Comprehensive Study Notes: The Respiratory System (Chapter 23, Anatomy & Physiology)

An Introduction to the Respiratory System

Overview of Respiratory Function

The respiratory system is essential for gas exchange, supplying oxygen to body tissues and removing carbon dioxide produced by cellular metabolism. It also plays roles in sound production, olfaction, and protection against environmental hazards.

• Aerobic metabolism in cells requires oxygen and produces carbon dioxide as a waste product.

• Oxygen is obtained from the air via diffusion across lung surfaces; blood transports oxygen to tissues and returns carbon dioxide to the lungs for exhalation.

Components and Organization of the Respiratory System

Primary Functions

• 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.

• Detects odors via olfactory receptors in the nasal cavity.

Anatomical Organization

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

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

Respiratory Tract Divisions

• Conducting portion: Nasal cavity to larger bronchioles; moves air but no gas exchange occurs here.

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

• Alveoli: Air-filled sacs where all gas exchange takes place.

Respiratory Mucosa and Defense System

• Lines the conducting portion; consists of an epithelium and underlying areolar tissue (lamina propria).

• Functions as a filtration system, removing particles and pathogens from inhaled air.

• Lamina propria: Contains mucous glands (upper system, trachea, bronchi) and smooth muscle (lower system bronchioles).

• Cilia: Propel mucus and trapped debris toward the pharynx for swallowing.

• Alveolar macrophages: Engulf small particles reaching the lungs.

Structure of Respiratory Epithelium

• Nasal cavity & superior pharynx: Pseudostratified ciliated columnar epithelium with mucous cells.

• Inferior pharynx: Stratified squamous epithelium (resists abrasion).

• Superior lower respiratory tract: Pseudostratified ciliated columnar epithelium.

• Smaller bronchioles: Cuboidal epithelium with scattered cilia.

• Alveoli: Simple squamous epithelium with specialized cells.

Upper Respiratory System

Nose and Nasal Cavity

• Primary entryway for air; air enters through nostrils (nares) into the nasal vestibule.

• Nasal hairs in the vestibule trap large particles.

• Nasal septum: Divides cavity into left and right; anterior portion is hyaline cartilage.

• Olfactory region: Superior portion; contains receptors for smell.

• Mucus from paranasal sinuses and tears cleans and moistens the cavity.

Air Flow and Meatuses

• Air passes from vestibule to choanae through superior, middle, and inferior meatuses.

• Meatuses create turbulence, trapping particles, warming, and humidifying air, and bringing odors to olfactory receptors.

Palates

• Hard palate: Forms the floor of the nasal cavity, separating it from the oral cavity.

• Soft palate: Extends posteriorly, dividing the nasopharynx from the rest of the pharynx.

Pharynx

• Shared by respiratory and digestive systems; extends from choanae to entrances of larynx and esophagus.

• Divided into three regions:

  • Nasopharynx: Superior; contains pharyngeal tonsil and auditory tube openings.

  • Oropharynx: Connects to oral cavity.

  • Laryngopharynx: Inferior; between hyoid bone and entrances to larynx and esophagus.

Lower Respiratory System

Larynx

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

• Three large, unpaired cartilages:

  • Thyroid cartilage: Hyaline; forms anterior/lateral walls; laryngeal prominence (Adam’s apple).

  • Cricoid cartilage: Hyaline; forms posterior portion; connects to first tracheal cartilage.

  • Epiglottis: Elastic cartilage; covers glottis during swallowing to prevent food/liquid entry.

• Three pairs of smaller cartilages: Arytenoid, corniculate, cuneiform—important for opening/closing glottis and sound production.

Vocal Folds and Sound Production

• Vestibular ligaments: Within vestibular folds; protect vocal folds.

• Vocal folds (vocal cords): Involved in sound production; air passing through vibrates them to produce sound waves.

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

• Phonation: Sound production at the larynx.

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

Trachea and Bronchial Tree

• Trachea: Tough, flexible tube from cricoid cartilage to mediastinum; branches into right and left main bronchi.

• Contains 15–20 C-shaped cartilages for support; open posteriorly to allow esophageal expansion.

• Bronchial tree: Main bronchi → lobar bronchi (to lung lobes) → segmental bronchi (to bronchopulmonary segments).

• Right lung: 10 segments; left lung: 8–9 segments.

• Bronchioles: No cartilage; dominated by smooth muscle; branch into terminal bronchioles (about 6500 per segmental bronchus).

Bronchial Regulation and Disorders

• Autonomic nervous system: Controls bronchodilation (sympathetic, increases diameter) and bronchoconstriction (parasympathetic or histamine, decreases diameter).

• Asthma: Excessive smooth muscle stimulation causes severe bronchoconstriction, restricting airflow.

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

Gas Exchange Structures

Alveolar Organization

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

• Alveolar sacs are clusters of alveoli; each alveolus is surrounded by capillaries and elastic fibers.

