Comprehensive study notes: MHS1102 Lecture 5 — Respiratory System 1

Introduction to the Respiratory System

  • Cells need oxygen for aerobic metabolism and produce carbon dioxide waste. Oxygen diffuses into the blood in the lungs; blood delivers oxygen to tissues and returns carbon dioxide to the lungs for exhalation.

  • Inhaled air reaches exchange surfaces; deoxygenated blood is routed to alveoli via pulmonary arteries, becomes oxygenated, and returns to the left atrium via pulmonary veins.

Functions of the Respiratory System

  • Provide large surface area for gas exchange.

  • Move air to and from lung exchange surfaces.

  • Protect respiratory surfaces from dehydration, temperature, and pathogens.

  • Produce sounds for communication.

  • Detect odors with nasal olfactory receptors.

Anatomy Overview

  • Organization:

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

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

  • Conducting vs. Respiratory Portion:

    • Conducting portion: nasal cavity to larger bronchioles (cleans, warms, moistens air).

    • Respiratory portion: smallest respiratory bronchioles and alveoli (gas exchange).

    • Alveoli: air-filled pockets, primary site of gas exchange.

The Upper Respiratory System

  • Nose:

    • Primary air entry via external nares (nostrils).

    • Air passes into nasal vestibule; nasal hairs trap large particles.

  • Nasal cavity:

    • Divided by nasal septum.

    • Anterior portion, supported by hyaline cartilage; apex of nose is cartilaginous.

    • Superior portion houses olfactory region.

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

  • Airflow and conditioning in the nasal cavity:

    • Air flows via meatuses; turbulence traps particles, warms/humidifies air, and delivers olfactory stimuli.

    • Nasal mucosa warms/humidifies inhaled air, crucial for lower respiratory system protection. Mouth breathing bypasses this.

  • Palates:

    • Hard palate: forms nasal cavity floor, separates nasal/oral cavities.

    • Soft palate: separates nasopharynx from rest of pharynx.

  • Nosebleeds (epistaxis): Common due to extensive nasal cavity vascularization.

  • Pharynx (throat):

    • Shared by digestive/respiratory systems; extends from choanae to larynx/esophagus.

    • Divided into: nasopharynx, oropharynx, laryngopharynx.

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

    • Oropharynx: Connects to oral cavity.

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

The Larynx and Lower Respiratory System (Basics)

  • Air flows from pharynx to larynx via the glottis (vocal cord opening).

  • Larynx cartilages (three unpaired):

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

    • Cricoid cartilage (hyaline): posterior; articulates with arytenoids.

    • Epiglottis (elastic): covers glottis during swallowing, preventing aspiration.

  • Ligaments connect laryngeal cartilages to hyoid bone, epiglottis.

  • Swallowing: Larynx elevates, epiglottis folds over glottis.

  • Smaller hyaline cartilages (paired): arytenoid, corniculate, cuneiform; aid glottis movement and sound production.

  • Larynx ligaments: Vestibular and vocal ligaments within vestibular/vocal folds; protect and produce sound.

  • Sound production:

    • Air vibrating vocal folds creates sound waves.

    • Intrinsic laryngeal muscles adjust vocal fold tension, changing pitch.

    • Phonation: sound production at larynx; articulation: modification by lips, tongue, teeth for speech.

  • Glottis: Can be open or closed.

  • Laryngeal muscles: Two sets: neck/pharynx muscles (position/stabilize larynx) and intrinsic muscles (control vocal fold tension, open/close glottis).

The Trachea

  • Trachea (windpipe) is a flexible tube from cricoid cartilage to mediastinum, branching into main bronchi.

  • Structure:

    • Submucosa: thick connective tissue with mucus-producing tracheal glands.

    • 15−2015-20 C-shaped tracheal cartilages stiffen the airway; ends connected by elastic annular ligaments and trachealis muscle, allowing distortion for swallowing.

    • Inner lining: ciliated respiratory epithelium with lamina propria.

