Chapter 9: Gas Exchange in Humans

📖 Chapter 9: Gas Exchange in Humans — IGCSE Biology

IGCSE Biology

Chapter 9
Gas Exchange in Humans

A complete guide to the human respiratory system, alveoli, breathing mechanisms, and gas exchange adaptations.

Pages 1 – 6

9.1 The Human Respiratory System

Gas exchange is the process by which oxygen (O₂) moves from the air into the bloodstream, while carbon dioxide (CO₂) moves from the blood back into the air.

Key Structures

  • Nose/Mouth — air entry; nose warms, filters and moistens air
  • Trachea — windpipe; reinforced with C-shaped cartilage rings
  • Bronchi — two branches, one entering each lung
  • Bronchioles — smaller tubes branching inside the lungs
  • Alveoli — tiny air sacs where gas exchange occurs
  • Diaphragm — muscle sheet below lungs; controls breathing
💡 Key Term: Ventilation — the movement of air into and out of the lungs (breathing). It is not the same as gas exchange.
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9.2 Alveoli — Structure & Adaptations

There are approximately 700 million alveoli in the human lungs, giving a total surface area of about 70 m²(the size of a tennis court).

Adaptations for Efficient Gas Exchange

  • Large surface area — millions of alveoli massively increase the area for diffusion
  • Thin walls — only one cell thick (squamous epithelium), so diffusion distance is minimal
  • Moist lining — gases dissolve in the film of moisture before diffusing
  • Rich capillary network — surrounded by blood capillaries to maintain concentration gradient
  • Ventilation — breathing constantly refreshes air, maintaining a steep gradient
Fick's Law: Rate of diffusion ∝ (Surface Area × Concentration Difference) ÷ Thickness of membrane
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9.3 The Breathing Mechanism

Inhalation (Breathing In)

  • Diaphragm contracts → flattens downward
  • Intercostal muscles contract → ribs move up and out
  • Chest volume increases
  • Air pressure inside lungs decreases(below atmospheric)
  • Air rushes into the lungs

Exhalation (Breathing Out)

  • Diaphragm relaxes → domes upward
  • Intercostal muscles relax → ribs move down and in
  • Chest volume decreases
  • Air pressure inside lungs increases(above atmospheric)
  • Air is pushed out of the lungs
💡 Remember: Breathing is a physical process (muscles + pressure). Gas exchange is a chemical/diffusion process. Don't confuse them!
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9.4 Composition of Inhaled vs. Exhaled Air

Gas Inhaled Air Exhaled Air
Oxygen (O₂) 21% 16%
Carbon Dioxide (CO₂) 0.04% 4%
Nitrogen (N₂) 78% 78%
Water Vapour Variable (low) Saturated (high)
Temperature Room temp (~20°C) Body temp (~37°C)

Why does exhaled air still contain oxygen?

Not all oxygen is absorbed in a single breath — only about 5% of the oxygen in each breath is extracted by the alveoli. The remaining 16% is exhaled unused.

💡 Exam Tip: Exhaled air is warmer , wetter , and contains more CO₂ — these three facts are commonly tested.
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9.5 Summary & Key Terms

Alveolus — tiny air sac in the lungs where gas exchange takes place by diffusion.
Diaphragm — dome-shaped muscle below the lungs that controls breathing movements.
Diffusion — net movement of molecules from high to low concentration, down a concentration gradient.
Intercostal muscles — muscles between the ribs that assist in breathing by moving the rib cage.
Trachea — the windpipe; a cartilage-ringed tube carrying air from the throat to the bronchi.
Ventilation rate — the number of breaths per minute (normal adult = 12–20 breaths/min at rest).
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