Alveolar Cell Types

• Pneumocytes type I: Simple squamous cells; site of gas exchange.

• Pneumocytes type II: Produce surfactant (oily secretion of phospholipids/proteins) to reduce surface tension and prevent alveolar collapse.

• Alveolar macrophages: Patrol and remove debris/pathogens.

Blood Air Barrier

• Composed of alveolar cell layer, capillary endothelial layer, and fused basement membrane.

• Gas exchange is rapid due to short diffusion distance and high lipid solubility of O2 and CO2.

• Pneumonia: Inflammation causes fluid leakage into alveoli, impairing gas exchange.

The Lungs

Gross Anatomy

• Right lung: 3 lobes (superior, middle, inferior); separated by horizontal and oblique fissures.

• Left lung: 2 lobes (superior, inferior); separated by oblique fissure; has cardiac notch for heart.

• Hilum: Entry/exit for pulmonary vessels, nerves, lymphatics; root of the lung anchors to mediastinum.

• Trabeculae: Fibrous partitions dividing lung into smaller compartments; smallest are pulmonary lobules.

Blood Supply

• Respiratory surfaces receive deoxygenated blood from pulmonary arteries; oxygenated blood returns via pulmonary veins.

• Bronchial arteries supply conducting passageways.

• Pulmonary embolism: Blockage of pulmonary artery branch, stopping blood flow to lung tissue.

Pleural Cavities and Membranes

• Each lung is in a pleural cavity lined by serous membrane (pleura).

• Parietal pleura: Lines thoracic wall; visceral pleura: covers lung surface.

• Pleural fluid: Lubricates space between layers, reducing friction.

External and Internal Respiration

Definitions

• External respiration: All processes exchanging O2 and CO2 with the environment.

• Internal respiration: Uptake of O2 and release of CO2 by cells (cellular respiration).

Steps of External Respiration

• Pulmonary ventilation (breathing)

• Gas diffusion across blood air barrier and capillary walls

• Transport of O2 and CO2 between lungs and tissues

Respiratory Disorders

• Hypoxia: Low tissue oxygen levels.

• Anoxia: Complete lack of oxygen in tissues.

Pulmonary Ventilation

Physical Principles

• Air moves from higher to lower pressure.

• Respiratory cycle: Inspiration (inhalation) and expiration (exhalation).

• Volume changes in thoracic cavity (via diaphragm/rib cage) create pressure changes.

Boyle’s Law

• Relationship between pressure and volume of a gas:

$P = \frac{1}{V}$

• Decreasing volume increases pressure; increasing volume decreases pressure.

Respiratory Muscles

• Primary: Diaphragm (75% of air movement), external intercostals (25%).

• Accessory: Sternocleidomastoid, scalenes, pectoralis minor, serratus anterior (active during forced breathing).

• Exhalation: Internal intercostals, transversus thoracis, abdominal muscles (active during forced exhalation).

Types of Breathing

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

• Forced breathing (hyperpnea): Active inhalation and exhalation, using accessory muscles.

• Diaphragmatic breathing: Deep breathing, dominated by diaphragm.

• Costal breathing: Shallow breathing, dominated by rib cage.

Pressure Changes

• Atmospheric pressure: 1 atm = 760 mm Hg.

• Intrapulmonary (intra-alveolar) pressure: Small changes during quiet breathing (−1 to +1 mm Hg); can be much larger during forced breathing.

• Intrapleural pressure: Always below atmospheric (−4 mm Hg average); helps keep lungs inflated.

• Pneumothorax: Air in pleural cavity causes lung collapse (atelectasis).

Compliance and Resistance

• Compliance: Measure of lung expandability; lower compliance means more force needed to inflate lungs.

• Affected by lung tissue, surfactant production, thoracic mobility.

• Resistance: Adjusted by bronchodilation/constriction.

Respiratory Volumes and Capacities

• Tidal volume (TV): Air moved per breath.

• Expiratory reserve volume (ERV): Additional air exhaled after normal exhalation.

• Inspiratory reserve volume (IRV): Additional air inhaled after normal inhalation.

• Residual volume: Air remaining after maximal exhalation.

• Vital capacity: ERV + TV + IRV.

• Total lung capacity: Vital capacity + residual volume.

Volume/Capacity	Definition
Tidal Volume (TV)	Air moved in one breath
Expiratory Reserve Volume (ERV)	Extra air exhaled after normal exhalation
Inspiratory Reserve Volume (IRV)	Extra air inhaled after normal inhalation
Residual Volume	Air left after maximal exhalation
Vital Capacity	ERV + TV + IRV
Total Lung Capacity	Vital Capacity + Residual Volume

Gas Exchange

Physical Principles

• Gas exchange depends on partial pressures and diffusion between gas and liquid phases.

• Dalton’s Law: Each gas in a mixture exerts its own partial pressure.

