cricoid, arytenoid, corniculate, cuneiform)

• Epiglottis: Elastic cartilage that covers the glottis during swallowing

• Vocal folds: True vocal cords; sound produced by air passing through

• Vestibular folds: False vocal cords; close glottis during swallowing

• Epithelium: Stratified squamous above vocal cords, pseudostratified ciliated columnar below

Lower Respiratory Tract

Trachea

The trachea (windpipe) is a tube about 2 cm in diameter and 10–12 cm long, extending from the larynx to the mediastinum. It is supported by C-shaped rings of hyaline cartilage that keep the airway open while allowing flexibility for swallowing.

• Mucosa: Contains goblet cells and cilia for trapping and moving debris

• Carina: Last tracheal ring; triggers cough reflex if stimulated

Bronchial Tree

The bronchial tree consists of branching airways that conduct air from the trachea to the alveoli. The primary bronchi branch into secondary and tertiary bronchi, which further divide into bronchioles and terminal bronchioles.

• Primary bronchi: Right is shorter, wider, and more vertical; left is longer and more horizontal

• Secondary bronchi: Three in right lung, two in left lung

• Tertiary bronchi: Ten in each lung (sometimes fewer in left)

• Bronchioles: Smallest airways, lack cartilage, have more smooth muscle

• Terminal bronchioles: Final conducting airways before respiratory zone

Airflow Pathway

The pathway of air from the external environment to the alveoli is as follows:

Order	Structure
1	Nares
2	Nasal cavity
3	Nasopharynx
4	Oropharynx
5	Laryngopharynx
6	Larynx
7	Trachea
8	Primary bronchi
9	Secondary bronchi
10	Tertiary bronchi
11	Multiple branches of bronchi
12	Bronchioles
13	Terminal bronchioles
14	Respiratory bronchioles
15	Alveolar ducts
16	Alveolar sacs

Alveoli and Respiratory Membrane

Alveoli are tiny air sacs where gas exchange occurs. They are surrounded by elastic fibers, smooth muscle, and pulmonary capillaries. The respiratory membrane is the barrier through which gases diffuse between alveoli and blood.

• Type I alveolar cells: Squamous cells, form most of the alveolar wall, allow rapid gas diffusion

• Type II alveolar cells: Cuboidal cells, secrete surfactant to reduce surface tension

• Alveolar macrophages: Phagocytes that remove debris and pathogens

• Respiratory membrane: Composed of type I alveolar cell, fused basal lamina, and capillary endothelial cell

Lungs and Pleural Cavities

Lung Structure

• Lobes: Right lung has three lobes; left lung has two lobes and a cardiac notch

• Hilum: Entry/exit point for bronchi, blood vessels, lymphatics, and nerves

• Bronchopulmonary segments: Each supplied by a tertiary bronchus

• Pleura: Double-layered serous membrane (parietal and visceral pleura) with pleural fluid to reduce friction

Clinical Application: Tuberculosis

• Cause: Airborne bacteria, spread by coughing/sneezing

• Latent TB: 90% of cases, asymptomatic

• Active TB: 10% of cases, can be fatal if untreated

• Symptoms: Persistent cough, bloody sputum, fever, night sweats, weight loss

• Pathology: Bacteria trigger immune response, forming granulomas in alveoli

Summary Table: Key Structures and Functions

Structure	Main Function	Epithelium Type
Nasal cavity	Warm, humidify, filter air	Pseudostratified ciliated columnar
Pharynx	Passage for air and food	Stratified squamous (oropharynx, laryngopharynx)
Larynx	Sound production, airway protection	Stratified squamous (above cords), pseudostratified ciliated columnar (below cords)
Trachea	Conduct air to bronchi	Pseudostratified ciliated columnar
Bronchi	Conduct air, filter particles	Pseudostratified ciliated columnar (large), columnar (smaller)
Bronchioles	Conduct air, regulate airflow	Simple cuboidal
Alveoli	Gas exchange	Simple squamous

Additional info: For exam preparation, focus on the order of airflow, cell types in each region, and the functional significance of each anatomical structure. Consider how diseases or injuries can disrupt normal respiratory function.