Respiratory System – Chapter 23

Respiratory System Overview

  • Respiration = gas exchange of O2 and CO2 between atmosphere and body cells
    • Cells need O2 for aerobic ATP production; must expel metabolic CO2
  • Respiratory system provides the passageways and structures for this exchange
    • Located in head, neck, trunk, plus lungs

General Functions

  • Air passageway (conduits for airflow)
  • Site of external gas exchange (alveolar–capillary interface)
  • Detection of odors (olfactory epithelium in superior nasal cavity)
  • Sound production (larynx & vocal folds)

Structural & Functional Organization

  • Structural
    • Upper tract: nose, nasal cavity, pharynx, larynx
    • Lower tract: trachea → bronchi → bronchioles → alveolar ducts → alveoli
  • Functional
    • Conducting zone: nose → terminal bronchioles (air transport only)
    • Respiratory zone: respiratory bronchioles, alveolar ducts, alveoli (gas exchange)

Respiratory Mucosa

  • Mucosa = epithelium + basement membrane + areolar lamina propria
  • Epithelium thins from nasal cavity (pseudostratified ciliated columnar) → alveoli (simple squamous)
  • Mucus
    • Secreted by goblet cells + mucous/serous glands
    • Contains mucin (↑ viscosity), lysozyme, defensins, IgA
    • Daily production: ≈ 1–7 tbsp; called sputum when expectorated with saliva & debris

Upper Respiratory Tract

Nose & Nasal Cavity

  • Nose = first conducting structure; external nares (nostrils) → internal choanae
  • Nasal septum divides cavity; conchae (superior, middle, inferior) create turbulence for conditioning
  • Functions: warm, humidify, filter air; extensive vascular plexus (epistaxis origin)

Paranasal Sinuses

  • Lined by pseudostratified ciliated columnar epithelium; mucus swept to pharynx
  • Blocked drainage → mucus accumulation → infection (sinusitis)

Pharynx

  • Muscular funnel posterior to nasal/oral cavities & larynx
    • Nasopharynx: air only; pseudostratified ciliated columnar
    • Oropharynx & Laryngopharynx: air + food; non-keratinized stratified squamous (abrasion resistance)

Larynx (Voice Box)

  • Cylindrical airway connecting laryngopharynx → trachea
  • Functions
    • Air passage & protective sphincter (epiglottis, vestibular folds)
    • Sound production & speech modulation
    • Assists Valsalva maneuver (↑ abdominal pressure)
    • Participates in sneeze/cough reflexes

Vocal & Vestibular Ligaments

  • Vocal ligaments (true cords): thyroid → arytenoid cartilages; covered by mucosa → vocal folds
    • Vibrate when air forced through rima glottidis → sound
  • Vestibular ligaments (false cords): superior; protective; create rima vestibuli

Intrinsic Muscles & Sound Mechanics

  • Skeletal muscles adjust tension & position of cords (adduction = narrower, abduction = wider)
  • Range: cord length/thickness (males deeper voice)
  • Pitch: ↑ tension → ↑ frequency
  • Loudness: force of air across folds

Clinical – Laryngitis

  • Inflammation from infection or overuse → hoarseness; severe pediatric cases may obstruct airway

Lower Respiratory Tract

Trachea

  • Flexible, patent “windpipe”; mucosa = pseudostratified ciliated columnar

Bronchial Tree

  • Main (primary) bronchi → lobar (secondary) → segmental (tertiary) → smaller bronchi → bronchioles → terminal bronchioles
  • Cartilage rings/plates diminish; bronchioles lack cartilage & possess proportionally thick smooth muscle → bronchoconstriction/dilation

Clinical – Asthma

  • Episodic bronchoconstriction + inflammation + excess mucus
  • Triggers: allergens, exercise; chronic remodeling thickens walls
  • Treatment: inhaled steroids & bronchodilators

Respiratory Zone

  • Respiratory bronchioles → alveolar ducts → alveolar sacs (300–400 million alveoli/lung)
  • Alveolar pores equalize pressure & provide collateral ventilation
  • Pulmonary capillary network envelops alveoli