• Atmospheric air (760 mm Hg): N2 78.6% (597 mm Hg), O2 20.9% (159 mm Hg), H2O 0.5% (3.7 mm Hg), CO2 0.04% (0.3 mm Hg).

• Henry’s Law: Amount of gas dissolved in liquid is proportional to its partial pressure.

Efficiency of Gas Exchange

• Large partial pressure differences, short diffusion distances, lipid solubility of gases, large surface area, and coordinated blood/air flow all enhance efficiency.

External and Internal Respiration

• External: O2 enters blood, CO2 leaves blood at alveoli.

• Internal: O2 leaves blood, CO2 enters blood at tissues.

Gas Transport

Oxygen Transport

• O2 binds reversibly to iron in hemoglobin (Hb) to form oxyhemoglobin.

• Each Hb molecule binds up to 4 O2 molecules; each RBC has ~280 million Hb molecules.

• Hemoglobin saturation: % of heme units with bound O2; affected by partial pressure, pH, temperature, and BPG levels.

Oxygen–Hemoglobin Saturation Curve

• Shows relationship between Hb saturation and O2 partial pressure.

• Curve shifts right (more O2 released) with lower pH or higher temperature; shifts left (less O2 released) with higher pH or lower temperature.

Bohr Effect

• Decrease in pH (from CO2 entering RBCs and forming carbonic acid) reduces Hb’s O2 affinity, enhancing O2 release.

• Equation for carbonic acid formation:

$\mathrm{CO}_2 + \mathrm{H}_2O \xrightarrow{\text{carbonic anhydrase}} \mathrm{H}_2\mathrm{CO}_3 \rightarrow \mathrm{H}^+ + \mathrm{HCO}_3^-$

Other Factors Affecting O2 Release

• Temperature: Higher temperature increases O2 release (important in active tissues).

• BPG (2,3-bisphosphoglycerate): Increased BPG promotes O2 release; low BPG prevents O2 release.

• Fetal hemoglobin: Binds O2 more strongly than adult Hb, facilitating O2 transfer from mother to fetus.

• Carbon monoxide (CO): Binds Hb more strongly than O2, causing poisoning.

Carbon Dioxide Transport

• CO2 is transported in three forms:

  • As bicarbonate ions (70%)

  • Bound to Hb as carbaminohemoglobin (23%)

  • Dissolved in plasma (7%)

• Bicarbonate formation involves the chloride shift (exchange of HCO3− for Cl− in RBCs).

Control of Respiration

Local and Neural Regulation

• Local changes in O2 delivery and ventilation-perfusion ratio adjust gas exchange efficiency.

• Respiratory centers in the brain (medulla oblongata and pons) regulate rate and depth of breathing.

• Voluntary control from cerebral cortex; involuntary control from brainstem centers.

Respiratory Centers

• Medulla oblongata: Respiratory rhythmicity centers (dorsal and ventral respiratory groups, DRG and VRG).

• DRG: Controls inspiration during quiet and forced breathing.

• VRG: Controls forced inspiration and expiration.

• Pons: Apneustic and pneumotaxic centers adjust output of medullary centers.

Reflexes and Sensory Input

• Chemoreceptors: Monitor CO2, O2, and pH in blood/CSF; stimulate respiratory centers as needed.

• Baroreceptors: Detect blood pressure changes; low BP increases respiratory rate, high BP decreases it.

• Stretch receptors: Prevent overexpansion (inflation reflex) and stimulate inspiration during deflation (deflation reflex).

• Protective reflexes: Sneezing, coughing, laryngeal spasm in response to irritants.

Disorders and Voluntary Control

• Sudden infant death syndrome (SIDS): Disruption of normal respiratory reflexes in infants.

• Hypercapnia: High CO2 due to hypoventilation; increases respiratory rate.

• Hypocapnia: Low CO2 due to hyperventilation; decreases respiratory rate.

• Emotions and anticipation of exercise can alter breathing via hypothalamic and autonomic pathways.

Age-Related Changes in Respiration

Newborn Adaptations

• Before birth: Lungs are collapsed, no air present.

• At birth: First breath inflates lungs, closes fetal circulatory shunts (foramen ovale, ductus arteriosus).

Aging Effects

• Elastic tissue deteriorates, reducing lung compliance and vital capacity.

• Arthritic changes restrict chest movement.

• Emphysema: Progressive alveolar damage, common in smokers and elderly.

Integration with Other Systems

Homeostatic Regulation

• Respiratory and cardiovascular systems coordinate to maintain O2 and CO2 balance in tissues.

• Adjustments in lung perfusion, respiratory rate, blood pressure, and cardiac output ensure efficient gas exchange.

Additional info: These notes synthesize and expand upon the provided lecture content, filling in missing terms (e.g., O2, CO2, BPG, etc.) and providing academic context for clarity and completeness.