Cambridge O Level5090

Human gas exchange

Biology 5090 Chapter Notes

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Human gas exchange
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In this chapter

  1. 1Introduction to Gas Exchange
  2. 2The Breathing System Anatomy
  3. 3Features for Efficient Gas Exchange
  4. 4The Mechanism of Breathing (Ventilation)
  5. 5Inspired vs. Expired Air
  6. 6Protection of the Gas Exchange System
  7. 7Control of Breathing Rate

1. Introduction to Gas Exchange

Every cell in your body needs energy to function. This energy is released from food through a chemical process called aerobic respiration. Aerobic respiration requires a constant supply of oxygen and produces carbon dioxide as a waste product. The human gas exchange system is responsible for getting oxygen from the air into the blood, and removing waste carbon dioxide from the blood into theair. This vital exchange of gases happens in the lungs.

Glucose + Oxygen → Carbon Dioxide + Water (+ Energy)

Key term

Aerobic Respiration: The chemical process in cells that uses oxygen to release energy from glucose, producing carbon dioxide and water as waste products.

Examiner insight

Examiners reward students who can clearly distinguish between breathing (also called ventilation) and cellular respiration, explaining the link between them.

Common pitfall

Confusing 'respiration' (the chemical process in cells) with 'breathing' (the physical act of moving air in and out of the lungs).

Worked example 13 marks

A student states, 'We breathe to get energy'. Explain why this statement is not entirely accurate and provide a more precise explanation.

  1. 1

    Step 1: Identify the inaccuracy. Breathing (ventilation) is the physical movement of air. It does not directly provide energy.

  2. 2

    Step 2: State the correct process for energy release. Energy is released by aerobic respiration, a chemical reaction that happens inside cells (specifically, mitochondria).

  3. 3

    Step 3: Link breathing to respiration. Breathing provides the oxygen needed for aerobic respiration to occur and removes the waste carbon dioxide that respiration produces. So, breathing enables respiration, which in turn releases energy.

Recap

  • Gas exchange supplies oxygen for aerobic respiration.
  • Aerobic respiration releases energy from food in cells.
  • Gas exchange removes waste carbon dioxide produced by respiration.
  • Breathing is the physical process, while respiration is the chemical process.

Quick check

  1. What is the primary waste gas removed by the gas exchange system?1 mark
  2. Which reactant for aerobic respiration is obtained via the lungs?1 mark

2. The Breathing System Anatomy

Air enters the body through the nose or mouth and travels down the trachea (windpipe). The trachea is kept open by C-shaped rings of cartilage. At the bottom, the trachea splits into two bronchi (singular: bronchus), one leading to each lung. Each bronchus branches into smaller and smaller tubes called bronchioles. These bronchioles end in millions of tiny air sacs called alveoli. It is in the alveoli that gas exchange with the blood actually takes place.

Key term

Alveoli: Microscopic air sacs in the lungs where the exchange of oxygen and carbon dioxide takes place between the air and the blood.

Fun fact

If you could lay all the alveoli in your lungs out flat, their total surface area would be about 70 square metres, roughly the size of a tennis court!

Worked example 14 marks

Trace the path of an oxygen molecule from the atmosphere until it reaches the blood.

  1. 1

    Step 1: Air is inhaled into the nasal cavity or mouth.

  2. 2

    Step 2: It passes down the trachea.

  3. 3

    Step 3: It enters one of the two bronchi.

  4. 4

    Step 4: It travels through progressively smaller bronchioles.

  5. 5

    Step 5: It enters an alveolus.

  6. 6

    Step 6: It dissolves in the moist lining of the alveolus and diffuses across the alveolar wall and the capillary wall into a red blood cell.

Recap

  • The pathway of air is: trachea → bronchi → bronchioles → alveoli.
  • The trachea is supported by rings of cartilage to prevent it from collapsing.
  • The bronchi lead to the left and right lungs.
  • Gas exchange occurs in the alveoli.

