Cambridge IGCSE0970

Gas exchange in humans

Biology 0970 Chapter Notes

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Gas exchange in humans
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In this chapter

  1. 1The Link Between Respiration and Gas Exchange
  2. 2Features of an Efficient Gas Exchange Surface
  3. 3The Human Respiratory System
  4. 4Gas Exchange in the Alveoli
  5. 5The Mechanics of Breathing (Ventilation)
  6. 6Air Composition and the Effects of Exercise

2. Features of an Efficient Gas Exchange Surface

For gases to be exchanged efficiently, the respiratory surface must have specific features. These adaptations are all designed to maximise the rate of diffusion. In humans, the alveoli in the lungs provide this surface. The key features are: a large surface area, a thin lining, a good blood supply, a moist surface, and good ventilation.

Key term

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

Examiner insight

Examiners look for answers that explicitly link each adaptive feature of the alveoli to its effect on the rate of diffusion.

Common pitfall

Stating a feature without explaining how it helps diffusion. For example, just writing 'thin walls' is not enough; you must link it to a 'short diffusion distance'.

Fun fact

If you could spread out the total surface area of all the alveoli in an adult's lungs, it would cover an area of about 70 square metres, the size of half a tennis court.

Worked example 14 marks

Explain how the alveoli in the lungs are adapted for efficient gas exchange. (4 marks)

  1. 1

    Step 1: Large surface area. There are millions of alveoli, which collectively provide a huge surface area (like a tennis court) for gases to diffuse across simultaneously.

  2. 2

    Step 2: Thin walls. The walls of the alveoli and the surrounding capillaries are each only one cell thick, providing a very short diffusion distance for gases.

  3. 3

    Step 3: Good blood supply. A dense network of capillaries surrounds each alveolus, which quickly transports gases to and from the lungs, maintaining a steep concentration gradient.

  4. 4

    Step 4: Moist lining. The surface is covered in a thin layer of moisture, which allows oxygen to dissolve before it diffuses across the membrane.

Recap

  • A large surface area allows more gas molecules to diffuse at the same time.
  • Thin walls create a short diffusion distance, speeding up gas exchange.
  • A good blood supply maintains a steep concentration gradient by carrying gases away.
  • A moist surface is needed for oxygen to dissolve before diffusing into the blood.
  • Good ventilation ensures a high concentration of oxygen and a low concentration of carbon dioxide in the alveoli.

Quick check

  1. List two features of an ideal gas exchange surface.2 marks

3. The Human Respiratory System

The human respiratory system is a complex set of organs and tubes that transports air to the gas exchange surface. 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 branches into two bronchi (one for each lung), which further divide into smaller and smaller tubes called bronchioles. These bronchioles end in tiny, clustered air sacs called alveoli, where gas exchange occurs. The inside of the airways is lined with cells that produce mucus to trap dust and pathogens, and cilia (tiny hairs) that sweep this mucus up and out of the lungs.

Key term

Alveoli: Tiny, balloon-like air sacs in the lungs where the exchange of oxygen and carbon dioxide with the blood takes place.

Worked example 14 marks

Trace the path of an oxygen molecule from the air outside the body to an alveolus.

  1. 1

    Step 1: The oxygen molecule enters the body through the nose or mouth.

  2. 2

    Step 2: It passes down the pharynx and larynx into the trachea (windpipe).

  3. 3

    Step 3: It travels down the trachea, which branches into a bronchus.

  4. 4

    Step 4: The bronchus branches into progressively smaller bronchioles.

  5. 5

    Step 5: The oxygen molecule finally reaches an alveolus at the end of a bronchiole.

Worked example 22 marks

Explain the function of the cartilage rings in the trachea.

  1. 1

    Step 1: The cartilage provides structural support to the trachea.

  2. 2

    Step 2: This support prevents the trachea from collapsing when the pressure inside it is low, such as during inhalation, ensuring the airway remains open for air to pass through.

Recap

  • Air travels through the trachea, bronchi, and bronchioles to reach the alveoli.
  • The trachea is supported by C-shaped rings of cartilage to prevent collapse.
  • The airways are lined with mucus and cilia to trap and remove debris.
  • The alveoli are the site of gas exchange.

