5.1 Breathing and Exchange of Gases

Human Respiratory System

  • Pathway: Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli.
  • Lungs: Paired, spongy organs in the thoracic cavity, covered by pleura.
  • Diaphragm: A dome-shaped muscle essential for breathing.

Mechanism of Breathing

  • Inspiration (Inhalation):
    • Active process.
    • Diaphragm and external intercostal muscles contract.
    • Thoracic volume increases, pressure decreases.
  • Expiration (Exhalation):
    • Passive process.
    • Diaphragm and external intercostal muscles relax.
    • Thoracic volume decreases, pressure increases.

Exchange of Gases

  • Occurs by simple diffusion based on partial pressure gradients.
  • Alveoli: O₂ diffuses from alveoli to blood, CO₂ from blood to alveoli.
  • Tissues: O₂ diffuses from blood to tissues, CO₂ from tissues to blood.

Transport of Oxygen

  • 97% is transported by haemoglobin as oxyhaemoglobin.
  • Oxyhaemoglobin Dissociation Curve:
    • Sigmoid (S-shaped) curve.
    • Right shift (lower O₂ affinity): High pCO₂, high H⁺ (low pH), high temperature.
    • Left shift (higher O₂ affinity): Low pCO₂, low H⁺ (high pH), low temperature.

Transport of Carbon Dioxide

  • 70% is transported as bicarbonate ions (HCO₃⁻).
  • 20-25% as carbamino-haemoglobin.
  • 7% dissolved in plasma.
  • Chloride Shift: Exchange of HCO₃⁻ and Cl⁻ between RBCs and plasma.

Regulation of Breathing

  • Respiratory Rhythm Centre (Medulla): Controls the basic rhythm of breathing.
  • Pneumotaxic Centre (Pons): Moderates the rhythm centre, altering the respiratory rate.
  • Chemoreceptors: Sensitive to CO₂ and H⁺ levels, signaling the rhythm centre to adjust breathing.

Respiratory Volumes & Capacities

  • Tidal Volume (TV): Normal breath (500 mL).
  • Vital Capacity (VC): Max air breathed out after forced inspiration (TV + IRV + ERV).
  • Residual Volume (RV): Air remaining after forced expiration.
  • Total Lung Capacity (TLC): Total air in lungs after forced inspiration (VC + RV).

Respiratory Disorders

  • Asthma: Inflammation of bronchi and bronchioles, causing wheezing.
  • Emphysema: Damage to alveolar walls, reducing respiratory surface area (often due to smoking).
  • Occupational Respiratory Disorders: Lung damage from dust exposure in certain industries (e.g., Silicosis, Asbestosis).