Respiratory Mucosa and Epithelium

  • Respiratory mucosa lines the conducting portion; consists of epithelium and lamina propria.

  • Respiratory defense system functions: filtration, mucus production, ciliary transport, alveolar macrophages.

  • Epithelial types by region:

    • Nasal cavity, superior pharynx, larger airways (bronchi): pseudostratified ciliated columnar with goblet cells.

    • Inferior pharynx: stratified squamous epithelium.

    • Smaller bronchioles: cuboidal epithelium with scattered cilia.

  • Alveolar epithelium:

    • Type I pneumocytes: simple squamous, lines gas-exchange surface.

    • Type II pneumocytes: produce surfactant.

    • Alveolar macrophages: patrol alveolar space.

  • Surfactant: Oily secretion (phospholipids, proteins) reduces surface tension, preventing alveolar collapse during exhalation.

  • Respiratory Distress Syndrome (RDS): Insufficient surfactant causes alveolar collapse.

Gas Exchange Structures and the Blood-Air Barrier

  • Gas exchange occurs across the thin blood–air barrier in alveoli, composed of:

    • Alveolar cell layer (Type I pneumocytes).

    • Capillary endothelial layer.

    • Fused basement membranes.

  • Efficiency: Due to short diffusion distance and small, lipid-soluble O2\text{2} and CO2\text{2}.

  • Pneumonia: Fluid in alveoli injures the barrier, compromising gas exchange.

The Lungs and Pleura

  • Pleura and pleural fluid:

    • Lungs are in pleural cavities, lined by serous membranes (pleura), which reduce friction, create a pressure gradient for inflation, and compartmentalize to limit infection.

    • Parietal pleura: lines thoracic wall.

    • Visceral pleura: covers lung surfaces.

    • Pleural fluid lubricates the space.

  • The Lungs:

    • Each lung in its own pleural cavity; mediastinum separates them.

    • Base rests on diaphragm; fissures separate lobes.

    • Right lung: three lobes (superior, middle, inferior) with horizontal and oblique fissures; broader.

    • Left lung: two lobes (superior, inferior) with oblique fissure and cardiac notch; longer.

    • Hilum: entry/exit for pulmonary vessels, nerves, lymphatics.

    • Root of the lung: connective tissue anchoring lung to mediastinum.

  • Pulmonary vasculature:

    • Deoxygenated blood from pulmonary arteries to alveolar capillaries; oxygenated blood returns via pulmonary veins to left atrium.

    • Bronchial arteries supply conducting passages.

    • Pulmonary circuit blood pressure: Lower than systemic; susceptible to blockages (pulmonary embolism).

    • Expressed as PextpulmonaryP_{ ext{pulmonary}}.

The Bronchial Tree and Airways

  • Branching pattern:

    • Main bronchi (right/left) to lobar bronchi (to each lobe), then to segmental bronchi (to bronchopulmonary segments).

    • Right lung: 1010 segments; Left lung: 88 or 99 segments.

  • Bronchi and bronchioles:

    • Segmental bronchi lead to bronchioles; terminate into terminal bronchioles (approx. 6,5006,500 per segmental bronchus).

    • Bronchioles lack cartilage; smooth muscle dominates distally.

  • Structural changes with branching:

    • Bronchial walls have decreasing cartilage and increasing smooth muscle, affecting airway resistance.

  • Autonomic control of bronchioles:

    • Sympathetic: bronchodilation (increased diameter, decreased resistance).

    • Parasympathetic: bronchoconstriction (decreased diameter).

    • Histamine also causes bronchoconstriction.

  • Asthma: Excess smooth muscle stimulation leads to severe bronchoconstriction.

  • Alveolar connections:

    • Terminal bronchioles branch to respiratory bronchioles, connecting to alveolar ducts and sacs; alveoli are gas exchange sites.

The Alveolar Structure and Capillary Network

  • Alveolar organization:

    • Alveolar ducts end in alveolar sacs; alveoli cluster into pulmonary lobules.

    • Each alveolus has a capillary network surrounded by elastic fibers.

