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Cambridge IGCSE Biology · 0610

Chapter 11: Gas exchange in humans

Features of gas exchange surfaces

The surfaces in humans where gas exchange occurs (the alveoli) possess key adaptations to allow for efficient diffusion:

  1. Large surface area: Allows more space for the diffusion of gases. The alveoli provide a total surface area of 80–100 square metres.
  2. Thin surface: Alveoli have very thin walls, providing a short diffusion distance so exchange occurs rapidly.
  3. Good blood supply: A dense network of capillaries surrounds the alveoli to maintain a steep concentration gradient by carrying away oxygenated blood and bringing deoxygenated blood.
  4. Good ventilation with air: Ensures that waste gases can diffuse out of the blood while fresh oxygen diffuses in.
  5. Moist: Surfaces are moist to allow gases to dissolve before diffusing across the membrane.

Exam Traps

  • Do not say a thin wall increases surface area — it shortens diffusion distance.
  • Avoid claiming gas exchange occurs in the trachea or bronchi — it happens in alveoli only.

The human breathing system

The system is composed of several structures that facilitate the movement of air:

Lungs
The main organs where gas exchange occurs.
Diaphragm
A sheet of muscle at the base of the thorax.
Ribs and intercostal muscles
Protect the lungs and heart; muscles work antagonistically to move the rib cage during breathing.
Larynx
Contains the vocal cords.
Trachea
Connects the throat to the bronchi.
Bronchi
Tubes that carry air from the trachea into each lung.
Bronchioles
Smaller branches of the bronchi that lead to the alveoli.
Alveoli
Tiny air sacs where gas exchange between the air and the capillary network takes place.
The human respiratory system showing lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, and alveoli with capillaries
Diagram 1: The human respiratory system. The lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, and alveoli with their associated capillaries are labelled.

Cartilage and protection

Cartilage
Present as C-shaped rings in the trachea and bronchi to provide structural strength, keeping the airways open so air can pass through.
Protection from pathogens
  • Goblet cells: Found in the trachea and bronchi; they secrete mucus to trap foreign pathogens and dust.
  • Ciliated cells: Possess hair-like projections called cilia that move in a coordinated way to transport the mucus upwards toward the throat to be swallowed.

Exam Traps

  • Do not say cilia secrete mucus — goblet cells secrete mucus; cilia move it.
  • Avoid describing cartilage as moving mucus — it provides structural support only.

Ventilation of the lungs

Ventilation involves changing the volume and pressure in the thorax (chest cavity) to move air.

Breathing in (inhalation)
  1. The external intercostal muscles contract and the internal intercostal muscles relax, pulling the ribs up and out.
  2. The diaphragm contracts and flattens.
  3. The volume of the thorax increases, which decreases the pressure.
  4. Air diffuses into the lungs from the higher pressure outside.
Breathing out (exhalation)
  1. Usually a passive process where muscles relax, and the thorax volume decreases.
  2. During forced exhalation, the internal intercostal muscles contract to pull the ribs down and in.
Mechanism of ventilation showing inhalation and exhalation with rib, diaphragm, and air movement arrows
Diagram 2: Mechanism of ventilation. Two stages show inhalation and exhalation. Arrows indicate the movement of the ribs (up/out vs. down/in), the diaphragm (flattening vs. doming), and the resulting movement of air.

Exam Traps

  • Do not say pressure increases during inhalation — volume increases so pressure decreases.

Composition of inspired and expired air

Component Inspired air (%) Expired air (%) Reason for difference
Oxygen Higher (~21%) Lower (~16%) Absorbed into the blood at the alveoli for respiration.
Carbon dioxide Lower (~0.04%) Higher (~4%) Produced as a waste product of respiration and released from the blood.
Water vapour Variable Higher (saturated) Evaporates from the moist surfaces of the respiratory tract.

Investigation: Limewater is used to test for carbon dioxide. When expired air is bubbled through it, it turns cloudy/milky, showing a higher CO2 concentration than inspired air.

Exam Traps

  • Do not say expired air has more oxygen than inspired air — oxygen concentration decreases.
  • Avoid claiming limewater tests for oxygen — it detects carbon dioxide.

Physical activity and breathing

Effects
Physical activity increases both the rate and depth of breathing.
Explanation
  1. During exercise, muscles increase the rate of aerobic respiration, producing more carbon dioxide.
  2. This increase in CO2 concentration in the blood is detected by the brain.
  3. The brain signals the breathing muscles to work faster and harder to expel CO2 and take in more oxygen.

Exam Traps

  • Do not say low oxygen alone triggers faster breathing — raised CO2 is the main detected change.
  • Avoid describing breathing changes as voluntary muscle planning rather than automatic brain response to CO2.

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