The Respiratory System

Introduction to the Respiratory System

The respiratory system is essential for gas exchange, supplying oxygen to body tissues and removing carbon dioxide produced by cellular metabolism. It consists of specialized structures that facilitate the movement and exchange of gases between the atmosphere and the bloodstream.

• Oxygen is obtained from the air by diffusion across the exchange surfaces in the lungs.

• Carbon dioxide produced by tissues is released into the air across the same surfaces.

• Blood transports oxygen from the lungs to peripheral tissues and returns carbon dioxide from tissues to the lungs.

Functions of the Respiratory System

• Provides a large surface area for gas exchange between air and blood.

• Moves air to and from the exchange surfaces of the lungs.

• Protects respiratory surfaces from dehydration, temperature changes, and pathogens.

• Produces sounds for communication (speech).

• Detects odors via olfactory receptors in the nasal cavity.

Organization of the Respiratory System

Anatomical Divisions

• Upper respiratory system: Nose, nasal cavity, paranasal sinuses, and pharynx.

• Lower respiratory system: Larynx, trachea, bronchi, bronchioles, and alveoli.

Functional Divisions

• Conducting portion: Nasal cavity to larger bronchioles; moves air to the sites of gas exchange.

• Respiratory portion: Smallest respiratory bronchioles and alveoli; site of gas exchange.

• Alveoli: Air-filled sacs where gas exchange occurs.

Respiratory Mucosa and Defense System

• Respiratory mucosa: Lines the conducting portion; consists of respiratory epithelium and underlying lamina propria (areolar tissue).

• Defense mechanisms:

  • Mucus traps particles and pathogens.

  • Cilia move mucus and debris toward the pharynx (mucociliary escalator).

  • Alveolar macrophages engulf small particles in the alveoli.

Upper Respiratory System

Nose and Nasal Cavity

• Entry point for air; air enters through nostrils (nares) into the nasal vestibule.

• Nasal hairs trap large particles.

• Nasal septum: Divides the cavity into left and right sides.

• Olfactory region: Superior portion lined by olfactory epithelium for smell.

• Nasal conchae (turbinates): Superior, middle, and inferior projections that create turbulent airflow, enhancing warming, humidification, and filtration.

• Meatuses: Passages between conchae where air flows.

• Hard palate: Floor of the nasal cavity, separates nasal and oral cavities (formed by maxillae and palatine bones).

• Soft palate: Posterior to hard palate, separates nasopharynx from the rest of the pharynx.

• Nasal mucosa: Highly vascularized lamina propria warms and humidifies air; moisture condenses on exhalation.

• Nosebleeds: Common due to rich blood supply.

Pharynx

• Shared chamber for respiratory and digestive systems.

• Divided into:

  • Nasopharynx: Superior portion.

  • Oropharynx: Posterior to oral cavity.

  • Laryngopharynx: Inferior portion, lined with stratified squamous epithelium for abrasion resistance.

Lower Respiratory System

Larynx

• Air passes from pharynx into larynx through the glottis (opening between vocal cords).

• Cartilages of the larynx:

  • Thyroid cartilage: Forms anterior/lateral walls; Adam’s apple.

  • Cricoid cartilage: Forms posterior portion.

  • Epiglottis: Covers glottis during swallowing to prevent aspiration.

• Ligaments:

  • Vestibular ligaments (within vestibular folds) protect vocal folds.

  • Vocal ligaments (within vocal folds) produce sound (vocal cords).

Sound Production

• Air passing through glottis vibrates vocal folds, producing sound waves.

• Pitch is controlled by tension in vocal folds (adjusted by arytenoid cartilages).

• Phonation: Sound production at the larynx.

• Articulation: Modification of sound by lips, tongue, and teeth.

Trachea

• Tough, flexible tube branching into right and left main bronchi.

• Tracheal cartilages: C-shaped rings stiffen and protect airway; open posteriorly to allow esophageal expansion during swallowing.

• Ends of cartilage connected by annular ligament and trachealis muscle (contraction reduces tracheal diameter).

Bronchial Tree

• Main bronchi: Right and left, enter respective lungs at the hilum.

• Carina: Ridge separating main bronchi at tracheal bifurcation.

• Main bronchi branch into lobar bronchi (supplying lung lobes), then segmental bronchi (supplying bronchopulmonary segments).

• Bronchi walls: Less cartilage, more smooth muscle as branches get smaller; easier to change diameter in smaller bronchi.

• Bronchitis: Inflammation and constriction of bronchi/bronchioles, causing breathing difficulty.

Bronchioles

• Smallest branches; terminal bronchioles are the smallest conducting branches.

• No cartilage, dominated by smooth muscle.

• Bronchodilation: Sympathetic activation enlarges airway diameter, reducing resistance.

• Bronchoconstriction: Parasympathetic activation or histamine release narrows airway, increasing resistance.

• Asthma: Severe bronchoconstriction due to excessive smooth muscle stimulation, restricting airflow.

Gas Exchange Structures

Alveoli and Associated Structures

• Terminal bronchioles branch into respiratory bronchioles, which connect to alveoli via alveolar ducts.

• Alveolar sacs are clusters of alveoli surrounded by elastic fibers and capillaries.

• Pneumocytes type I: Simple squamous cells forming alveolar wall.

