Respiratory System - Comprehensive Notes

Course Information

  • Course: FLG 221
  • Lecture 1
  • Lecturer: Ms MT Lebelo
  • Location: Tswelopele Building
  • abbreviations: AD
  • Slogans/affiliations emerging on page: PERSEQUOR, DESTINATUM PERSED
  • University: UNIVERSITEIT VAN PRETORIA / UNIVERSITY OF PRETORIA / YUNIBESITHI YA PRETORIA
  • Faculty: Faculty of Health Sciences / Fakulteit Gesondheidswetenskappe / Lefapha la Disaense tša Maphelo
  • Motto: Make today matter
  • Website: www.up.ac.za

Respiratory System: Structural and Functional Overview

  • Respiratory system classification (structural):
    • Nasal cavity
    • Upper respiratory tract
    • Pharynx
    • Larynx
    • Trachea
    • Lower respiratory tract
    • Primary bronchi
    • Lungs
  • Source/reference note: https://www.ck12.org/c/life-science/respiratory-system-organs/lesson/Respiratory-System-Organs-MS-LS/

Functional Classification: Conducting vs Respiratory Zones

  • Conducting zone: conducts air to the lungs
    • Components: Nasal cavity, Pharynx, Larynx, Trachea, Bronchi, Bronchioles, Alveoli, Respiratory bronchioles, Alveolar duct
  • Respiratory zone: gas exchange regions

Anatomy: Overall Structure of the Respiratory System

  • Structures listed: Nose and nasal cavity; Pharynx; Larynx; Trachea; Bronchi tree; Upper and lower respiratory airways

Nose and Nasal Cavity: Structure and Immediate Function

  • Lined with mucous membranes
  • Contains ciliated epithelium and goblet cells (mucus secretion)
  • Rich blood supply
  • Dilation of nasal blood vessels (e.g., during cold/flu, allergy, toxins) → edema of mucous membranes → obstruction of airways
  • Source: https://courses.lumenlearning.com/wm-biology2/chapter/epithelial-tissues/

Functions of the Mucous Membrane

  • Warms air to ~37°C
  • Moistens incoming air
  • Filters incoming air: nose hairs guard nostrils; Cilia and sticky mucus entrap dust and microorganisms

Pharynx (Throat): Functional Roles

  • Common passage for:
    • Transport of food from oral cavity to esophagus
    • Transport of air from nasal cavity to larynx
  • Swallowing reflex: soft palate rises to prevent food from entering nasal cavity
  • Larynx elevates and breathing is inhibited to prevent food from entering the trachea (reduces choking risk)

Pharynx and Larynx: Spatial Relationships (diagrammatic reference)

  • Regions referenced: Oro-/pharynx, Nasopharynx, Hypopharynx
  • Associated structures: Tongue

Larynx (Voice Box): Functions

  • Continuous connection to trachea posteriorly
  • Functions:
    • Switching mechanism to route air and food into proper channels
    • Voice production
  • Inner surface lined with mucous membrane

Trachea (Windpipe): Structure and Function

  • Path: Larynx → Trachea → Primary bronchi
  • Structural feature: C-shaped cartilaginous rings provide wall firmness and prevent airway collapse
  • Lining: Mucous membrane with ciliated columnar epithelium
  • Additional role: Filtering incoming air

Bronchial Tree: Branching and Structure

  • Trachea divides into Right and Left primary bronchi
  • Bronchi lined with ciliated columnar epithelium
  • Progression: Secondary bronchi → tertiary bronchi → bronchioles → terminal bronchioles → respiratory bronchioles (few alveoli buds at this level)
  • Supporting tissue: Smooth muscle and elastic fibres

Branching of the Airways: Conducting vs Exchange Systems

  • Conducting system components (in order): Trachea, Primary bronchi, Smaller bronchi, Bronchioles
  • Exchange system components: Alveoli
  • Design principle noted: As branching becomes more numerous, the walls thin out; alveolar design increases surface area for gas exchange
  • Cross-sectional area and diameter trends are described in the table on the page, illustrating progressive thinning of walls with branching and a large total surface area in the alveolar region

