Breathing and Exchange of Gases

1. Chapter at a Glance

• Breathing (respiration) is the exchange of atmospheric O₂ with CO₂ produced by cells during catabolism.
• Respiratory organs vary with habitat and organisation: simple diffusion in lower invertebrates, moist cuticle in earthworms, tracheal tubes in insects, gills in aquatic forms, lungs in terrestrial vertebrates, and cutaneous respiration in amphibians.
• Human respiratory system consists of conducting part (external nostrils to terminal bronchioles) and exchange part (alveoli and their ducts).
• Breathing occurs by creation of pressure gradients: inspiration when intrapulmonary pressure < atmospheric pressure; expiration when intrapulmonary pressure > atmospheric pressure, mediated by diaphragm and external/internal intercostal muscles.
• Exchange of O₂ and CO₂ occurs by simple diffusion across the alveolar membrane and between blood and tissues, driven by partial pressure gradients.
• O₂ is transported mainly (97 %) as oxyhaemoglobin in RBCs; CO₂ is transported mainly (70 %) as bicarbonate, 20–25 % as carbamino-haemoglobin.
• Respiratory rhythm is regulated by the respiratory rhythm centre in the medulla, moderated by the pneumotaxic centre in the pons and chemosensitive areas sensitive to CO₂ and H⁺.
• Disorders include asthma (inflammation of bronchi/bronchioles), emphysema (damage to alveolar walls) and occupational respiratory disorders (fibrosis from dust).

2. Definitions and Laws

• Breathing/respiration: The process of exchange of O₂ from the atmosphere with CO₂ produced by the cells.
• Conducting part: The part starting with the external nostrils up to the terminal bronchioles; transports atmospheric air to the alveoli, clears foreign particles, humidifies and brings air to body temperature.
• Exchange (respiratory) part: The alveoli and their ducts; site of actual diffusion of O₂ and CO₂ between blood and atmospheric air.
• Partial pressure: Pressure contributed by an individual gas in a mixture of gases (pO₂ for oxygen, pCO₂ for carbon dioxide).
• Tidal Volume (TV): Volume of air inspired or expired during a normal respiration. It is approx. 500 mL.
• Inspiratory Reserve Volume (IRV): Additional volume that can be inspired by a forcible inspiration. This averages 2500 mL to 3000 mL.
• Expiratory Reserve Volume (ERV): Additional volume that can be expired by a forcible expiration. This averages 1000 mL to 1100 mL.
• Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL.
• Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration (TV + IRV).
• Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration (TV + ERV).
• Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration (ERV + RV).
• Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration (ERV + TV + IRV) or breathe out after a forced inspiration.
• Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration (RV + ERV + TV + IRV) or vital capacity + residual volume.
• Oxygen dissociation curve: Sigmoid curve obtained when percentage saturation of haemoglobin with O₂ is plotted against pO₂.
• Carbonic anhydrase reaction (exact text):
  CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ (facilitated by carbonic anhydrase in both directions).

3. Important Diagrams and Activities

• Figure 14.1: Diagrammatic view of human respiratory system (sectional view of left lung) – shows external nostrils, larynx, trachea, bronchi, bronchioles, alveoli, pleura and diaphragm.
• Figure 14.2: Mechanism of breathing showing (a) inspiration and (b) expiration – demonstrates pressure-gradient driven thoracic volume changes.
• Figure 14.3: Diagrammatic representation of exchange of gases at the alveolus and body tissues with blood and transport of O₂ and CO₂ – illustrates partial-pressure gradients.
• Figure 14.4: Diagram of a section of an alveolus with a pulmonary capillary – shows the three-layer diffusion membrane.
• Figure 14.5: Oxygen dissociation curve – shows sigmoid relationship between % saturation of haemoglobin and pO₂.

4. Common Misconceptions and Exam Pitfalls

• Assuming O₂ is the primary regulator of respiration (text states its role is “quite insignificant”).
• Confusing IRV (inspiration after normal expiration) with ERV (expiration after normal inspiration) or mixing IC and EC.
• Stating that diffusion occurs throughout the respiratory tract instead of only across the thin alveolar-capillary membrane.
• Forgetting exact average values (TV = 500 mL, IRV = 2500–3000 mL, etc.) or omitting that every 100 mL oxygenated blood delivers ~5 mL O₂ and deoxygenated blood delivers ~4 mL CO₂.
• Overlooking that solubility of CO₂ is 20–25 times higher than O₂, allowing greater CO₂ diffusion per unit partial-pressure difference.

5. Formula Sheet