The Respiratory System

Overview and Functions

The respiratory system is essential for gas exchange, supplying oxygen to cells and removing carbon dioxide produced during cellular respiration. It works closely with the cardiovascular system to ensure efficient transport and exchange of gases.

• Major Functions: Gas exchange, olfaction, and speech.

• Processes: Pulmonary ventilation (breathing), pulmonary gas exchange, transport of respiratory gases, and tissue gas exchange.

Major Organs and Structural Divisions

The respiratory system is divided into upper and lower regions, each with specialized structures.

• Upper Respiratory System: Nose, paranasal sinuses, pharynx.

• Lower Respiratory System: Larynx, trachea, bronchi, lungs, alveoli.

• Respiratory Muscles: Classified as part of the muscular system.

Upper Respiratory System

Nose and Paranasal Sinuses

The nose is the only externally visible part of the respiratory system and serves multiple functions.

• Functions: Airway for respiration, moistens and warms air, filters and cleans air, chamber for speech, houses olfactory receptors.

• Regions: External nose (surface features and skeletal framework), nasal cavity (divided by septum, lined with mucosa).

• Paranasal Sinuses: Lighten skull, help warm/moisten air, secrete mucus.

Pharynx

The pharynx connects the nasal cavity to the larynx and mouth to the esophagus. It is divided into three regions:

• Nasopharynx: Airway only, lined with ciliated epithelium, contains pharyngeal tonsils.

• Oropharynx: Passageway for food and air, lined with stratified squamous epithelium, contains palatine and lingual tonsils.

• Laryngopharynx: Passageway for food and air, continuous with esophagus.

Lower Respiratory System

Larynx

The larynx, or voice box, is responsible for maintaining an open airway, routing air and food, and voice production.

• Framework: Nine cartilages (including thyroid, cricoid, arytenoid, cuneiform, corniculate, and epiglottis).

• Vocal Folds: True vocal cords produce sound; vestibular folds (false vocal cords) help close glottis during swallowing.

• Epithelium: Stratified squamous (superior), ciliated columnar (inferior).

Trachea

The trachea (windpipe) is a flexible tube supported by C-shaped cartilage rings.

• Layers: Mucosa (ciliated epithelium), submucosa (seromucous glands), adventitia (outer connective tissue).

• Trachealis Muscle: Contracts during cough to expel mucus.

• Carina: Last cartilage before trachea branches into bronchi.

Bronchi and Subdivisions

Airways branch about 23 times, forming the bronchial tree.

• Conducting Zone: Trachea → main bronchi → lobar bronchi → segmental bronchi → bronchioles → terminal bronchioles.

• Respiratory Zone: Terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs (clusters of alveoli).

• Structural Changes: Cartilage decreases, epithelium thins, smooth muscle increases.

Alveoli and Respiratory Membrane

Alveoli are the sites of gas exchange, surrounded by capillaries and elastic fibers.

• Respiratory Membrane: Thin barrier for gas exchange (simple squamous epithelium, shared basement membrane).

• Type II Alveolar Cells: Secrete surfactant and antimicrobial proteins.

• Alveolar Macrophages: Clean inner surfaces.

• Alveolar Pores: Equalize air pressure and provide alternate routes.

Lungs and Pleurae

Gross Anatomy of the Lungs

Each lung is surrounded by pleurae and divided into lobes and segments.

• Right Lung: Three lobes (superior, middle, inferior).

• Left Lung: Two lobes (superior, inferior), cardiac notch for heart.

• Bronchopulmonary Segments: Each served by its own artery, vein, and bronchus.

• Lobules: Smallest subdivisions, hexagonal shape.

Pleurae

Pleurae are double-layered serous membranes.

• Parietal Pleura: Lines thoracic wall and diaphragm.

• Visceral Pleura: Covers external lung surface.

• Pleural Fluid: Lubricates and creates surface tension, keeping lungs attached to thoracic wall.

Respiratory Physiology

Pulmonary Ventilation

Pulmonary ventilation consists of inspiration and expiration, driven by pressure changes.

• Boyle’s Law: Pressure (P) varies inversely with volume (V):

• Inspiration: Active process; diaphragm and external intercostals contract, increasing thoracic volume.

• Expiration: Passive during quiet breathing; active during forced expiration (abdominal and internal intercostal muscles).

Physical Factors Influencing Ventilation

• Airway Resistance: Friction in airways; greatest in medium-sized bronchi.

• Alveolar Surface Tension: Surfactant reduces surface tension, preventing alveolar collapse.

• Lung Compliance: Measure of lung distensibility;

Pulmonary Volumes and Capacities

Pulmonary Volumes

• Tidal Volume (TV): Air moved in/out per breath (~500 mL).

• Inspiratory Reserve Volume (IRV): Air forcibly inspired beyond TV.

• Expiratory Reserve Volume (ERV): Air forcibly expired beyond TV.

• Residual Volume (RV): Air remaining in lungs after forced expiration.

Pulmonary Capacities

• Inspiratory Capacity (IC):

• Functional Residual Capacity (FRC):

• Vital Capacity (VC):

• Total Lung Capacity (TLC):

Gas Exchange

Basic Properties of Gases

• Dalton’s Law: Total pressure of a gas mixture equals the sum of partial pressures.

• Henry’s Law: Gas dissolves in liquid in proportion to its partial pressure and solubility.

Pulmonary Gas Exchange

• Factors: Partial pressure gradients, membrane thickness/surface area, ventilation-perfusion coupling.

• Oxygen: Steep gradient drives diffusion into blood.

• Carbon Dioxide: Smaller gradient, but higher solubility allows efficient diffusion.

Tissue Gas Exchange

• Oxygen: Diffuses from blood to tissues.

• Carbon Dioxide: Diffuses from tissues to blood.

Transport of Gases

Oxygen Transport

• Hemoglobin: 98.5% of O2 is bound to hemoglobin; 1.5% dissolved in plasma.

• Oxygen-Hemoglobin Dissociation Curve: S-shaped curve shows relationship between partial pressure and saturation.

• Factors Affecting Saturation: Temperature, pH, CO2, BPG.

Carbon Dioxide Transport

• Forms: Dissolved in plasma (7–10%), bound to hemoglobin (20%), as bicarbonate ions (70%).

• Bicarbonate Buffer System: Maintains blood pH.

Control of Respiration

Neural Mechanisms

• Medullary Respiratory Centers: VRG (rhythm generation), DRG (integration).

• Pontine Centers: Smooth transitions between inspiration and expiration.

Chemical Factors

• CO2: Most powerful respiratory stimulant.

• O2: Peripheral chemoreceptors respond when levels fall below 60 mm Hg.

• pH: Influences ventilation via chemoreceptors.

Clinical Applications and Disorders

Chronic Obstructive Pulmonary Disease (COPD)

• Emphysema: Alveolar destruction, reduced elasticity.

• Chronic Bronchitis: Excess mucus, inflamed/fibrosed mucosa.

Asthma

• Reversible airway obstruction, inflammation, bronchospasms.

Tuberculosis

• Caused by Mycobacterium tuberculosis; symptoms include cough, fever, weight loss.

Lung Cancer

• Leading cause of cancer death; types include adenocarcinoma, squamous cell carcinoma, small cell carcinoma.

Sleep Apnea

• Temporary cessation of breathing during sleep; treated with CPAP.

Cystic Fibrosis

• Genetic disorder causing thick mucus, frequent infections.

Developmental Aspects

• Upper respiratory structures develop first; lungs mature until young adulthood.

• Respiratory efficiency decreases with age.