The Conducting Airways: Air Conditioning of Inhaled Air

  • The conducting airways condition air before it reaches alveoli:
    • Warming: air warmed to approximately 37ext°C37^ ext{°C} to maintain core body temperature and protect alveoli
    • Humidifying: addition of water vapor
    • Filtering: removal of foreign material (viruses, bacteria, inorganic particles) to prevent alveolar exposure

Cells of the Conducting Airway Epithelium

  • Ciliated columnar epithelial cells: mucociliary movement to sweep substances upward
  • Goblet cells: secrete mucus (mucins) contributing to innate immunity
  • Serous cells: produce lysozymes and IgA
  • Clara cells (in bronchioles): produce anti-inflammatory substances (phospholipase A2 inhibitor)
  • Other cells: neuroendocrine cells regulating smooth muscle function and growth

Lungs: Structure and Protective Surroundings

  • Lungs provide an exchange surface
  • Surface area ~75extm275 ext{ m}^2
  • Thin-walled and moist surface
  • Enclosed by pleural membranes with pleural fluid
  • Occupy most of the thoracic cavity; protected by ribs and skin
  • Right lung: three lobes; Left lung: two lobes

The Pleura: Pleural Membranes and Fluid

  • A double-layered sac around each lung:
    • Parietal pleura: outer membrane attached to inner surface of thoracic wall
    • Visceral pleura: membrane covering the lung surface
    • Pleural fluid: provides lubrication and forms a pleural seal
  • Function: Reduces friction between pleural membranes and helps hold lungs against the thoracic wall
  • Analogy on page: pleural sac compared to a fluid-filled balloon around an air-filled balloon

Chest Sectional View: Pleural Sack Arrangement

  • Each lung is enclosed by two pleural membranes
  • Esophagus and aorta pass through the thorax between the pleural sacs
  • Structures labeled: Esophagus, Aorta, Parietal pleural membrane, Right/Left lungs, Heart, Pericardial/pleural spaces

The Pleural Sac Analogy (Detail)

  • The pleural sac forms a double membrane surrounding the lung, similar to a fluid-filled balloon around an air-filled balloon
  • Note: Pleural fluid volume is smaller than the illustrative depiction suggests

Respiratory/Airway Organization: Conducting vs. Respiratory Zones (Numbers and Branching Overview)

  • Conducting zone includes major conduits: Trachea → Primary bronchi → Smaller bronchi → Bronchioles > Terminal bronchioles
  • Respiratory zone includes: Respiratory bronchioles → Alveolar ducts → Alveoli
  • Alveolar count:
    • Approximately Nextalveoli ≈ 3×108N_{ ext{alveoli}} \,\approx\, 3\times 10^{8}
  • Alveolar sacs count (as per page 19): approximately 8×1068\times 10^{6}
  • Alveolar-capillary network provides the site for gas exchange

Respiratory Zone: Gas Exchange Structures

  • Structures involved: Respiratory bronchiole, Alveolar duct, Alveoli
  • Primary site of gas exchange occurs here

Alveoli: Structure, Abundance, and Capillary Association

  • Series of interconnected sacs; ~300 million alveoli present
  • Rich supply of pulmonary capillaries; arrangement optimizes diffusion between air and blood
  • Each alveolus has a single epithelial cell layer

Alveolar Cell Types and Their Functions

  • Type I alveolar cells (Type I pneumocytes):
    • Large, occupy ~95% of alveolar surface area
    • Form the barrier between air and blood; very thin to facilitate diffusion
  • Type II alveolar cells (Type II pneumocytes):
    • Smaller but thicker; synthesize and secrete surfactant (phospholipid) that lines the alveolar surface to aid lung expansion during breathing
  • Alveolar macrophages (dust cells):
    • Phagocytose inhaled dust particles; can be coughed out or transported to lymphatics via lymph nodes in the hilum of the lung

Alveolar Wall Structure: Elasticity and the Respiratory Membrane

  • Alveolar walls are thin and lack muscle fibers
  • Connective tissue contains elastin and collagen fibers, enabling elastic recoil during lung expansion and return
  • The respiratory membrane is the air-blood barrier between alveolar air and pulmonary capillary blood
  • Thickness of the respiratory membrane is approximately 0.1−1.5 μm0.1-1.5\ \mu\text{m}
  • Components include: alveolar epithelium, capillary endothelium, and fused basal laminae
  • Macrophages can reside within alveolar spaces

