Gas exchange in humans

C_6H_{12O_6} + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}

This process occurs in the cells of the body to release energy for muscle contraction and other life processes. The breathing system facilitates this by moving air in and out of the lungs.

Air travels through a specific pathway. You must be able to identify these parts in diagrams:

1. Nose/Mouth: Air enters here. The nose has hairs and mucus to filter particles.
2. Trachea (Windpipe): A tube supported by C-shaped rings of cartilage. Cartilage is a flexible tissue that keeps the trachea open, preventing it from collapsing when pressure changes during breathing.
3. Bronchi: The trachea splits into two bronchi, one leading to each lung.
4. Bronchioles: The bronchi branch into smaller tubes called bronchioles. These have no cartilage support but are surrounded by smooth muscle.
5. Alveoli (Singular: Alveolus): Tiny air sacs at the end of the bronchioles where gas exchange occurs. They are surrounded by a dense network of capillaries.

The breathing system has mechanisms to protect the delicate alveoli from pathogens (bacteria/viruses) and dust particles. This involves three key components working together:

1. Goblet Cells: These are specialized epithelial cells found in the lining of the trachea and bronchi. Their specific function is to secrete mucus.
2. Mucus: A sticky substance produced by goblet cells that traps dust particles and pathogens, preventing them from reaching the alveoli.
3. Ciliated Cells: These are epithelial cells with hair-like projections called cilia. The cilia beat in a coordinated wave to move the mucus (with trapped particles) upwards towards the throat. This is often described as the 'mucociliary escalator'. Once in the throat, the mucus is swallowed and destroyed by stomach acid.

To understand ventilation, you must identify and know the function of:

1. Diaphragm: A dome-shaped muscle at the base of the thorax (chest cavity).
2. External Intercostal Muscles: Muscles located between the ribs that run downwards and forwards.
3. Internal Intercostal Muscles: Muscles located between the ribs that run downwards and backwards. These are primarily used during forced expiration.

Method to Investigate Differences:

1. Collect a sample of inspired air (or use ambient air) and bubble it through limewater in Test Tube A.
2. Collect a sample of expired air (by breathing out through a straw/tube) and bubble it through limewater in Test Tube B.

Observation:

• Test Tube A (Inspired): The limewater remains clear or turns cloudy very slowly because inspired air has a low concentration of CO_2 (~0.04%).
• Test Tube B (Expired): The limewater turns cloudy/milky/white precipitate forms rapidly and intensely because expired air has a much higher concentration of CO_2 (~4%).

This investigation proves that respiration produces carbon dioxide.

To fully explain the differences (Supplement level), you must link the changes to biological processes:

1. Oxygen Decrease: Oxygen is absorbed from the alveoli into the blood to be used for aerobic respiration in cells (C_6H_{12O_6} + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}). It is not 'used up' in the lungs, but diffuses away.
2. Carbon Dioxide Increase: Carbon dioxide is a waste product of aerobic respiration. It diffuses from the blood into the alveoli to be removed.
3. Water Vapour Increase: The lining of the trachea and alveoli is moist. Water evaporates from this lining into the air during breathing, so expired air is saturated with water vapour.
4. Nitrogen Unchanged: Nitrogen makes up the majority of air but is inert; it is not used by the body for respiration or other metabolic processes, so its percentage remains roughly constant.

1. Breathing Rate Increases: You take more breaths per minute.
2. Breathing Depth Increases: Each breath moves a larger volume of air.

Both changes result in an increased total ventilation (more air exchanged per minute).

To explain why breathing changes during exercise, you must follow this chain of events:

1. Increased Energy Demand: Muscles contract more frequently and vigorously during exercise, requiring more energy.
2. Increased Respiration: To release more energy, cells increase the rate of aerobic respiration (C_6H_{12O_6} + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}).
3. Increased CO_2 Production: This increased respiration produces more carbon dioxide as a waste product.
4. Detection by Brain: The carbon dioxide dissolves in the blood plasma, forming carbonic acid, which lowers the pH. Chemoreceptors detect this increased carbon dioxide concentration (or decreased pH) in the blood.
5. Nerve Impulses: These receptors send impulses to the respiratory centre in the brain (medulla oblongata).
6. Muscle Response: The brain sends increased nerve impulses to the diaphragm and intercostal muscles.
7. Result: The muscles contract more frequently and with greater force, increasing both the rate and depth of breathing to remove the excess CO_2 and take in more O_2.

Graphing Breathing Data:

• When plotting breathing rate against time during exercise, you should see a sharp increase in rate at the start of exercise, a plateau or continued rise depending on intensity, and a gradual decrease (recovery) after exercise stops.
• Always label axes with units (e.g., 'Breathing Rate (breaths/min)').
• Use a suitable scale that occupies at least half the grid.

Investigation Design:

• If asked to plan an investigation into the effect of exercise intensity on breathing rate:
  • Independent Variable: Intensity of exercise (e.g., speed of treadmill, height of step).
  • Dependent Variable: Breathing rate (measured in breaths per minute).
  • Control Variables: Duration of exercise, age/fitness of subject, type of exercise.
  • Method: Measure resting breathing rate. Perform exercise at intensity X for Y minutes. Measure breathing rate immediately after. Repeat at intensity Z.