Respiratory System

6.1 Anatomy and Mechanism of Breathing

Describes respiratory tract anatomy, breathing mechanics, lung volumes, and regulation of ventilation.

• Anatomy
  • Air enters through nares, filtered by vibrissae and mucous
  • Passes pharynx → larynx (air only, glottis covered by epiglottis during swallowing)
    • 2 vocal cords controlled by skeletal muscle/cartilage
  • Trachea → mainstem bronchi → smaller bronchioles → alveoli
    • Alveoli covered in surfactant (lowers surface tension, aids gas exchange)
  • Lungs in thoracic cavity, surrounded by pleura
    • Visceral pleura: adjacent to lung
    • Parietal pleura: outer layer
  • Diaphragm: separates thoracic & abdominal cavities, under somatic control
  • Intercostal muscles assist breathing
• Pathway of Air
  • Nares → pharynx → larynx → trachea → bronchi → bronchioles → alveoli

  

• Breathing Mechanics
  • Interpleural space contains fluid, lubricates pleura
  • Inhalation: active, diaphragm & external intercostals contract → intrathoracic volume ↑ → pressure ↓ → lungs expand
  • Exhalation: usually passive, internal intercostals assist, chest wall recoils
  • Emphysema: alveolar wall destruction → reduced elastic recoil
• Lung Capacities
  • TLC (total lung capacity): 6–7 L, cannot measure with spirometry (RV included)
  • RV (residual volume): air remaining after full exhale, cannot measure with spirometry
  • VC (vital capacity) = TLC - RV
  • TV (tidal volume): normal inhale/exhale
  • ERV (expiratory reserve volume): forcibly exhaled
  • IRV (inspiratory reserve volume): forcibly inhaled
  • Decreased in stiff lung tissue (e.g., pulmonary fibrosis)

• Regulation of Breathing
  • Controlled by medulla oblongata (ventilation center)
  • Chemoreceptors sensitive to CO2
    • Hypercapnia → ↑ respiratory rate
    • Severe hypoxemia → ↑ respiratory rate
  • Conscious control possible

6.2 Functions of the System

Covers primary functions of the respiratory system: gas exchange, thermoregulation, immune defense, and pH control.

• Gas Exchange
  • Capillaries bring deoxygenated blood from right ventricle to alveoli
  • Alveoli walls: 1 cell thick → efficient diffusion along concentration gradients
  • Higher altitudes: ↑ breathing rate, Hb unloading affected, ↑ RBC and blood vessels
• Thermoregulation
  • Vasodilation/vasoconstriction of nasal/tracheal capillaries
  • Shivering, sweating, panting
• Immune Function
  • Nasal cavity: first line of defense
    • Vibrissae trap particulates
    • Lysozyme attacks gram-positive bacteria
    • Cilia propel mucus to oral cavity (mucociliary elevator)
  • Macrophages engulf pathogens
  • IgA antibodies protect against pathogens
  • Mast cells with preformed antibodies → allergic reactions
• Control of pH
  • Bicarbonate buffer system: $CO_2 + H_2O ↔ H_2CO_3 ↔ H^+ + HCO_3^-$
  • Maintains blood pH 7.35–7.45
  • Acidemia → ↑ respiratory rate
  • Alkalemia → ↓ respiratory rate (retain CO2)

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