The Respiratory Membrane: Composition and Thickness

  • Key components across the alveolus-capillary interface:
    • Alveolar epithelium (Type I and II cells)
    • Capillary endothelium
    • Fused basal laminae between epithelium and endothelium
    • Alveolar macrophages and interstitial cells may be present
  • Thickness: 0.1−1.5 μm0.1-1.5\ \mu\text{m}

Specialized Cell Types Involved in Gas Exchange and Immunity

  • Capillary endothelial cells: form walls of capillaries (blood exchange surface)
  • Alveolar epithelial cells (Type I pneumocytes): low enzymatic activity to reduce O2 expenditure and diffusion resistance
  • Type II pneumocytes: synthesize surfactant; increased metabolic activity; contain lysosomes, microvilli, ER, vesicles; phagocytic capabilities; aid recovery after injury
  • Alveolar macrophages (dust cells): phagocytose inhaled dust particles; cleared via coughing or lymphatic drainage
  • Lymphocytes: T-cells, B-cells, Natural Killer (NK) cells – components of adaptive immunity
  • Pulmonary dendritic cells: antigen-presenting cells (APCs) of the lungs – participate in immunity
  • Neutrophils and mast cells: components of the innate immune system in the respiratory apparatus

Practical and Conceptual Takeaways

  • The respiratory system is organized into two functional zones, each with specialized structures and roles: the conducting zone (air passage and conditioning) and the respiratory zone (gas exchange)
  • The nose and nasal cavity perform warming, humidifying, and filtering of inhaled air; mucociliary clearance is a major defense mechanism
  • The pharynx and larynx coordinate passageways for air and food and contribute to voice production; protective reflexes prevent choking
  • The trachea and bronchi filter and condition air, with mucociliary clearance continuing into smaller airways
  • The lungs consist of a highly branched airway tree culminating in millions of alveoli, which provide a vast surface area for gas exchange
  • The alveolar walls and the surrounding capillaries form a thin respiratory membrane optimized for diffusion; surfactant from Type II cells reduces surface tension and aids lung expansion
  • The pleural membranes create a sealed, friction-reduced environment for lung expansion within the thoracic cavity; the pleural fluid helps maintain the lung’s position against the chest wall
  • A wide array of immune and support cells reside in the lungs to defend against inhaled pathogens and particulates, ranging from epithelial cells to macrophages, dendritic cells, lymphocytes, neutrophils, and mast cells

Key Numerical Highlights (from the Lecture Notes)

  • Lung surface area: S≈75 m2S \approx 75\ \text{m}^2
  • Alveolar population: Nalveoli≈3×108N_{\text{alveoli}} \approx 3\times 10^{8}
  • Alveolar sacs: Nalveolar sacs≈8×106N_{\text{alveolar sacs}} \approx 8\times 10^{6}
  • Respiratory membrane thickness: t≈0.1−1.5 μmt \approx 0.1-1.5\ \mu\text{m}
  • Alveolar temperature conditioning reference: T≈37∘CT \approx 37^{\circ}\text{C}
  • Alveolar surface area and diffusion arrangement support maximal gas exchange due to extensive capillary network

Conceptual Metaphors and Key Pointers

  • The pleural sac analogy: two pleural membranes around each lung with pleural fluid functioning like a suction-sealed, lubricated balloon system, reducing friction and maintaining lung position during breathing
  • The alveolar network is designed to maximize contact area with blood for efficient diffusion, much like a sponge with a vast interface
  • Surfactant production by Type II cells is essential to prevent alveolar collapse by reducing surface tension, enabling easier expansion during inspiration

Connections to Foundational Principles and Real-World Relevance

  • Gas exchange relies on diffusion across a very thin barrier (respiratory membrane) driven by partial pressure differences for O2 and CO2
  • Structural specializations (mucociliary clearance, surfactant, elastic recoil) are critical for maintaining airway patency, efficient breathing, and defense against inhaled agents
  • Clinical relevance: edema, mucous overproduction, impaired surfactant production, or damage to alveolar structures can compromise gas exchange and ventilation
  • Immunological components in the lung indicate its role in both innate and adaptive immunity, highlighting respiratory health’s broader implications for infection control and inflammatory conditions