Alveolar Cell Types

  1. Type I (squamous): 95 % surface; thin diffusion barrier
  2. Type II (septal): secrete surfactant → ↓ surface tension, prevent collapse
  3. Alveolar macrophages (dust cells): fixed or free, phagocytose debris & microbes

Respiratory Membrane

  • Type I cell epithelium + fused basement membrane + capillary endothelium
  • Thickness ≈ 0.5\,\mu m; large area ≈ 70\,m^2 → efficient diffusion

Lungs & Pleura

  • Paired organs flanking mediastinum; each in its own pleural cavity
  • Bronchopulmonary segments: autonomous units (10 right; 8–10 left) with own tertiary bronchus & vascular supply; surgically resectable

Pleural Membranes & Cavity

  • Visceral pleura (on lung) & parietal pleura (thoracic wall); serous fluid lubricates
  • Intrapleural pressure < intrapulmonary → lungs stay inflated (outward pull vs elastic recoil)

Inflation Mechanics

  • Chest wall tends to expand; lungs recoil inward; surface tension of fluid couples motions

Clinical – Smoking & Lung Cancer

  • Smoking ↑ risk of infections, emphysema, atherosclerosis, ulcers, cancers (lung 85 %, esophagus, stomach, pancreas), low-birth-weight infants, secondhand effects
  • Lung cancer: aggressive; symptoms—chronic cough, hemoptysis, excess mucus

Pulmonary Ventilation (Breathing)

  • Four integrated processes of respiration
    1. Pulmonary ventilation
    2. Pulmonary gas exchange
    3. Gas transport
    4. Tissue gas exchange
  • Breathing phases
    • Inspiration (inhalation)
    • Expiration (exhalation)
    • Quiet (eupnea, restful) vs forced (vigorous/exercise)

Mechanics & Boyle’s Law

  • Volume ↔ pressure (inverse) at constant T: P1 V1 = P2 V2
  • Thoracic cavity changes
    • Vertical (diaphragm), lateral, anterior-posterior (rib movements)
  • Air flows down pressure gradient until equalization

Pressures

  • Atmospheric P_{atm} ≈ 760\,mm\,Hg at sea level
  • Intrapulmonary (alveolar) P{alv} fluctuates; = P{atm} at end phases
  • Intrapleural P_{ip} about -4\,mm\,Hg (between breaths)

Quiet Breathing Sequence

  1. MRC inspiratory neurons fire (~2 s) → diaphragm & external intercostals contract → thoracic V ↑ → P_{alv} ↓ → air in
  2. Inspiratory neurons inhibited (~3 s) → muscles relax → thoracic V ↓ → P_{alv} ↑ → air out

Forced Breathing

  • Recruit accessory muscles (sternocleidomastoid, scalenes, abdominal, etc.) → larger ΔV and ΔP → greater airflow; visible chest movement

Nervous Control of Breathing

  • Respiratory center (brainstem)
    • Medullary Respiratory Center (MRC)
    • Ventral RG (VRG) – rhythm generation; motor output
    • Dorsal RG (DRG) – sensory integration
    • Pontine Respiratory Center (pneumotaxic) – smooth transitions
  • Neural pathways
    • Phrenic nerves → diaphragm; intercostal nerves → intercostals

Receptors & Reflexes

  • Chemoreceptors
    • Central (medulla) sense CSF pH (reflects P{CO2})
    • Peripheral (aortic, carotid bodies) respond to blood H^+, P{CO2}, P{O2}
  • Others: irritant receptors, baroreceptors (Hering–Breuer), proprioceptors
  • Higher centers: hypothalamus (temperature), limbic (emotion), cortex (voluntary override)

Key Stimulus

  • P{CO2} rise of 5\,mmHg → breathing rate doubles; P{O2} less sensitive (must fall to <60\,mmHg)
  • Hypoxic drive: in chronic hypercapnia (e.g.
    emphysema) P{O2} becomes primary stimulus; supplemental O_2 can depress respiration