  • Alveolar cells and support:

    • Type I pneumocytes: simple squamous, for gas exchange.

    • Type II pneumocytes: produce surfactant.

    • Alveolar macrophages: engulf debris.

  • Blood-air barrier efficiency: Rapid gas exchange due to thin barrier, lipid-soluble gases.

Breathing Mechanics and Primary Muscles of Respiration

  • Primary respiratory muscles: Diaphragm and external intercostals.

  • Accessory respiratory muscles: Activated for increased ventilation (exercise, distress).

  • Breathing mechanisms:

    • Inhalation: Always active (muscle contraction).

    • Exhalation: Can be passive (elastic recoil) or active (muscle-assisted).

  • Muscles used in inhalation:

    • Diaphragm: contraction draws air (\sim 75 \text{%} of normal movement).

    • External intercostal muscles: assist inhalation (\sim 25 \text{%} of normal movement).

    • Accessory muscles (elevate ribs): sternocleidomastoid, scalenes, pectoralis minor, serratus anterior.

  • Muscles used in exhalation:

    • Internal intercostal muscles, transversus thoracis: depress ribs.

    • Abdominal muscles: compress abdomen, force diaphragm upward.

  • Breathing patterns:

    • Quiet breathing (eupnea): Active inhalation, passive exhalation; diaphragmatic (deep) and costal (rib cage) movements.

    • Forced breathing (hyperpnea): Both active; requires accessory muscles.

  • Elastic rebound: Inhalation muscles relax, elastic components recoil, returning diaphragm/rib cage to resting positions.

Summary of Key Concepts and Connections

  • Gas exchange occurs by diffusion across the thin, lipid-soluble alveolar-capillary barrier.

  • The respiratory system filters, humidifies, detects odors, produces sound, and protects airways.

  • The bronchial tree's decreasing cartilage and increasing smooth muscle allow airflow regulation.

  • Surfactant from Type II pneumocytes prevents alveolar collapse.

  • Pleural cavity and fluid enable lung inflation and limit infection spread.

  • Distinct epithelia adapt conducting airways (defense) and gas-exchange regions.

  • Pulmonary blood flow maximizes gas exchange; lower pulmonary pressures risk emboli.

  • Bronchial tree segmentation (e.g., 1010 right, 8−98-9 left segments) aids disease localization.

Notation and Equations (Key Numerical References)

  • Gas exchange barrier layers:

    • Alveolar epithelium (Type I pneumocytes) →\rightarrow Capillary endothelium →\rightarrow Fused basement membranes

  • Lung segmentation and counts:

    • Right lung bronchopulmonary segments: 1010

    • Left lung bronchopulmonary segments: 8 or 98 \text{ or } 9

    • Tracheal cartilage count: 15–2015 \text{--} 20 C-shaped tracheal cartilages

    • Diaphragm contribution to inspiration: diaphragm→75 % of normal air movement\text{diaphragm} \rightarrow 75 \text{ \%} \text{ of normal air movement}

    • External intercostals contribution: 25 %25 \text{ \%} of normal air movement

    • Pulmonary vs systemic pressures (conceptual): P{\text{pulmonary}} < P{\text{systemic}}

    • Surfactant function: reduces surface tension to prevent alveolar collapse.

Connections to Clinical Contexts

  • Pneumonia: Fluid in alveoli impairs the blood-air barrier, hindering gas exchange.

  • Asthma: Excessive bronchial smooth muscle contraction causes bronchoconstriction.

  • Respiratory Distress Syndrome (RDS): Insufficient surfactant leads to alveolar collapse.

  • Pulmonary embolism: Blockage in pulmonary artery reduces blood flow to alveoli, impairing gas exchange.

Foundational and Real-World Relevance

  • The respiratory system integrates ventilation, circulation, immunity, and speech.

  • Understanding epithelial specialization, mucociliary clearance, and surfactant is crucial for respiratory pathophysiology and neonatal care.

  • Pleural mechanics explain lung inflation and impact of chest trauma/effusion on breathing.