Quick check

  1. What is the function of the cartilage rings in the trachea?1 mark
  2. What are the smaller branches of the bronchi called?1 mark

3. Features for Efficient Gas Exchange

The alveoli are perfectly adapted for efficient gas exchange. Their effectiveness relies on four key features that maximize the rate of diffusion: 1. Large surface area: Millions of alveoli create a vast area for gases to diffuse across. 2. Thin walls: The walls of the alveoli and the surrounding capillaries are each only one cell thick, creating a very short distance for gases to travel. 3. Good blood supply: A dense network of capillaries surrounds each alveolus, constantly carrying blood to the lungs. This maintains a steep concentration gradient by swiftly transporting oxygen away and bringing carbon dioxide. 4. Moist surface: The inner surface of the alveoli is covered in a thin layer of fluid, allowing oxygen to dissolve before it diffuses into the blood.

Key term

Concentration Gradient: The difference in the concentration of a substance between two areas, which drives the movement of that substance from a high to a low concentration.

Examiner insight

To get full marks, you must link each adaptive feature to how it increases the rate of diffusion. For example, state that 'thin walls provide a short diffusion distance'.

Worked example 13 marks

Explain how a good blood supply increases the efficiency of gas exchange in the alveoli.

  1. 1

    Step 1: State the role of the blood. Blood flowing to the lungs is low in oxygen and high in carbon dioxide.

  2. 2

    Step 2: Explain the gradient for oxygen. As oxygen diffuses from the alveoli into the blood, the blood is immediately transported away from the lungs. This ensures the blood arriving at the lungs always has a lower oxygen concentration than the air in the alveoli.

  3. 3

    Step 3: Explain the gradient for carbon dioxide. Similarly, blood constantly brings more carbon dioxide to the lungs, ensuring its concentration in the blood is always higher than in the alveolar air.

  4. 4

    Step 4: Conclude the effect. This continuous transport of blood maintains a steep concentration gradient for both gases, which maximises the rate of diffusion.

Recap

  • Efficient gas exchange requires a large surface area.
  • A short diffusion distance is achieved by one-cell-thick walls.
  • A good blood supply maintains a steep concentration gradient.
  • A moist surface allows gases like oxygen to dissolve before diffusing.

Quick check

  1. State two features of an alveolus that helps with gas exchange.2 marks

4. The Mechanism of Breathing (Ventilation)

Ventilation is the process of moving air into (inhalation) and out of (exhalation) the lungs. It is controlled by the diaphragm and the intercostal muscles. Inhalation: The external intercostal muscles contract and the internal intercostal muscles relax, pulling the rib cage up and outwards. The diaphragm muscle contracts and flattens. These actions increase the volume of the thorax (chest cavity). This increase in volume causes a decrease in pressure inside the lungs, drawing air in. Exhalation (at rest): The external intercostal muscles relax and the internal intercostal muscles relax. The rib cage moves down and inwards due to gravity. The diaphragm muscle relaxes and domes upwards. These actions decrease the volume of the thorax, increasing the pressure inside the lungs and forcing air out.

Key term

Ventilation: The movement of air into and out of the lungs, brought about by the actions of the intercostal muscles and the diaphragm.

Common pitfall

Mixing up the relationship between volume and pressure. Remember: when volume goes up, pressure goes down, and vice versa.

Worked example 13 marks

Describe the changes that occur to the diaphragm and rib cage during inhalation.

  1. 1

    Step 1: Describe the diaphragm's action. The diaphragm muscle contracts and moves down (flattens).

  2. 2

    Step 2: Describe the intercostal muscles' action. The external intercostal muscles contract, while the internal intercostal muscles relax.

  3. 3

    Step 3: Describe the rib cage's movement. The contraction of the external intercostal muscles pulls the rib cage upwards and outwards.

Worked example 23 marks

Explain how the changes during inhalation lead to air entering the lungs, with reference to volume and pressure.

  1. 1

    Step 1: Link muscle action to volume. The contraction of the diaphragm and external intercostal muscles increases the volume of the thorax.

  2. 2

    Step 2: Link volume to pressure. According to Boyle's law, increasing the volume of the thorax causes the pressure inside the lungs to decrease, becoming lower than the atmospheric pressure outside the body.

  3. 3

    Step 3: Link pressure to air movement. Because air moves from an area of higher pressure to an area of lower pressure, air is drawn into the lungs from the atmosphere.