Quick check

  1. What is the name of the tubes that branch off the trachea?1 mark
  2. What is the role of cilia in the airways?1 mark

4. Gas Exchange in the Alveoli

At the alveoli, gas exchange happens by diffusion. 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 pulmonary artery capillaries has come from the respiring body tissues, so it has a low concentration of oxygen and a high concentration of carbon dioxide. This difference in concentrations, known as the concentration gradient, drives diffusion. Oxygen diffuses from the alveoli into the red blood cells in the capillaries. At the same time, carbon dioxide diffuses from the blood plasma into the alveoli to be breathed out.

Key term

Concentration Gradient: The difference in the concentration of a substance between two areas, which provides the driving force for diffusion.

Examiner insight

To gain full marks on questions about gas exchange, students must refer to diffusion and the relative concentrations (or concentration gradient) for both oxygen and carbon dioxide.

Common pitfall

Only describing the movement of oxygen and forgetting to mention the simultaneous movement of carbon dioxide in the opposite direction.

Worked example 14 marks

Describe how oxygen moves from the alveoli into the blood.

  1. 1

    Step 1: Inhaled air in the alveoli has a high concentration (or partial pressure) of oxygen.

  2. 2

    Step 2: Blood arriving at the lungs in the capillaries has a low concentration of oxygen because it has been used by body cells for respiration.

  3. 3

    Step 3: This creates a steep concentration gradient between the alveoli and the blood.

  4. 4

    Step 4: Oxygen diffuses across the thin walls of the alveolus and the capillary, down its concentration gradient, into the red blood cells.

Recap

  • Gas exchange at the alveoli occurs via diffusion.
  • Oxygen diffuses from the alveoli (high concentration) to the blood (low concentration).
  • Carbon dioxide diffuses from the blood (high concentration) to the alveoli (low concentration).
  • These processes happen simultaneously across the alveolar and capillary walls.
  • A steep concentration gradient is essential for rapid diffusion.

Quick check

  1. Which gas moves from the blood to the alveoli?1 mark
  2. Name the process by which gases move between the alveoli and the blood.1 mark

5. The Mechanics of Breathing (Ventilation)

Breathing, or ventilation, is the physical 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 outwards. The diaphragm contracts and flattens (moves down). Both actions increase the volume of the chest cavity (thorax). This causes the pressure inside the lungs to fall below atmospheric pressure, and air is drawn into the lungs.

Exhalation (breathing out): This is largely a passive process at rest. The external intercostal muscles relax, allowing the rib cage to move down and inwards. The diaphragm relaxes and domes upwards. Both actions decrease the volume of the thorax, increasing the pressure inside the lungs above atmospheric pressure, forcing air out.

Key term

Ventilation: The mechanical process of moving air into (inhalation) and out of (exhalation) the lungs to maintain the concentration gradients needed for gas exchange.

Examiner insight

Clear, sequential descriptions are essential. Examiners reward answers that correctly link muscle action -> volume change -> pressure change -> air movement.

Common pitfall

Confusing the pressure and volume changes. Remember the rule: as volume increases, pressure decreases, and vice versa.

Worked example 15 marks

Describe and explain the process of inhalation.

  1. 1

    Step 1: The external intercostal muscles contract, and the internal intercostal muscles relax. This pulls the rib cage upwards and outwards.

  2. 2

    Step 2: The diaphragm muscle contracts and flattens.

  3. 3

    Step 3: These movements increase the volume of the thorax (chest cavity).

  4. 4

    Step 4: The increase in volume causes a decrease in pressure inside the lungs to below atmospheric pressure.

  5. 5

    Step 5: Air flows into the lungs from the higher external pressure to the lower internal pressure until the pressures are equal.

Recap

  • Inhalation: Ribs move up and out, diaphragm moves down, volume increases, pressure decreases, air moves in.
  • Exhalation: Ribs move down and in, diaphragm moves up, volume decreases, pressure increases, air moves out.
  • Inhalation is an active process involving muscle contraction.
  • Quiet exhalation is a passive process involving muscle relaxation.
  • Ventilation maintains the concentration gradients for O2 and CO2 in the alveoli.

