Cambridge IGCSE0610

Gas exchange in humans

Biology 0610 Chapter Notes

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Gas exchange in humans
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In this chapter

  1. 1The Need for Gas Exchange
  2. 2Features of Gas Exchange Surfaces
  3. 3The Human Respiratory System
  4. 4Gas Exchange in the Alveoli
  5. 5The Mechanism of Breathing
  6. 6Composition of Inhaled and Exhaled Air

1. The Need for Gas Exchange

All living cells need a constant supply of energy to carry out life processes like growth, movement, and repair. This energy is released through a chemical process called aerobic respiration. Aerobic respiration uses oxygen to break down glucose (from food) and produces carbon dioxide and water as waste products. Gas exchange is the vital process of taking in oxygen from the environment and releasing the waste carbon dioxide. Without it, cells would run out of energy and be poisoned by their own waste.

glucose + oxygen → carbon dioxide + water + energy (released)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Key term

Aerobic Respiration: The chemical process in cells that uses oxygen to break down nutrient molecules, such as glucose, to release energy.

Examiner insight

Examiners look for a clear link between the body's need for energy, the process of respiration in cells, and the role of gas exchange in supplying reactants and removing waste.

Common pitfall

Confusing breathing with respiration. Breathing (or ventilation) is the physical movement of air in and out of the lungs, whereas respiration is the chemical reaction inside cells to release energy.

Worked example 14 marks

Explain why a person's breathing rate increases during exercise. [4]

  1. 1

    During exercise, muscles contract more frequently and require more energy. [1]

  2. 2

    To supply this energy, the rate of aerobic respiration in muscle cells increases. [1]

  3. 3

    This increased respiration requires more oxygen and produces more carbon dioxide as a waste product. [1]

  4. 4

    Therefore, the breathing rate increases to supply more oxygen to the lungs and remove the excess carbon dioxide more quickly. [1]

Recap

  • Gas exchange supplies oxygen for aerobic respiration.
  • Aerobic respiration releases energy for all life processes.
  • Oxygen is a reactant in respiration, and carbon dioxide is a waste product.
  • The body must take in oxygen and remove carbon dioxide to survive.

Quick check

  1. What are the two main gases involved in gas exchange in humans?1 mark
  2. State the chemical word equation for aerobic respiration.1 mark

2. Features of Gas Exchange Surfaces

For gas exchange to happen quickly and efficiently, the surface where it occurs must have specific adaptations. These features maximise the rate of diffusion of oxygen into the body and carbon dioxide out of it. In humans, this specialised surface is found in the alveoli of the lungs.

Key term

Diffusion: The net movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient.

Examiner insight

To get full marks, you must not only state the feature (e.g., 'large surface area') but also explain how it helps gas exchange (e.g., 'so more gas can diffuse at once').

Fun fact

The total surface area of all the alveoli in an adult's lungs is about 70 square metres, roughly the size of half a tennis court!

Worked example 16 marks

The alveoli in the lungs are adapted for efficient gas exchange. Describe and explain three of these adaptations. [6]

  1. 1

    Adaptation 1: Thin walls. The walls of the alveoli and the surrounding capillaries are each only one cell thick. [1] Explanation: This creates a very short diffusion distance for gases, allowing for a rapid rate of exchange. [1]

  2. 2

    Adaptation 2: Large surface area. There are millions of alveoli in the lungs, which collectively provide a huge surface area. [1] Explanation: A larger surface area allows more gas molecules to diffuse across the surface at the same time. [1]

  3. 3

    Adaptation 3: Good blood supply. Each alveolus is covered in a dense network of capillaries. [1] Explanation: This constant flow of blood quickly carries oxygen away from the lungs and brings carbon dioxide to them, maintaining a steep concentration gradient for diffusion. [1]

Recap

  • Efficient gas exchange surfaces are thin to provide a short diffusion path.
  • They have a large surface area to maximise the rate of diffusion.
  • A moist surface allows gases like oxygen to dissolve before diffusing.
  • A good blood supply maintains a steep concentration gradient.
  • Good ventilation ensures the air in the lungs is constantly refreshed.

Quick check

  1. Why is it important for a gas exchange surface to be moist?1 mark

3. The Human Respiratory System

The human respiratory system is a series of organs responsible for taking in oxygen and expelling carbon dioxide. Air enters through the nose or mouth and travels down the trachea (windpipe). The trachea is kept open by C-shaped rings of cartilage. It then splits into two bronchi (singular: bronchus), one going to each lung. Inside the lungs, the bronchi branch into progressively smaller tubes called bronchioles. These bronchioles end in tiny, microscopic air sacs called alveoli, which is where gas exchange actually occurs.

