Respiratory System: Structure, Function, and Physiology

Respiratory System Overview

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is structurally and functionally organized to optimize the conditioning and exchange of respiratory gases.

Functions of the Respiratory System

• Primary Function: Gas exchange (O2 uptake, CO2 elimination)

• Secondary Functions:

  • Conditioning air (cleaning, warming, humidifying)

  • Regulation of blood pH (via CO2 elimination)

  • Protection (against pathogens and debris)

  • Speech production

Structural Organization of the Respiratory System

Upper Respiratory Tract (Outside Thorax)

• Nose → Pharynx

• Conditions and cleans air, conducts airflow

Lower Respiratory Tract (Inside Thorax)

• Larynx → Lungs

• Continues conditioning, conducts air, and is the site of gas exchange

Functional Organization

• Conducting Zone: Nose → Terminal bronchioles (air passageways, no gas exchange)

• Respiratory Zone: Respiratory bronchioles → Alveoli (sites of gas exchange)

Upper Respiratory Tract

Components

• Nose

• Nasal cavity

• Paranasal sinuses (frontal, sphenoid, ethmoid, maxillary)

• Pharynx (throat)

Conducts and conditions air before it reaches the lower tract.

Nose and Nasal Cavity

• External Nares: Anterior nasal apertures (nostrils)

• Nasal Cavity:

  • Hairs trap debris

  • Ciliated mucosa conditions air; cilia move mucus to pharynx

  • Divided by nasal septum

  • Nasal conchae create turbulence

  • Olfactory epithelium for smell

  • Receives tear drainage

Paranasal Sinuses

• Four bone cavities (frontal, ethmoid, sphenoid, maxillary)

• Reduce skull weight, contribute to sound resonance

• Lined with mucous membrane; conditions air

• Empty into nasal cavity

Pharynx (Throat)

• Passageway for air and food/water

• Protected by tonsils and mucosa

• Three divisions:

  • Nasopharynx: Posterior to nasal cavity; air passageway; contains pharyngeal and tubal tonsils; soft palate prevents food entry during swallowing

  • Oropharynx: Posterior to oral cavity; shared passageway for air, food, water; contains palatine and lingual tonsils

  • Laryngopharynx: Base of tongue/hyoid bone to esophagus; shared passageway; continuous with larynx and esophagus

Lower Respiratory Tract

Components

• Larynx

• Trachea

• Bronchial tree

• Lungs

Conducts air, conditions air, and is the site of gas exchange.

Larynx

• Extends from hyoid bone through cricoid cartilage

• Air passageway; prevents food/water from entering lower tract

• Sound production (speech)

• Supported by cartilage (epiglottis, thyroid, cricoid)

Glottis

• Slit-like opening into larynx

• Produces speech with vocal cords

Laryngeal Cartilages

• Epiglottis: Covers glottis, prevents food from entering trachea

• Thyroid cartilage: Includes "Adam's apple"

• Cricoid cartilage: Inferior, encircles vocal cords

Trachea

• 5 inches long, extends to 5th thoracic vertebra

• Conducts and conditions air

• Reinforced with C-shaped tracheal cartilages (no cartilage posteriorly, allows swallowing)

• Extends from larynx to primary bronchi, anterior to esophagus

• Carina: internal ridge, highly sensitive, triggers cough reflex

Bronchial Tree

• Branching passageways conducting air

• Supported by cartilage

• Branching: Primary (main) → Secondary (lobar) → Tertiary (segmental)

• Right primary bronchus is more vertical, shorter, and wider (aspirated objects more likely to enter)

• Different branches serve different lung regions (lobes, bronchopulmonary segments)

Structural Modifications (Bronchial Tree to Terminal Bronchioles)

• Ciliated epithelium transitions to non-ciliated (macrophages remove particles in alveoli)

• Cartilage decreases: incomplete rings → plates → scattered pieces → none (terminal/respiratory bronchioles)

• Smooth muscle increases as cartilage decreases

Respiratory Zone

• Sites of gas exchange (diffusion)

• Includes: respiratory bronchioles, alveolar ducts, alveoli, alveolar sacs

• Thinner epithelium for efficient gas exchange

Alveoli

• Main site of gas exchange

• Microscopic out-pouchings

• Simple squamous epithelium, thin elastic basement membrane

• Source of spongy lung texture

Alveolar Cells

• Type I alveolar cells: Squamous epithelial, main site of gas exchange

• Type II alveolar cells: Secrete alveolar fluid and surfactant (reduces surface tension, prevents collapse), secrete anti-bacterial proteins

• Alveolar macrophages (dust cells): Remove debris

• Fibroblasts: Produce reticular and elastic fibers

Lungs

Location and Structure

• Paired, spongy organs within pleural cavities, separated by mediastinum

• Contain bronchial tree and respiratory zone

• Enclosed by pleura (serous membrane):

  • Visceral pleura: Surface of lungs

  • Parietal pleura: Attached to thoracic cavity wall

  • Pleural fluid lubricates, allows movement during ventilation

Regions and Surfaces

• Apex: Superior extension, above clavicle

• Base: Inferior, rests on diaphragm

• Hilum: Vertical slit on mediastinal surface

• Cardiac notch: Recess on left lung

Gross Anatomy

• Lobes: Right (superior, middle, inferior); Left (superior, inferior)

• Fissures: Right (horizontal, right oblique); Left (left oblique)

Lung Lobules

• Smallest visible division of lungs

• Wrapped in elastic connective tissue

• Contain bronchiole and its branches, arteriole, venule, lymphatic vessel

Ventilation (Breathing)

Ventilation is the process of moving air in and out of the lungs, driven by pressure differences.

• Depends on gas moving from high to low pressure (Boyle's Law)

• Boyle's Law: There is an inverse relationship between volume and pressure: $P \propto \frac{1}{V}$

• Forceful inspiration involves contraction of scalenes and sternocleidomastoid muscles

Mechanics of Breathing

• Quiet Inspiration: Diaphragm and external intercostals contract, thoracic cavity size increases, pressure decreases, air flows in

• Quiet Expiration: Muscles relax, thoracic cavity size decreases, pressure increases, air flows out

• Forceful Expiration: Involves internal intercostals and abdominal muscles

Spirometry

Spirometry measures respiratory volumes:

• Tidal Volume (TV): Amount in a single, relaxed breath (~500 ml)

• Inspiratory Reserve Volume (IRV): Amount forcibly inhaled above TV (~3,000 ml)

• Expiratory Reserve Volume (ERV): Amount forcibly exhaled after TV (~1,000 ml)

• Residual Volume (RV): Always remains in lungs

• Vital Capacity (VC): Max amount exhaled (TV + IRV + ERV = ~4,500 ml)

Respiration (Gas Exchange)

• External Respiration: Between alveoli and pulmonary capillaries; O2 diffuses into blood, CO2 diffuses out

• Internal Respiration: Between systemic capillaries and interstitial spaces; O2 diffuses out of blood, CO2 diffuses in

CO2 Transport

• Most (60%) as bicarbonate ion (HCO3-) in RBCs and plasma

• Attached to hemoglobin in RBCs (30%)

• Dissolved as CO2 in RBCs and plasma (10%)

Control of Breathing

• Medulla: Sets basic rate and rhythm; CO2 changes (pH) are primary stimulus

• Pons: Smooths respiratory pattern

• Chemoreceptors:

  • Central (medulla oblongata): Monitors CO2 and pH of CSF

  • Peripheral (aorta, carotid arteries): Detects changes in O2 concentration