• Pneumocytes type II: Large cells producing surfactant (reduces surface tension, prevents alveolar collapse).

• Alveolar macrophages: Patrol and remove debris/pathogens.

• Respiratory distress syndrome: Inadequate surfactant leads to alveolar collapse after exhalation.

Blood-Air Barrier

• Site of gas exchange; consists of three layers:

  1. Alveolar cell layer

  2. Capillary endothelial layer

  3. Fused basement membrane

• Gas exchange is efficient due to short diffusion distance and lipid solubility of gases.

• Pneumonia: Inflammation causes fluid buildup, impairs gas exchange.

The Lungs and Pleura

Lung Anatomy

• Right and left lungs in respective pleural cavities; base rests on diaphragm.

• Right lung: Three lobes (superior, middle, inferior) separated by horizontal and oblique fissures; wider, displaced upward by liver.

• Left lung: Two lobes (superior, inferior) separated by oblique fissure; longer, has cardiac notch for heart.

Pleural Cavities and Membranes

• Two pleural cavities separated by mediastinum.

• Pleura: Serous membrane with two layers:

  • Parietal pleura: Lines thoracic wall.

  • Visceral pleura: Covers lung surface.

  • Pleural fluid: Lubricates, reduces friction.

Respiration: Processes and Mechanisms

External and Internal Respiration

• External respiration: Exchange of O2 and CO2 between interstitial fluid and external environment.

• Internal respiration: Absorption of O2 and release of CO2 by cells.

Steps in External Respiration

• Pulmonary ventilation (breathing): Moves air in and out of lungs.

• Gas diffusion:

  • Between alveolar air and pulmonary capillary blood.

  • Between systemic capillary blood and tissues.

• Transport of O2 and CO2 in blood.

• Hypoxia: Low tissue oxygen levels.

• Anoxia: Complete lack of oxygen in tissues (can cause cell death).

Pulmonary Ventilation

Mechanics of Breathing

• Pulmonary ventilation is the physical movement of air into and out of the respiratory tract, providing alveolar ventilation.

• Air flows from higher to lower pressure (down a pressure gradient).

• Boyle’s Law: Pressure and volume of a gas are inversely related. Equation:

• Movement of gas molecules exerts pressure on the container (lungs/thoracic cavity).

Respiratory Cycle

• Consists of inspiration (inhalation) and expiration (exhalation).

• Volume changes in the thoracic cavity (via diaphragm and rib cage) create pressure changes that drive airflow.

• At rest: Atmospheric and intrapulmonary pressures are equal; no air movement.

• Inhalation: Thoracic volume increases, intrapulmonary pressure decreases; air flows in.

• Exhalation: Thoracic volume decreases, intrapulmonary pressure increases; air flows out.

Muscles of Respiration

• Primary muscles (active during quiet breathing):

  • Diaphragm (contracts and flattens during inhalation)

  • External intercostals (elevate ribs)

• Accessory muscles (active during forced breathing):

  • Sternocleidomastoid, scalenes, pectoralis minor, serratus anterior (assist inhalation)

  • Internal intercostals, abdominal muscles (assist exhalation)

• Inhalation is always active (requires muscle contraction); exhalation is passive at rest, active during forceful breathing.

Types of Breathing

• Quiet breathing (eupnea): Active inhalation, passive exhalation.

• Diaphragmatic (deep) breathing: Driven by diaphragm.

• Costal (shallow) breathing: Driven by rib cage movements.

• Elastic rebound: Relaxation of inspiratory muscles returns lungs and chest to original position.

• Forced breathing (hyperpnea): Both inhalation and exhalation are active, using accessory muscles.

Pressure Changes During Breathing

• Normal atmospheric pressure: 1 atm = 760 mm Hg.

• Intrapulmonary (intra-alveolar) pressure: Pressure inside alveoli; varies slightly during breathing (−1 mm Hg during inhalation, +1 mm Hg during exhalation).

• Intrapleural pressure: Pressure in pleural cavity; usually negative relative to atmospheric pressure.

• Pneumothorax: Air enters pleural cavity (due to injury or alveolar rupture), breaking fluid bond and causing lung collapse (atelectasis). Treatment involves removing air to restore negative pressure and reinflate lung.

Factors Affecting Pulmonary Ventilation

• Airway resistance: Opposes airflow; decreased by bronchodilation, increased by bronchoconstriction.

• Compliance: Measure of lung expandability.

  • Lower compliance = harder to fill lungs.

  • Increased by loss of lung tissue (e.g., emphysema).

  • Decreased by low surfactant (alveolar collapse) or reduced thoracic mobility (injury/disease).

Summary Table: Key Structures and Functions of the Respiratory System

Example: Application of Boyle's Law in Breathing

• During inhalation, the diaphragm contracts, increasing thoracic volume and decreasing intrapulmonary pressure below atmospheric pressure, causing air to flow into the lungs.

• During exhalation, the diaphragm relaxes, thoracic volume decreases, intrapulmonary pressure rises above atmospheric pressure, and air flows out.

• Boyle's Law equation:

Additional info: The notes above expand on the original content by providing definitions, examples, and a summary table for clarity and completeness. All key terms are explained, and the physiological mechanisms are described in a student-friendly, academic style.