Recap

  • Inhalation is an active process involving muscle contraction.
  • During inhalation, thorax volume increases and pressure decreases.
  • Exhalation at rest is a largely passive process involving muscle relaxation.
  • During exhalation, thorax volume decreases and pressure increases.
  • Air always moves from a region of higher pressure to a region of lower pressure.

Quick check

  1. Which set of intercostal muscles contracts to lift the ribs during inhalation?1 mark
  2. Does the volume of the thorax increase or decrease during exhalation?1 mark

5. Inspired vs. Expired Air

The composition of the air we breathe in (inspired air) is different from the air we breathe out (expired air).

  • Oxygen: Inspired air is about 21% oxygen. In the lungs, oxygen diffuses into the blood. Expired air contains less oxygen, about 16%.
  • Carbon Dioxide: Inspired air has very little carbon dioxide, about 0.04%. Carbon dioxide diffuses from the blood into the lungs to be removed. Expired air contains a much higher concentration, about 4%.
  • Water Vapour: The amount in inspired air is variable depending on the weather. The air passages are moist, so expired air is always saturated with water vapour.
  • Temperature: Expired air is warmed to body temperature (37°C).

We can test for carbon dioxide using limewater (calcium hydroxide solution). When you bubble expired air through limewater, it turns cloudy or milky, showing the presence of CO2.

Key term

Limewater: An aqueous solution of calcium hydroxide, Ca(OH)₂, used to test for carbon dioxide gas, which turns it from clear to milky/cloudy.

Examiner insight

When comparing inspired and expired air, using approximate percentages will gain more credit than vague terms like 'more' or 'less'.

Worked example 13 marks

A student bubbles both inspired and expired air through two separate test tubes of limewater. Predict and explain the results.

  1. 1

    Step 1: State the result for inspired air. The limewater bubbled with inspired air will show no change or a very slow, slight cloudiness. This is because inspired air contains a very low concentration of carbon dioxide (0.04%).

  2. 2

    Step 2: State the result for expired air. The limewater bubbled with expired air will turn cloudy or milky quickly.

  3. 3

    Step 3: Explain the result for expired air. This is because expired air contains a much higher concentration of carbon dioxide (about 4%), which is a waste product of cellular respiration transported to the lungs for removal.

Recap

  • Inspired air has more oxygen (~21%) than expired air (~16%).
  • Expired air has much more carbon dioxide (~4%) than inspired air (~0.04%).
  • Expired air is saturated with water vapour and is warmer than inspired air.
  • Limewater turns milky in the presence of carbon dioxide.

Quick check

  1. State the approximate percentage of oxygen in the air you breathe out.1 mark
  2. What observation is made when testing expired air with limewater?1 mark

6. Protection of the Gas Exchange System

The air we breathe contains dust, pollen, and pathogens like bacteria and viruses. The gas exchange system has a clever cleaning system to stop these from reaching the delicate alveoli. The trachea and bronchi are lined with two special types of cells: goblet cells and ciliated cells. Goblet cells secrete sticky mucus, which traps inhaled particles. Ciliated cells are covered in tiny hair-like structures called cilia. These cilia beat in a coordinated, wave-like motion to sweep the mucus (along with the trapped debris) upwards, away from the lungs and towards the throat. This 'mucus escalator' allows the mucus to be either swallowed or coughed out, keeping the airways clear.

Key term

Cilia: Tiny hair-like structures on the surface of epithelial cells in the airways that beat rhythmically to move mucus upwards and out of the lungs.

Fun fact

Cigarette smoke contains chemicals that paralyse and eventually destroy cilia. This is why smokers are more prone to lung infections.

Worked example 14 marks

Explain the roles of goblet cells and ciliated cells in protecting the lungs.

  1. 1

    Step 1: Describe the role of goblet cells. Goblet cells produce and secrete a sticky fluid called mucus.

  2. 2

    Step 2: Explain the function of mucus. This mucus lines the airways (trachea and bronchi) and traps particles such as dust, pollen, and pathogens that are inhaled.

