Summary of Breathing and Exchange of Gases

• Human Respiratory System: Comprises two lungs and associated air passages for gas exchange.
• Breathing Process: Involves inspiration (inhaling) and expiration (exhaling) of air.
• Gas Exchange: Occurs in alveoli where O₂ is absorbed into blood and CO₂ is expelled.
• Diffusion Mechanism: Gases diffuse based on partial pressure gradients; O₂ moves from alveoli to blood, CO₂ from tissues to alveoli.
• Transport of Gases: O₂ is mainly transported as oxyhemoglobin; CO₂ is transported as bicarbonate (70%) and carbaminohemoglobin (20-25%).
• Regulation of Respiration: Controlled by the respiratory center in the medulla and pneumotaxic center in the pons; sensitive to CO₂ and H+ levels.
• Respiratory Volumes: Includes tidal volume, vital capacity, residual volume, etc., important for clinical diagnosis.
• Disorders: Conditions like asthma and emphysema affect breathing efficiency.

Chapter 14: Breathing and Exchange of Gases

14.1 Respiratory Organs

• Oxygen (O₂) is utilized by organisms to break down molecules for energy.
• Carbon dioxide (CO₂) is released during catabolic reactions.
• Breathing is the exchange of O₂ from the atmosphere with CO₂ produced by cells.
• Mechanisms of breathing vary among animals:
  • Lower invertebrates: Simple diffusion over body surface.
  • Earthworms: Use moist cuticle.
  • Insects: Tracheal tubes for air transport.
  • Aquatic arthropods/molluscs: Gills for gas exchange.
  • Terrestrial forms: Lungs for gas exchange.
  • Amphibians: Can also respire through moist skin.

14.2 Mechanism of Breathing

• Breathing involves:
  • Inspiration: Drawing in air.
  • Expiration: Releasing air.
• Pressure gradients are created for air movement:
  • Inspiration occurs when intra-pulmonary pressure < atmospheric pressure.
  • Expiration occurs when intra-pulmonary pressure > atmospheric pressure.
• Muscles involved:
  • Diaphragm: Increases thoracic volume.
  • External intercostal muscles: Lift ribs.

14.3 Exchange of Gases

• Alveoli are primary sites for gas exchange.
• Gases diffuse based on pressure/concentration gradients.
• Factors affecting diffusion:
  • Solubility of gases.
  • Thickness of membranes.
• Partial Pressure:
  • pO₂ for oxygen.
  • pCO₂ for carbon dioxide.

Table 14.1: Partial Pressures (in mm Hg) of Oxygen and Carbon Dioxide

14.4 Transport of Gases

• Oxygen is mainly transported as oxyhaemoglobin.
• CO₂ is transported as:
  • 70% as bicarbonate (HCO₃) with carbonic anhydrase.
  • 20-25% as carbamino-haemoglobin.

14.5 Regulation of Respiration

• Regulated by the respiratory centre in the medulla of the brain.
• Pneumotaxic centre in the pons can modify respiratory functions.
• Chemosensitive area responds to CO₂ and H+ levels.

14.6 Disorders of Respiratory System

• Asthma: Difficulty in breathing due to bronchi inflammation.
• Emphysema: Damage to alveolar walls, reducing respiratory surface, often caused by smoking.

Important Diagrams

• Figure 14.1: Diagrammatic view of the human respiratory system.
• Figure 14.3: Gas exchange process in the respiratory and circulatory systems.
• Figure 14.4: Structure of an alveolar cavity highlighting gas exchange barriers.

Common Mistakes and Exam Tips

Common Pitfalls

• Misunderstanding Gas Exchange: Students often confuse the sites of gas exchange. Remember that gas exchange primarily occurs in the alveoli, not in other parts of the respiratory system.
• Confusing Respiratory Volumes: Be clear about different respiratory volumes and capacities. For example, Residual Volume (RV) is the air remaining after forced expiration, while Vital Capacity (VC) is the maximum air that can be exhaled after a maximum inhalation.
• Ignoring Partial Pressures: Students may overlook the significance of partial pressures (pO₂ and pCO₂) in gas exchange. Understand that these pressures drive the diffusion of gases.
• Overlooking Regulation Mechanisms: The regulation of respiration is often misunderstood. Focus on the role of the medulla and pons in controlling the respiratory rhythm.

Tips for Exam Preparation

• Diagrams and Figures: Familiarize yourself with diagrams such as the oxygen dissociation curve and the structure of the respiratory system. Being able to label these can help in exams.
• Definitions and Functions: Make sure you can define key terms like Vital Capacity, Tidal Volume, and explain their significance in respiratory physiology.
• Practice with Questions: Use practice questions to test your understanding of concepts like the mechanisms of breathing and the transport of gases in the blood.
• Understand Disorders: Be aware of common respiratory disorders like asthma and emphysema, including their causes and effects on the respiratory system.