Airflow, Resistance & Compliance

  • Airflow formula: F = \dfrac{\Delta P}{R}
    • \Delta P = P{atm} - P{alv} (pressure gradient)
    • R = resistance (airways, lung/chest elasticity, alveolar state)
  • Factors ↑ R
    • ↓ chest wall elasticity (age, disease)
    • Bronchoconstriction or lumen occlusion (mucus, inflammation)
    • Alveolar collapse (surfactant deficiency)
  • Compliance = ease of expansion; ↓ compliance → ↑ work (respiratory disorders may raise energy cost from 5 % to 25 % of total metabolism)

Gas Exchange Principles

Partial Pressures & Dalton’s Law

  • P_{gas} = \text{Total pressure} \times \text{\% of gas}
  • Each gas diffuses independently down its own gradient
  • Alveolar gas differs from atmospheric due to dead space mixing, diffusion, humidity

Henry’s Law & Solubility

  • Gas in liquid ∝ P_{gas} & solubility coefficient
  • CO2 24× more soluble than O2; N_2 least soluble (requires high pressure to dissolve)
  • Decompression sickness: rapid surfacing → N2 bubbles; treated with hyperbaric chambers (↑P{O_2})

Pulmonary Gas Exchange (External Respiration)

  • Efficiency aided by huge surface area & thin membrane
  • Ventilation–perfusion coupling: local control matches airflow (bronchioles) with blood flow (arterioles)
    • ↓ airflow → bronchoconstriction & arteriole constriction
    • ↑ airflow → bronchodilation & arteriole dilation

Tissue Gas Exchange (Internal Respiration)

  • Systemic cells consume O2, produce CO2 → gradients drive diffusion opposite of lungs

Gas Transport in Blood

Oxygen

  • 98 % bound to hemoglobin (HbO2); 2 % dissolved
  • Saturation curve (sigmoid)
    • Cooperative binding; steep portion facilitates unloading in tissues
    • Sea-level arterial P{O2} = 104\,mmHg → 98 % saturation
    • Oxygen reserve: after systemic circuit at rest, Hb still 75 % saturated; provides margin for ↑ demand
  • Influences on Hb-O2 affinity
    • ↑CO_2, ↑H^+ (Bohr effect), ↑temperature, 2,3-BPG → right shift (easier unloading)

Carbon Dioxide

  • Transport forms
    1. Dissolved in plasma (7 %)
    2. Carbaminohemoglobin (23 %) – binds globin amino groups
    3. Bicarbonate in plasma (70 %)
    • In RBCs: CO2 + H2O \xrightarrow{carbonic\ anhydrase} H2CO3 \rightarrow H^+ + HCO_3^-
    • Chloride shift exchanges HCO_3^- with Cl^-

Hemoglobin as Multi-solute Carrier

  • Binds O2 (iron), CO2 (globin), and H^+
  • Binding of one modulates affinity for others (allosteric regulation)

Breathing & Homeostasis

  • Normal set-point: 12–15 breaths/min, tidal volume ≈ 500\,mL

Hyperventilation

  • Breathing > metabolic need → ↓P{CO2} (hypocapnia) & respiratory alkalosis
  • Cerebral vasoconstriction → dizziness, paresthesias; prolonged → coma/death

Hypoventilation

  • Bradypnea/hypopnea → ↑P{CO2} (hypercapnia) & ↓P{O2} (hypoxemia)
  • Leads to respiratory acidosis, cyanosis, polycythemia; severe → convulsions/death

Exercise (Hyperpnea)

  • Depth ↑, rate stable; proprioceptors, motor commands, and anticipation stimulate respiratory center
  • Enhanced ventilation + cardiac output keeps arterial P{O2} & P{CO2} stable despite ↑ metabolism

Clinical Correlations Summary

  • Asthma: reversible bronchoconstriction; immune hypersensitivity
  • Emphysema: smoking-induced destruction of alveoli & elastic tissue → ↓ surface area & recoil
  • Respiratory diseases (CHF, PE, cancer) impair diffusion, surface area, or V/Q matching → ↓P{O2}, ↑P{CO2}
  • Hypoxic drive caution in chronic CO2 retainers; oxygen therapy must be titrated