HSSLIVE Plus One Zoology Chapter 6: Breathing and Exchange of Gases Notes

Chapter 6: Breathing and Exchange of Gases

Respiratory System in Humans

Respiratory Organs:

1. Nose: Air entry, filtering, warming, moistening
2. Pharynx: Common passage for air and food
3. Larynx: Voice box with vocal cords
4. Trachea: Windpipe reinforced with C-shaped cartilage rings
5. Bronchi: Two branches of trachea entering lungs
6. Bronchioles: Smaller branches within lungs
7. Alveoli: Tiny air sacs where gas exchange occurs
8. Lungs: Paired organs in thoracic cavity

Protective Mechanisms:

• Nasal Hair: Filters large particles
• Mucus: Traps dust and microorganisms
• Cilia: Sweeps mucus toward pharynx
• Epiglottis: Prevents food from entering trachea

Lungs Structure:

• Located in thoracic cavity
• Protected by ribcage
• Covered by double-layered pleural membrane
• Right lung has three lobes, left lung has two lobes
• Contains about 300-500 million alveoli

Mechanism of Breathing

Inspiration (Breathing In):

1. Diaphragm contracts and flattens
2. External intercostal muscles contract
3. Ribs move upward and outward
4. Thoracic volume increases
5. Pressure in lungs decreases below atmospheric pressure
6. Air moves into lungs

Expiration (Breathing Out):

1. Diaphragm relaxes and moves up
2. External intercostal muscles relax
3. Ribs move downward and inward
4. Thoracic volume decreases
5. Pressure in lungs increases above atmospheric pressure
6. Air moves out of lungs

Respiratory Volumes and Capacities:

• Tidal Volume (TV): Air inspired or expired during normal breathing (500 ml)
• Inspiratory Reserve Volume (IRV): Additional air that can be inspired (2500-3000 ml)
• Expiratory Reserve Volume (ERV): Additional air that can be expired (1000-1100 ml)
• Residual Volume (RV): Air remaining in lungs after forced expiration (1100-1200 ml)
• Vital Capacity (VC): Maximum air that can be expired after maximum inspiration (TV + IRV + ERV = 4500 ml)
• Total Lung Capacity (TLC): Total air contained in lungs after maximum inspiration (VC + RV = 5700 ml)

Exchange of Gases

Diffusion of Gases:

• Partial Pressure: Pressure exerted by individual gas in a mixture
• Alveolar Air (mm Hg): O₂ (104), CO₂ (40)
• Blood (mm Hg): Deoxygenated – O₂ (40), CO₂ (45); Oxygenated – O₂ (95), CO₂ (40)
• Tissue (mm Hg): O₂ (40), CO₂ (45)

External Respiration (Lungs):

• O₂ diffuses from alveoli to blood due to pressure gradient
• CO₂ diffuses from blood to alveoli due to pressure gradient
• Thin respiratory membrane (0.5 µm) facilitates diffusion
• Large surface area (70-80 m²) enhances gas exchange

Internal Respiration (Tissues):

• O₂ diffuses from blood to tissues
• CO₂ diffuses from tissues to blood

Transport of Gases

Oxygen Transport:

• Dissolved in Plasma: About 3%
• Bound to Hemoglobin: About 97%
• Oxyhemoglobin Formation: Hb + O₂ ⟶ HbO₂
• Oxygen-Hemoglobin Dissociation Curve: S-shaped curve showing relationship between pO₂ and % saturation of Hb

Factors Affecting Oxygen Binding:

• pO₂: Higher pressure increases binding
• pCO₂: Higher CO₂ reduces O₂ binding (Bohr effect)
• Temperature: Higher temperature reduces binding
• pH: Lower pH reduces binding
• 2,3-BPG: Increases oxygen release to tissues

Carbon Dioxide Transport:

• Dissolved in Plasma: About 7%
• As Bicarbonate: About 70%
  • CO₂ + H₂O ⟶ H₂CO₃ ⟶ H⁺ + HCO₃⁻ (catalyzed by carbonic anhydrase)
• Bound to Hemoglobin: About 23% (as carbaminohemoglobin)