Quick check

  1. During inhalation, does the volume of the thorax increase or decrease?1 mark
  2. Which set of muscles contracts to pull the rib cage upwards and outwards?1 mark

6. Air Composition and the Effects of Exercise

The composition of the air we breathe in (inhaled) is different from the air we breathe out (exhaled). Inhaled air contains about 21% oxygen and 0.04% carbon dioxide. Exhaled air contains less oxygen (around 16-18%) and more carbon dioxide (around 4%) because of gas exchange in the lungs. Exhaled air is also saturated with water vapour and is warmed to body temperature (37°C).

During exercise, muscles respire more to release more energy. This increases the demand for oxygen and produces more carbon dioxide. The body responds by increasing both the breathing rate (breaths per minute) and the depth of breathing (tidal volume). This increases the total volume of air ventilated, allowing more oxygen to be supplied to the blood and more carbon dioxide to be removed, maintaining safe levels in the blood.

Key term

Tidal Volume: The volume of air moved into or out of the lungs during a single, normal breath at rest.

Examiner insight

When asked to explain data about air composition, students must link the changes in gas percentages directly to the processes of respiration and gas exchange.

Worked example 13 marks

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

GasInhaled Air (%)Exhaled Air (%)
Oxygen2116
Carbon Dioxide0.044
Nitrogen7878

Explain the difference in the percentage of oxygen.

  1. 1

    Step 1: The percentage of oxygen is lower in exhaled air (16%) than in inhaled air (21%).

  2. 2

    Step 2: This is because oxygen is needed by the body's cells for aerobic respiration.

  3. 3

    Step 3: In the lungs, oxygen diffuses from the alveoli, where its concentration is high, into the blood to be transported to the cells. This removes it from the air that is then exhaled.

Worked example 24 marks

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

  1. 1

    Step 1: During exercise, muscles contract more frequently and require more energy.

  2. 2

    Step 2: To release this energy, the rate of aerobic respiration in muscle cells increases.

  3. 3

    Step 3: This increased respiration uses up oxygen more quickly and produces more carbon dioxide as a waste product.

  4. 4

    Step 4: The breathing rate increases to take in more oxygen and remove the excess carbon dioxide from the body more quickly, maintaining the concentration gradients at the alveoli.

Recap

  • Exhaled air has less oxygen and more carbon dioxide than inhaled air.
  • Exhaled air is warmer and more moist than inhaled air.
  • Nitrogen concentration does not change as it is not used by the body.
  • Exercise increases the demand for energy, leading to a higher rate of respiration.
  • The body responds to exercise by increasing breathing rate and depth.

Quick check

  1. Which gas is found in a higher percentage in exhaled air compared to inhaled air?1 mark
  2. State two ways breathing changes during exercise.2 marks

End-of-chapter exercise

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

  1. Distinguish between aerobic respiration and ventilation (breathing).2 marks
  2. List four features that make the alveoli an efficient surface for gas exchange.4 marks
  3. Describe the path taken by a molecule of carbon dioxide from a respiring muscle cell to the atmosphere.5 marks
  4. Explain the mechanical process that causes air to be forced out of the lungs during quiet exhalation.4 marks
  5. A student uses limewater to test samples of inhaled and exhaled air. Predict the results and explain the reason for any difference.3 marks
  6. Explain why a constant flow of blood through the capillaries surrounding the alveoli is essential for maintaining a high rate of gas exchange.3 marks
  7. During a long-distance run, an athlete's tidal volume and breathing rate both increase. Explain the physiological importance of these changes.4 marks
  8. A lung disease causes the breakdown of the walls between adjacent alveoli, forming larger, irregular air sacs. Explain how this would affect the efficiency of gas exchange.3 marks
  9. Describe the roles of the diaphragm and the external intercostal muscles during inhalation.4 marks
  10. Why is the percentage of nitrogen in inhaled air the same as in exhaled air?2 marks

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