Key term

Trachea: The main tube, also known as the windpipe, that carries air from the throat towards the lungs, supported by rings of cartilage.

Common pitfall

Confusing the terms 'bronchus' (the large tube) and 'bronchiole' (the small tube). Remember that 'iole' means small.

Worked example 14 marks

Describe the path taken by an oxygen molecule from the air outside the body to an alveolus in the lung. [4]

  1. 1

    The oxygen molecule is first inhaled through the nose or mouth. [1]

  2. 2

    It then travels down the trachea (windpipe). [1]

  3. 3

    From the trachea, it passes into one of the two bronchi. [1]

  4. 4

    It then moves through smaller and smaller bronchioles, finally reaching an alveolus (air sac). [1]

Worked example 24 marks

The trachea contains rings of cartilage and is lined with cells that produce mucus. Explain the function of these two features. [4]

  1. 1

    Cartilage: The C-shaped rings of cartilage provide support and prevent the trachea from collapsing when the air pressure inside is low during inhalation. [2]

  2. 2

    Mucus: The mucus is sticky and traps dust, pollen, and pathogens from the inhaled air. Cilia (tiny hairs) then sweep this mucus upwards to the throat to be swallowed, cleaning the air before it reaches the lungs. [2]

Recap

  • The pathway of air is: trachea → bronchi → bronchioles → alveoli.
  • The trachea is held open by rings of cartilage.
  • The airways are lined with mucus and cilia to trap and remove debris.
  • The lungs are the primary organs of the respiratory system.

Quick check

  1. What is the name of the tiny air sacs where gas exchange happens?1 mark
  2. What is the name of the two major tubes that branch off the trachea?1 mark

4. Gas Exchange in the Alveoli

The actual 'swap' of gases happens in the millions of alveoli. When you breathe in, the air in the alveoli has a high concentration of oxygen and a low concentration of carbon dioxide. The blood arriving at the lungs in the capillaries has the opposite: a low concentration of oxygen (as it's been used by body cells) and a high concentration of carbon dioxide (as it's a waste product). Due to these concentration gradients, oxygen diffuses from the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli to be breathed out. The thin walls of the alveoli and capillaries make this diffusion process extremely fast and efficient.

Key term

Concentration Gradient: The difference in the concentration of a substance between two areas, which causes particles to move from high to low concentration.

Examiner insight

Top-level answers always specify the direction of movement for both gases and clearly state that this movement is driven by diffusion down a concentration gradient.

Worked example 13 marks

Explain how oxygen moves from the alveoli into the blood. [3]

  1. 1

    The concentration of oxygen is higher in the air inside the alveoli than it is in the deoxygenated blood arriving at the lungs. [1]

  2. 2

    This creates a steep concentration gradient. [1]

  3. 3

    As a result, oxygen diffuses across the thin walls of the alveolus and the capillary into the red blood cells. [1]

Recap

  • Gas exchange occurs between the alveoli and the surrounding capillaries.
  • Oxygen diffuses from the alveoli into the blood down its concentration gradient.
  • Carbon dioxide diffuses from the blood into the alveoli down its concentration gradient.
  • The walls of the alveoli and capillaries are only one cell thick, creating a short diffusion path.
  • A constant blood flow maintains the concentration gradients.

Quick check

  1. Which gas moves from the blood to the alveoli?1 mark
  2. By what process do gases move between the alveoli and the blood?1 mark

5. The Mechanism of Breathing

Breathing, or ventilation, is the mechanical process of moving air into and out of the lungs. It involves the diaphragm (a sheet of muscle below the lungs) and the intercostal muscles (between the ribs).

Inhalation (Breathing In): This is an active process. The external intercostal muscles contract, pulling the rib cage up and out. The diaphragm contracts and flattens. These actions increase the volume of the chest cavity (thorax). This increased volume causes the pressure inside the lungs to drop below atmospheric pressure, and air rushes in.

Exhalation (Breathing Out): At rest, this is a passive process. The external intercostal muscles relax, and the rib cage moves down and in. The diaphragm relaxes and domes upwards. These actions decrease the volume of the thorax, increasing the pressure inside the lungs above atmospheric pressure, forcing air out.

Key term

Diaphragm: A large, dome-shaped sheet of muscle at the base of the rib cage that plays a crucial role in the mechanism of breathing.

