• Air enters through nostrils.
• Fine hairs and mucus filter dust particles.
• Air passes through:
  • Nostrils → Throat → Lungs.
• Cartilage rings in the trachea prevent collapse of air passage.

8. Alveoli

• Lungs contain millions of alveoli.
• Alveoli are balloon-like structures.
• Provide large surface area for gas exchange.
• Surrounded by a network of blood capillaries.

9. Breathing Mechanism

Inhalation

• Ribs move upward and outward.
• Diaphragm flattens.
• Chest cavity expands.
• Air enters lungs.

Exhalation

• Ribs move downward.
• Diaphragm becomes dome-shaped.
• Chest cavity decreases.
• Air is expelled.

10. Exchange of Gases in Alveoli

• Oxygen diffuses from alveoli into blood.
• Carbon dioxide diffuses from blood into alveoli.
• Residual air always remains in lungs for continuous gas exchange.

11. Transport of Oxygen

• Large animals cannot rely only on diffusion.
• Haemoglobin transports oxygen.
• Present in Red Blood Cells (RBCs).
• Has high affinity for oxygen.

12. Transport of Carbon Dioxide

• Carbon dioxide is more soluble in water than oxygen.
• Most CO₂ is transported in dissolved form in blood.