  3. 3

    Step 3: Describe the role of ciliated cells. Ciliated cells have cilia that beat in a coordinated rhythm.

  4. 4

    Step 4: Explain how they work together. This beating action moves the mucus upwards, away from the lungs towards the throat, where it can be swallowed. This process prevents the particles from reaching and damaging the alveoli.

Recap

  • Goblet cells in the trachea and bronchi produce mucus.
  • Mucus traps dust, pollen, and pathogens.
  • Ciliated cells have cilia that sweep the mucus upwards.
  • This 'mucus escalator' keeps the lungs clean.
  • Smoking can damage cilia, leading to mucus build-up and a 'smoker's cough'.

Quick check

  1. Which cells produce mucus in the airways?1 mark
  2. In which direction do cilia move mucus?1 mark

7. Control of Breathing Rate

Your breathing rate changes automatically depending on your body's needs. This is controlled by the brain, specifically a region called the medulla oblongata. During exercise, your muscles respire more quickly to produce extra energy. This increases the concentration of carbon dioxide in your blood. Receptors in the medulla and in the walls of major arteries detect the increased CO2 level (which makes the blood slightly more acidic). In response, the medulla sends more frequent nerve impulses to the diaphragm and intercostal muscles. This causes them to contract more frequently (increasing breathing rate) and more forcefully (increasing breathing depth). As a result, you breathe faster and deeper, which expels the excess CO2 more quickly and takes in more oxygen for your working muscles.

Key term

Medulla Oblongata: The part of the brainstem that controls vital involuntary functions, including the rate and depth of breathing, by monitoring blood carbon dioxide levels.

Examiner insight

Clear, logical explanations that link increased exercise to increased respiration, then to increased CO2 in the blood, and finally to the brain's response, score the highest marks.

Common pitfall

Stating that the body increases breathing rate primarily because of a lack of oxygen. While oxygen levels do have an effect, the main driver is the concentration of carbon dioxide in the blood.

Worked example 14 marks

Explain why a sprinter's breathing rate remains high for a short period after they have finished a 100m race.

  1. 1

    Step 1: State the initial cause. During the race, the sprinter's muscles respire at a very high rate, producing a large amount of carbon dioxide.

  2. 2

    Step 2: Link to blood composition. This CO2 diffuses into the blood, causing its concentration to rise significantly.

  3. 3

    Step 3: Describe the body's response. Receptors in the brain detect the high CO2 level and maintain a high rate and depth of breathing.

  4. 4

    Step 4: Explain the purpose. The continued fast, deep breathing is necessary to remove the excess CO2 from the blood and replenish the oxygen used during the sprint, allowing the body's chemistry to return to its resting state.

Recap

  • Breathing rate is controlled by the medulla in the brain.
  • The primary trigger for an increase in breathing rate is an increase in blood carbon dioxide concentration.
  • During exercise, increased respiration leads to more CO2 in the blood.
  • The brain responds by increasing the rate and depth of breathing.
  • This helps to remove CO2 faster and supply more O2 to the muscles.

Quick check

  1. Which chemical in the blood is the main stimulus for changing the breathing rate?1 mark
  2. Which part of the brain controls breathing?1 mark

End-of-chapter exercise

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

  1. Define the term ventilation.2 marks
  2. State three features of the alveoli that make them efficient gas exchange surfaces.3 marks
  3. Describe the path taken by an oxygen molecule from the trachea to a red blood cell.3 marks
  4. Compare the composition of inspired and expired air. Include approximate percentage values for oxygen and carbon dioxide.3 marks
  5. Explain the roles of the external intercostal muscles and the diaphragm during inhalation.4 marks
  6. Describe how you would use limewater to show that expired air contains more carbon dioxide than inspired air. Include the expected results.4 marks
  7. Explain, in terms of volume and pressure, the mechanism of exhalation at rest.4 marks
  8. Explain how goblet cells and ciliated cells work together to protect the gas exchange system from pathogens.4 marks
  9. A person starts jogging. Explain the sequence of events that causes their rate and depth of breathing to increase.5 marks
  10. Long-term smoking damages the walls of the alveoli, causing them to break down and merge. Explain how this would affect the process of gas exchange.3 marks

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