Common pitfall

Mixing up the actions for inhalation and exhalation. A good way to remember is 'Inhalation Increases': the volume of the thorax Increases.

Worked example 14 marks

Describe the roles of the intercostal muscles and the diaphragm during inhalation. [4]

  1. 1

    During inhalation, the external intercostal muscles contract, while the internal intercostal muscles relax. [1]

  2. 2

    This contraction pulls the rib cage upwards and outwards. [1]

  3. 3

    At the same time, the diaphragm muscle contracts and flattens (moves down). [1]

  4. 4

    Both of these actions increase the volume of the thorax, which causes air to enter the lungs. [1]

Recap

  • Ventilation is the movement of air into and out of the lungs.
  • Inhalation is an active process involving muscle contraction.
  • During inhalation, the diaphragm contracts (flattens) and the ribs move up and out.
  • Exhalation at rest is a passive process.
  • During exhalation, the diaphragm relaxes (domes) and the ribs move down and in.
  • Changes in volume lead to changes in pressure, which drives airflow.

Quick check

  1. Is inhalation an active or passive process? Explain your answer.2 marks

6. Composition of Inhaled and Exhaled Air

The air we breathe in (inhaled air) is not the same as the air we breathe out (exhaled air). The differences are a direct result of gas exchange in the lungs and metabolic processes in our cells. Exhaled air contains less oxygen, because it has diffused into the blood. It contains much more carbon dioxide, because this waste gas has diffused out of the blood. It is also saturated with water vapour, as water evaporates from the moist surfaces of the alveoli. The temperature of exhaled air is typically body temperature (around 37°C). The amount of nitrogen remains unchanged because it is not used by the body.

Key term

Limewater: A solution of calcium hydroxide used to test for carbon dioxide, which turns it from clear to a milky or cloudy white.

Examiner insight

Examiners expect you to be able to quote the approximate percentages of oxygen and carbon dioxide in inhaled and exhaled air and explain the reasons for the change.

Worked example 14 marks

The table shows the percentage composition of some gases in inhaled and exhaled air.

GasInhaled Air (%)Exhaled Air (%)
Oxygen2116
Carbon Dioxide0.044
Nitrogen7979

Explain the reason for the difference in the percentage of oxygen and carbon dioxide. [4]

  1. 1

    Oxygen: The percentage of oxygen decreases from 21% to 16%. [1] This is because oxygen diffuses from the alveoli into the blood to be used by the body's cells for aerobic respiration. [1]

  2. 2

    Carbon Dioxide: The percentage of carbon dioxide increases from 0.04% to 4%. [1] This is because carbon dioxide is a waste product of aerobic respiration and diffuses from the blood into the alveoli to be exhaled. [1]

Recap

  • Inhaled air has about 21% oxygen; exhaled air has about 16%.
  • Inhaled air has about 0.04% carbon dioxide; exhaled air has about 4%.
  • The percentage of nitrogen gas is unchanged (~79%).
  • Exhaled air is saturated with water vapour and is at body temperature.
  • These differences are caused by gas exchange in the alveoli.

Quick check

  1. Why is there more water vapour in exhaled air than in inhaled air?1 mark
  2. Name a chemical you could use to prove that exhaled air contains more carbon dioxide than inhaled air.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. List four features that make the alveoli an effective surface for gas exchange. [4]4 marks
  2. Describe the process of exhalation when at rest. [4]4 marks
  3. Explain why the concentration of carbon dioxide in the blood arriving at the lungs is higher than in the air in the alveoli. [3]3 marks
  4. A student blows through a straw into a test tube of limewater. The limewater turns milky. What conclusion can be drawn from this observation, and why? [2]2 marks
  5. Distinguish between breathing and cellular respiration. [4]4 marks
  6. Describe the roles of mucus and cilia in the trachea and bronchi. [3]3 marks
  7. Explain the changes that occur in the thorax to cause inhalation. You should refer to the diaphragm, intercostal muscles, volume and pressure. [5]5 marks
  8. The blood contains red blood cells, which are full of haemoglobin. Explain how haemoglobin helps to maximise the rate of oxygen uptake in the lungs. [3]3 marks
  9. Emphysema is a disease where the walls of the alveoli break down, creating larger, irregular air sacs. Explain how this would affect a person's ability to absorb oxygen into their blood. [4]4 marks
  10. During an asthma attack, the muscles in the walls of the bronchioles contract, making the airways narrower. Explain why this makes it difficult for a person to breathe and can lead to a lower concentration of oxygen in the blood. [5]5 marks

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